Playas of the Great Plains 9780292798847

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PLAYAS OF THE GREAT PLAINS

NUMBER THREE

Peter T. Flawn Series in Natural Resource Management and Conservation

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PLAYAS OF THE GREAT PLAINS

loren m. smith

UNIVERSITY OF TEXAS PRESS AUSTIN

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The Peter T. Flawn Series in Natural Resource Management and Conservation is supported by a grant from the National Endowment for the Humanities and by gifts from the following donors:

Jenkins Garrett Edward H. Harte Houston H. Harte Jess T. Hay Mrs. Lyndon B. Johnson Bryce & Jonelle Jordan Ben F. & Margaret Love Wales H. & Abbie Madden

Sue Brandt McBee Charles Miller Beth R. Morian James L. & Nancy H. Powell Tom B. Rhodes Louise Saxon Edwin R. & Molly Sharpe Larry E. & Louann Temple

Copyright © 2003 by the University of Texas Press All rights reserved Printed in the United States of America First edition, 2003 Requests for permission to reproduce material from this work should be sent to Permissions, University of Texas Press, Box 7819, Austin, TX 78713-7819.
The paper used in this book meets the minimum 䊊 requirements of ANSI/NISO Z39.48-1992 (R1997) (Permanence of Paper). LIBR ARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA

Smith, Loren M. Playas of the Great Plains / Loren M. Smith. p. cm. — (Peter T. Flawn series in natural resource management and conservation ; no. 3) Includes bibliographical references and index. ISBN 0-292-70534-4 (cloth : alk. paper) — ISBN 0-292-70177-2 (pbk : alk. paper) 1. Wetland ecology—High Plains (U.S.) 2. Playas— High Plains (U.S.) I. Title. II. Series. QH104.5.G7 S65 2003 577.680978 — dc21 2003002153 Cover photo by Wyman Meinzer, courtesy of the U.S. Fish and Wildlife Service.

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FOR JANIECE

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CONTENTS

List of Illustrations ix List of Tables xiii Preface xv PLAYAS AND THEIR ENVIRONMENT CHAPTER 1

What Is a Playa? 3

CHAPTER 2

Origin and Development 29 ECOSYSTEM ASPECTS

1

43

CHAPTER 3

Flora 45

CHAPTER 4

Fauna 66

CHAPTER 5

Structure, Function, and Diversity 108 CONSERVATION ASPECTS

139

CHAPTER 6

Historical, Cultural, and Current Societal Value of Playas 141

CHAPTER 7

Threats to Proper Function of Playas 160

CHAPTER 8

Conservation Past, Present, and Future 177 Appendix 202 References 219 Index 247

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LIST OF ILLUSTR ATIONS

FIGURE 1.1

FIGURE 1.2 FIGURE 1.3 FIGURE 1.4

FIGURE 1.5 FIGURE 1.6 FIGURE 1.7 FIGURE 1.8 FIGURE 1.9 FIGURE 1.10 FIGURE 2.1

FIGURE 2.2 FIGURE 2.3

An area of the central Southern High Plains showing the highest density of playas in the Great Plains 4 The Great Plains of North America with three major grassland zones 5 Aerial view of an individual playa illustrating its circular form 9 Cross-section drawing of typical Southern High Plains playa showing little elevation change in the basin 10 Location of the Southern High Plains, or Llano Estacado, in the Great Plains 11 Nebraska’s four areas of playas 16 Playas with pits excavated in them to aid with irrigation of the surrounding watershed 17 A modified playa in the Comanche National Grassland, Baca County, Colorado 18 Modified playa in Rainwater Basin of southcentral Nebraska 19 Agriculture in the western Great Plains is dependent on irrigation 27 A typical circular-shaped playa in the Southern Great Plains and typical oblong-shaped playa of the Rainwater Basin 30 The northwestern escarpment, or “caprock,” of the Southern High Plains 32 Sections of topographic maps from the vicinity of Petersburg, Texas 34

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FIGURE 2.4

A “buffalo wallow” in the eastern portion of the Texas Panhandle (1904) 37 Lunette on the southeast side of a Rainwater Basin playa 41 A typical Southern Great Plains playa showing two vegetation zones 52 Windrow of seeds in a playa 53 Counties sampled for flora description in the Southern Great Plains 60 Vegetation zones in Rainwater Basin wetlands 64 Common invertebrates sampled in playas 68 Three of the most common amphibians in Great Plains playas 73 Playas throughout the Great Plains serve as important migration habitat for more than 30 species of shorebirds 84 Generalized shape of migration corridor for many waterfowl species in the Central Flyway during spring 86 Dabbling ducks often occur in tremendous densities on playas during migration 88 Dabbling duck species form pair bonds while wintering in playas 92 Grasshopper sparrows, mallards, and American coots nesting in playa basins 99 Hispid pocket mice and black-tailed prairie dogs, common mammals in the playa watershed 106 Suggested food web of a wet playa 121 Decomposition rates of pink smartweed 123 Graphical illustration of the weak relationship between playa area and total species richness 132 Southern High Plains escarpment 143 Archaeological site in a saline lake wetland 147 Cattle watering in a Texas Panhandle playa (1904) 152

FIGURE 2.5 FIGURE 3.1 FIGURE 3.2 FIGURE 3.3 FIGURE 3.4 FIGURE 4.1 FIGURE 4.2 FIGURE 4.3

FIGURE 4.4

FIGURE 4.5 FIGURE 4.6 FIGURE 4.7 FIGURE 4.8

FIGURE 5.1 FIGURE 5.2 FIGURE 5.3

FIGURE 6.1 FIGURE 6.2 FIGURE 6.3

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LIST OF ILLUSTRATIONS

FIGURE 6.4 FIGURE 6.5 FIGURE 7.1 FIGURE 7.2 FIGURE 8.1 FIGURE 8.2 FIGURE 8.3

Abandoned farmstead in a wheat field in the western High Plains 153 Urban playa in Lubbock, Texas 157 Cross section of a playa showing sediment influence 163 Playas associated with cattle feedlots receive significant fecal runoff 170 Education programs for schoolchildren to promote playa conservation 182 A native vegetation buffer strip being established in a playa watershed 188 A playa in Texas that has been “protected” through a conservation easement 195

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LIST OF TABLES

TABLE 1.1 TABLE 1.2

TABLE 3.1 TABLE 3.2 TABLE 4.1 TABLE 4.2 TABLE 4.3 TABLE 4.4

TABLE 4.5 TABLE 5.1 TABLE 5.2 TABLE 5.3 TABLE 5.4

Number and total area of playas in 54 counties 14 Mean daily temperature and mean monthly precipitation for selected playa regions, 1961– 1990 22 Common playa algae and macroalgae in the Southern High Plains 46 Playa vegetation classified into 14 physiognomic types 57 Amphibians inhabiting playas of the Great Plains 74 Shorebirds associated with Great Plains playas during migration 81 Waterfowl species observed migrating through the playa lakes 85 Esophageal foods from hunter-shot northern pintails and from ducks collected while observed feeding 94 Mammal species likely associated with Great Plains playas 104 Range of water variables found in playas of the Southern High Plains 111 Mean aboveground standing crop of five common playa wetland plants 117 Relationship between plant species diversity and playa area in the Southern Great Plains 131 Relationship between plant species diversity and playa area when only wetland plant species are included in the analysis 133

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TABLE 5.5

Mean plant species diversity and richness for all plant species and wetland species in playas 135 Sediment depth, sediment volume, and volumeloss ratio of playas in the fine- and mediumtextured soil zones 164

TABLE 7.1

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PREFACE

I

n December 1983, the first time I visited the Southern High Plains, I was interviewing for an assistant professor position at Texas Tech University. Upon landing at the airport in Lubbock, I remarked to my host, Henry Wright, “Wow, this is a flat place.” I had been warned by others before my visit that “you will probably find this one of the flattest places you’ve ever been.” Certainly many people find flat landscapes less visually appealing than, for example, mountains. But it is this very “level” landscape that promotes the formation of playa wetlands and what draws me to the Plains. From a natural history perspective they are one of the least studied ecoregions in North America. I have had much help in the past 19 years studying playas and prairie ecology. In particular I want to thank the United States Fish and Wildlife Service and the Caesar Kleberg Foundation for Wildlife Conservation. Their financial support was there at the beginning and continues through to this day. We owe a great deal of what we know about playas to these two groups. Jeff Haskins with the Fish and Wildlife Service has been very supportive and helpful especially from this financial standpoint. Other agencies or groups that have funded some of our studies on playas and their ecology include the Texas Parks and Wildlife Department and the Playa Lakes Joint Venture, through some of its partners (in addition to those listed above): Kansas Wildlife and Parks Department, Colorado Division of Wildlife, New Mexico Department of Game and Fish, Oklahoma Department of Wildlife Conservation, Phillips Petroleum, and Ducks Unlimited. I have been fortunate to have been associated with a number of hardworking graduate students over the years whose work on playas and their associated biota has made much of this book possible. In reverse chronological order they include Doug Sheeley, David Price,

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Jim Bergan, David Haukos, Gene Rhodes, Hong-Ren Luo, Craig Davis, Jim Cathey, Jim Anderson, Warren Conway, Matt Gray, Lisa Brennan, and Dana Ghioca. Craig Davis and Jim Anderson reviewed some of the invertebrate information, and Matt Gray helped with computer graphics. Jim Anderson also reviewed decomposition information. David Haukos and Ted LaGrange of the U.S. Fish and Wildlife Service and Nebraska Game and Parks Commission, respectively, provided essential data on regional playa issues and fine camaraderie on numerous field trips. My wife, Janiece, and our children, Clayton and Jessica, helped on numerous occasions in the field and tracked down essential information contained in this book. Janiece also proofed several tables. John Taylor helped with some historical and cultural insight especially by directing me to his “Auntee” de Baca’s book. I thank Rick Gilliland for the gift of the frontispiece. Kay Arellano graciously typed the book manuscript. Several individuals provided help with maps, figures, or photographs including Ted LaGrange, Randy Stutheit, Carlton Britton, Jerry Winslow, Jim Ray, Jim Steiert, David Haukos, Mike Gilbert, and Monte Monroe of the Southwest Collections at Texas Tech. Mike Fritz and Rick Schneider checked Nebraska fauna lists. Dianne Hall provided helpful comments on playa invertebrates and biogeography. Most of the published literature on playas exists on studies conducted in the Southern Great Plains. Playas farther north in the Plains deserve more study, especially those in Nebraska, and I hope I have not slighted them too much. The more extensive treatment of some topics may appear unbalanced compared to others (e.g., birds vs. algae), but this is primarily related to availability of information. C. C. (Tex) Reeves Jr., Gene Wilde, Mike Gilbert, and Jim Ray read portions of the manuscript, while David Haukos, Bob Fullilove, and Ted LaGrange provided comments on the entire manuscript. I certainly appreciate their thoughts. Lynne Chapman and William Bishell at the University of Texas Press provided assistance throughout the editorial process. I am thankful for the help of all listed above, but as always I accept responsibility for potential errors that are present.

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CHAPTER 1

WHAT IS A PLAYA?

“P

laya” and its synonym “playa lake” are a couple of those vague terms like “swamp” or “marsh” that are generally used to describe some type of wetland. Playa is also a Spanish word with an English translation of shore or beach. The translation provides little help in describing a playa. If the titular question is asked relative to a particular geographic region, such as New Mexico, it becomes somewhat easier to answer, though the result is still not certain. When individuals use wetland terms like swamp, playa, or lake, their intended meaning generally applies to a local region, but to someone from outside these local areas the terms may carry a different sense. A farmer in Wisconsin, for example, would likely form a much different mental image of “lake” than a West Texas farmer. The Texas farmer might envision a wet, shallow low spot in a field or pasture (such as the playa in fig. 1.1), whereas the Wisconsinite would likely see a deep fishing lake. Many wetland ecologists also have little understanding of what a playa is. Mitsch and Gosselink in their book Wetlands define playa as a “term used in the southwestern United States (U.S.) for marshlike ponds similar to potholes but with a different geologic origin” (2000, 41). In the National Research Council report Wetlands characteristics and boundaries, a playa lake is defined as a “shallow depression similar to a prairie pothole, abundant on the Southern High Plains on a tableland south of the Canadian River in Texas and New Mexico, characterized by annual or multiyear cycles of drydown and filling” (1995, 288). Among other incorrect assumptions, both of these definitions make the naive assertion that playas are similar to prairie potholes. Although Mitsch and Gosselink were quite correct that playas and prairie potholes differ in their geologic formation and the National Research Council report was correct that the hydroperiod (the

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Figure 1.1 An area of the central Southern High Plains showing the highest density of playas ( 1/sq mi) in the Great Plains. (Photo by author.)

length of time a wetland has surface water) of a playa is erratic, playas, as this description will illustrate, are uniquely different from prairie potholes and any other wetland system. The need to equate playas to prairie potholes probably arose from the fact that more has been written about the latter, and most other wetlands in the United States, than about playas and that both of these wetland types are found in the Great Plains (fig. 1.2). Indeed, the paucity of literature on playas is likely because they occur primarily on private land in the more sparsely populated portions of the Great Plains, where there is little governmental ownership of wetlands. Thus, they have received less study than wetlands in more densely populated regions. Although one of the most endangered ecoregions in North America, the Great Plains itself is poorly understood relative to other U.S. ecoregions (Samson and Knopf 1996). Geologists also have not reached a general consensus on the meaning of the term playa. Motts stated, “Most American geologists would probably consider a playa to have four characteristics: (1) an area occupying a basin or topographic valley of interior drainage, (2) a smooth barren surface that is extremely flat and has a low gradient, (3) an area infrequently containing water that occurs in a region of low

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rainfall where evaporation exceeds precipitation, and (4) an area of fairly large size (generally more than 2,000 –3,000 feet [610 –914 m] in diameter)” (1970, 9). Motts continued, “The barren surface, devoid of vegetation and abundant gravel, is a distinctive feature of a ‘playa’. . . . Thousands of small, topographically enclosed areas ranging from a few feet to several hundred feet in diameter are scattered throughout western United States, yet one would hesitate to call them playas” (9). This definition would exclude most of the playas in the Great Plains, where playas are the most numerous. More recently Rosen defined playas from a geologic perspective: “as an intracontinental basin where the water balance of the lake (all sources of precipitation, surface water flow, and groundwater flow minus evaporation and evapotranspiration) is negative for more than

Figure 1.2 The Great Plains of North America with three major grassland zones (after Küchler 1975). The highest density of playas occurs in the southern short-grass prairie gradually decreasing into the mixed-grass prairie. However, some playas exist in the tall-grass prairie of Nebraska.

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half the year, and the annual water balance is also negative” (1994, 1). He further stated: “The playa surface must act as a local or regional discharge zone. Evidence of evaporite minerals will generally be present in parts of the basin” (1). This definition also would exclude those playas of the Great Plains. Noting that this classification would not encompass playas in western Texas and eastern New Mexico (but not other portions of the Great Plains) he created a “special case,” which he termed “recharge playa.” As the special case name implies, these playas would not receive water (discharge) from ground sources (e.g., springs) but could supply water (recharge) to underground aquifers. This “special case” is true for the overwhelming majority of playas not just in Texas and New Mexico but for all the Great Plains. From a numbers and area perspective, the most abundant group of playas in the world, those of the Great Plains, should not be listed as a “special case”; rather, those that occur elsewhere covering less area probably should be the exception to the rule. Moreover, most geologists that have studied the playas in the Great Plains have not adopted Motts’s or Rosen’s definition (e.g., Osterkamp and Wood 1987; Gustavson et al. 1994; Reeves and Reeves 1996). Regardless of these previously mentioned misunderstandings, the terms “playa” and “playa lake” have generally referred to various types of shallow wetlands in prairie, semiarid, or arid environments throughout the world (Neal 1975; Bolen et al. 1989; Rosen 1994). As noted above, their ecology, hydrology, and geology, however, varies greatly among geographic regions. Playas have even been hypothesized to have once existed on Mars (Hartmann 1998, 24, 26). However, for the most numerous group of playas, those found in the Great Plains, I define them as shallow, depressional recharge wetlands occurring in the Great Plains region that are formed through a combination of wind, wave, and dissolution processes with each wetland existing in its own watershed. As the words depressional and recharge imply, Great Plains playas only receive water from precipitation and runoff. Naturally water is only lost through evaporation, transpiration, and recharge. Within the Great Plains, playas as thus defined, occur with highest densities in the High Plains portion of the Southern Great Plains of eastern New Mexico, western Texas, the Panhandle of Oklahoma, southeastern Colorado, and southwestern Kansas (fig. 1.2) but are also scattered throughout some northern portions of the Plains and

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western mountain states (Motts 1970; Osterkamp and Wood 1987; MacKay et al. 1990; Brough 1996; LaGrange 1997). Because the vast majority of playas occur in the Great Plains, from Wyoming and Nebraska to Texas and New Mexico, and most scientific study of playas has occurred there, the description of playas is focused on that geographic region. DESCRIPTIONS OF GREAT PLAINS PLAYAS CLASSIFICATION

The most commonly used system to classify wetlands in the United States today is the Department of Interior’s Cowardin et al. (1979) system. It is a hierarchical method similar to taxonomic classification with wetlands being categorized by system, class, plant community and substrate, water regime, and water chemistry. Other modifiers exist to note physical alterations to wetlands such as excavations and dikes. Following the Cowardin et al. classification, playas in the Great Plains are categorized as palustrine or lacustrine systems. Palustrine wetlands generally are dominated by woody plants or persistent emergent plants. (Rooted herbaceous plants that protrude above the water’s surface are emergent vegetation.) Any nontidal wetland with more than 30% persistent emergent vegetation is palustrine. Palustrine wetlands that do not have greater than 30% persistent vegetation must be less than 8 hectares (20 ac), less than 2 meters (about 6.6 ft) in the deepest portion of the basin, and contain no active wave-formed shoreline. Lacustrine wetlands are generally larger than 8 hectares but this system cannot have persistent emergent plants exceeding 30% of the basin. This type of wetland can be placed in either littoral (less than 2 m water depth with no persistent plants) or limnetic (greater than 2 m deep) classes. These wetlands can be less than 8 hectares if an active wave-formed shoreline is part of the wetland boundary or if the deepest part of the basin exceeds 2 meters in water depth. Because as Guthery and Bryant (1982) noted, the average area of a playa in the Southern Great Plains is 6.3 hectares (15.5 ac), and most playas are shallow ( 2 m), the majority of Great Plains playas are palustrine. Fewer are lacustrine littoral playas, and even fewer are lacustrine limnetic playas; but exact percentages have not been determined. Most playas are further classified as “emergent vegetated wet-

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land.” Again, however, lacustrine littoral wetlands cannot have persistent emergent vegetation exceeding 30% of the basin, so their vegetation is necessarily classified as nonpersistent (e.g., annual). Some playas also may be classified as possessing “aquatic bed” vegetation, which refers to plants living completely in or on the water. Rooted submerged and/or other floating plants, including algae, are aquatic bed vegetation. Within the palustrine system, there are even a few playas that can be placed into the “scrub/shrub” or “forested” wetland class. Scrub/shrub are woody plants less than 2 meters (6.6 ft) high, whereas forested wetlands have trees taller than 2 meters. A few may not have any vegetation but have open water with “unconsolidated bottoms.” Today, the water regime in most playas is termed “temporarily” or “seasonally” flooded. Temporarily flooded playas may contain water for only a few weeks during the growing season while seasonally flooded playas have water present during extended periods during the growing season. Historically playas probably held water for longer periods than today; this will be discussed later. There are a few “semipermanently” flooded playas. These wetlands will have water in them throughout most years. Other commonly used modifiers for playas, in this classification system, are related to human-caused physical modifications such as “excavations” and “dikes.” Indeed, for many playas, portions of the same wetland can be classified differently due to physical modifications, such as excavations. APPEARANCE

Although the Cowardin et al. classification system allows various wetland types to be clearly categorized by function, and compared among regions, it does not greatly enhance the reader’s ability to develop an accurate vision of a Great Plains playa. To recount, Great Plains playas are mainly freshwater wetlands, dependent on precipitation from storms (or irrigation runoff) for surface water, self-contained in their own closed watershed, and not recharged by elevated groundwater (e.g., figs. 1.1 and 1.3). Wetlands in the Great Plains that have springs or receive groundwater additions to their surface water are not generally considered to be playas. Because playa watersheds are not connected to one another and storms can be very localized in the Great Plains, a playa in one location may be full of water while only a short distance away other playas will be

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Figure 1.3 Aerial view of an individual playa illustrating its circular form. (Photo by author.)

dry. They are shallow, usually only 1.5 meters (5 ft) deep, at most. Playas have erratic hydroperiods, drying and filling with water frequently within most years. These water fluctuations usually promote diverse herbaceous plant growth. However, whether the vegetation is annual or perennial, terrestrial or aquatic, depends on how long the playa has been with or without water (Guthery et al. 1982; Haukos and Smith 1997). Playas are also small, with 87% being less than 12 hectares (30 ac) in area in the Southern Great Plains (Guthery and Bryant 1982). Although the average playa is small, the range in individual playa area is large with some less than a hectare to some more than 4 square kilometers ( 1 sq mi). Although scientists know little about the area of individual playas elsewhere in the Great Plains, the playas of southwest Nebraska are also small (LaGrange 1997). The remaining Rainwater Basin playas of south-central Nebraska may be a bit larger than those farther south in the Great Plains. Most playas in the Southern Great Plains appear almost perfectly circular (fig. 1.3). Indeed, Luo (1994) devised equations to calculate playa wetland area while studying sedimentation rates; the resulting

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Figure 1.4 Cross-section drawing of typical Southern High Plains playa showing little elevation change in the basin.

formulas were very similar to the simple geometric equation used to calculate the area of a circle. North of the Southern Great Plains, playas often do not appear as circular and may be more irregularly shaped. This suggests playas in northern portions of the Great Plains possibly may have been formed through a combination of different processes than those in the south. Interestingly, though, some playas in the central and northern Plains share some unique physical features with those in the south. Many of the playas in the Rainwater Basin of Nebraska and the Southern Great Plains of Texas and New Mexico have small ridges or dunes on their east and south sides termed lunettes (Kuzila and Lewis 1993; Sabin and Holliday 1995). These are addressed in Chapter 2 when playa formation is discussed. Western Great Plains playas are also structurally simple in that there is generally a gentle slope starting from the edge of the hydric soil of the wetland to a level bottom, from which the elevation does not change (Luo et al. 1997; fig. 1.4). This is different from many other wetland types, such as riverine oxbows or prairie potholes, which have relatively irregular horizontal shapes and at least some elevational heterogeneity as one travels across the wetland basin. DISTRIBUTION AND NUMBERS

Until relatively recently, few playas were thought to exist in the Great Plains outside the Southern Great Plains (USFWS 1988). The Southern Great Plains of southeastern Colorado, southwestern Kansas, western Texas, the Oklahoma Panhandle, and eastern New Mexico was traditionally referred to as the “Playa Lakes Region” by numerous agencies and in many scientific papers (e.g., Nelson et al. 1983; USFWS 1994). Although not occurring in the same high density as farther south, playas do exist, essentially continuously, into northwestern Kansas, northeastern Colorado, eastern Wyoming, and western Nebraska (Holpp 1977; Osterkamp and Wood 1987; Brough 1996; LaGrange 1997). The majority of these

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Great Plains playas occur in the High Plains extending from western Nebraska and eastern Wyoming south to western Texas and eastern New Mexico. True to playa form, however, they cannot be categorized that easily because some playas also exist farther east in the Low Plains. Indeed, the Rainwater Basin wetlands of south-central Nebraska were classified as playas by LaGrange (1997). The highest density of playas occurs in the Southern High Plains, in an area south of the Canadian River known as the Llano Estacado (fig. 1.5). The largest plateau in North America at 82,000 square kilometers (31,700 sq mi), the Llano Estacado has been described as one

Figure 1.5 Location of the Southern High Plains, or Llano Estacado, in the Great Plains. This region contains more than 20,000 playas, the most in the Great Plains. (Modified from Sabin and Holliday 1995, courtesy of Annals of the Association of American Geographers, Blackwell Science Ltd.)

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of the largest featureless landscapes in the United States (Holliday 1991). The plateau is surrounded by relatively abrupt escarpments on the west, north, and east sides ranging from 50 to 200 meters (160 – 660 ft). On the south side, however, the plain gradually fades into the Permian Basin enough so that it is difficult to determine the Llano edge. Elevation of the Southern High Plains declines from approximately 1,500 meters (5,000 ft) in the northwest to 725 meters (2,300 ft) in the southeast. Playas are the most ubiquitous geomorphic and hydrological feature on the Llano Estacado (Sabin and Holliday 1995) with playa densities approaching 1 per 2.6 square kilometers (1/sq mi) (Guthery et al. 1981). It is difficult to imagine such a wetland density when one is at ground level, where the Llano is so flat that a person may not be able to see a playa that exists just a few hundred meters distant. From the air, however, especially after a rain when most playas have water, the sight is revealing (fig. 1.1). Within the Southern High Plains, playa density and area varies as a result of differences in soil texture and annual precipitation. The average area of playas increases from the southwestern portion of the region to the northeast, which follows precipitation patterns (Grubb and Parks 1968; Allen et al. 1972). However, soil texture also varies across this gradient with the southern one-third of the Llano being coarse textured, the middle being medium textured, and the northern third being fine textured (Allen et al. 1972; Sabin and Holliday 1995). Although the size of playas is greatest in the northeast Llano, the density of playas is highest in the medium-textured soil zone according to Guthery et al. (1981) or the coarse soils according to Sabin and Holliday (1995). This contradiction in playa density between Guthery et al. and Sabin and Holliday might simply be related to the manner in which playa density and soil zones were delineated in the two studies. Although a few other wetland types also occur on the Llano, including several riparian areas called “draws” and approximately 40 large “saline lakes” (Reeves 1976, 1990), they do not approach the numerical importance of playas. Sometimes saline lakes have been called playas, but they are not similar in hydrology, origin, or form to Great Plains playas and therefore are not typically considered to be playas by ecologists or regional geologists (Sabin and Holliday 1995). The exact number of playas existing in the Great Plains is unknown but certainly exceeds 25,000. Most of the estimates (or guesses) of the number of playas have been made for the Southern

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Great Plains or that area traditionally called the Playa Lakes Region, as defined above. Osterkamp and Wood (1987) suggested there were about 30,000 whereas Curtis and Beierman (1980) counted 24,600, and Reddell (1965) listed 37,000. These suggestions did not include playas outside the Southern Great Plains, such as in northeastern Colorado, northwestern Kansas, eastern Wyoming, and Nebraska. The estimate of 25,390 playas derived by Guthery and Bryant (1982) appears defendable for a portion of the so-called Playa Lakes Region because they actually counted playas in a 54-county region of the Southern Great Plains (table 1.1), using county soil-survey maps and other studies (Schwiesow 1965; Dvoracek and Black 1973), and conducted field checks to verify map information. These playas comprise an area of approximately 165,000 hectares (410,000 ac). Areas that had been mapped with Randall, Lofton, or Ness clays were included as were those designated as “intermittent lakes” on soils maps. Guthery and Bryant (1982) did not include some potential playas that had Randall fine sandy loam soils, or count playas in all counties of the Playa Lakes Region. Therefore, these estimates, because of their limited geographic coverage and other soil restrictions, should be considered conservative for the Southern Great Plains. The conservative nature of this estimate is further supported by more recent estimates of playa abundance by Sabin and Holliday (1995, 300), who derived playa numbers from topographic maps. They suggested that 25,000 playas was a realistic estimate for the Southern High Plains, an area smaller than that surveyed by Guthery and Bryant (1982). But as Sabin and Holliday noted, playas continue to defy accurate estimation by scientists: “Not all playas appear on topographic maps and not all depressions are seasonally dry lake basins. Soil surveys are helpful because of the unique soils found in the playas, but not all regions of the Southern High Plains have reasonably current published surveys” (1995, 290). Further, the Department of Interior’s National Wetlands Inventory, which has determined numbers and area of many of the different wetland types in the United States, has yet to complete the inventory of playas existing throughout the Great Plains. If playas outside the traditionally defined Playa Lakes Region are included, estimates of playa numbers and area further increase. Four areas of playas are reported to exist in Nebraska: the Southwest Playas, Rainwater Basin, Todd Valley, and Central Table (LaGrange

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Table 1.1 Number and total area of playas in 54 counties of the Southern Great Plains

State, County

Number of Playas

Hectares

Texas Andrews Armstrong Bailey Briscoe Carson Castro Cochran Crosby Dallam Dawson Deaf Smith Donley Floyd Gaines Garza Gray Hale Hansford Hartley Hemphill Hockley Howard Hutchinson Lamb Lipscomb Lubbock Lynn Moore Ochiltree Oldham Parmer Potter Randall Roberts Sherman Swisher Terry

298 676 598 787 535 621 395 925 220 702 451 114 1,783 65 283 752 1,383 345 123 9 1,171 185 167 1,280 18 934 842 195 590 75 455 69 564 20 219 910 532

1,877 5,746 1,932 4,966 7,132 7,998 734 7,401 1,157 2,864 5,694 682 16,439 85 1,893 5,054 9,418 2,805 1,289 37 3,396 1,513 1,081 5,422 95 6,280 3,715 1,747 6,260 1,200 4,022 1,960 6,723 40 2,048 8,145 1,225

Acres

4,636 14,193 4,772 12,266 17,615 19,756 1,815 18,278 2,858 7,074 14,069 1,684 40,605 210 4,676 12,482 23,263 6,928 3,184 91 8,388 3,738 2,669 13,405 235 15,503 9,172 4,316 15,462 2,964 9,935 4,840 16,606 99 5,058 20,117 3,022

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Table 1.1 (continued) State, County

Number of Playas

Wheeler Yoakum Colorado Baca Kansas Grant Haskell Meade Morton Seward Stanton Stevens New Mexico Curry Lea Quay Roosevelt Oklahoma Beaver Cimarron Texas Total

Hectares

Acres

10 38

— 76

— 187

198

675

1,668

232 701 712 58 294 676 133

752 2,755 3,645 430 1,734 1,900 746

1,857 6,805 9,004 1,062 4,284 4,692 1,843

524 1,175 228 535

3,553 2,036 2,002 2,140

8,775 5,030 4,945 5,286

84 264 237

732 1,193 1,951

1,807 2,947 4,818

25,390

166,395

410,994

Source: Modified from Guthery et al. 1981.

1997; fig. 1.6). The area of Rainwater Basin playas remaining in southcentral Nebraska is estimated at 13,807 hectares (34,103 ac; Raines et al. 1990) in approximately 400 relatively large wetlands and an unknown number of smaller basins (Schildman and Hurt 1984). In the Todd Valley and Central Table regions, there are approximately 716 hectares (1,769 ac) and 1,418 hectares (3,503 ac) remaining, respectively (LaGrange 1997, 13). The amount of playa habitat remaining in southwestern Nebraska is unknown, as is the area in northwestern Kansas, northeastern Colorado, and eastern Wyoming. Brough (1996), however, noted that there were at least 450 playas in the Powder River Basin of Wyoming. Regardless of the exact number and area, when one considers there are more than 25,000 playa wetlands, covering more than 180,000 hectares (445,000 ac), in a mostly semiarid

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Figure 1.6 Nebraska has four areas of playas. (Modified from LaGrange 1997, courtesy of Nebraska Game and Parks Commission.)

to arid, highly agriculturalized environment with few other wetlands, it is obvious playas are a keystone ecosystem central to the ecological integrity of the entire Great Plains. PLAYA MODIFICATIONS

Most playas have been hydrologically and ecologically altered in some form or fashion in the Great Plains. Unlike playas in the Rainwater Basin of Nebraska (LaGrange 1997) and a large expanse of wetlands throughout the midwestern United States (Prince 1997), however, it has been difficult to tile or gravity-drain playas in the western Plains. Because the western Plains are so flat and distance to surface drainage features, such as creeks or draws so great, it has been difficult to effectively divert water completely away from these playas. However, terraces established in the watershed, in the name of soil conservation, have prevented significant amounts of water from entering playas. Most commonly, land managers wishing to eliminate playas have leveled some with soil or intentionally pro-

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moted soil erosion into others. The extent of this problem is just now being realized but the actual magnitude of wetland loss has not been quantified. Throughout the Great Plains most playas less than 4 hectares (10 ac) are farmed when they are dry. Other playas have been hydrologically modified. Guthery and Bryant (1982) estimated that 33% of all playas in the Southern Great Plains had been modified. Playas have been trenched or filled for road construction, used for catchment of cattle feedlot runoff, and for urban wastewater and stormwater storage. They have even been used as dumps for municipal and agricultural trash. However, the most common form of hydrological modification has been pit excavation. Many of the larger playas have had deep pits dug into them to aid in irrigation of crops in the surrounding watershed (fig. 1.7). As noted below, irrigation agriculture is practiced extensively throughout the region, and initially many playas were integrated into a row-flood system (Bolen and Guthery 1982). Because playas are downhill from everything in the watershed, water collects there from precipitation and irrigation runoff (often termed

Figure 1.7 Many playas have had pits excavated in them to aid with irrigation of crops in the surrounding watershed. Note irrigation pump in the background. (Photo by author.)

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Figure 1.8 A modified playa with pits and trenches in the Comanche National Grassland, Baca County, Colorado. (Photo by author.)

“tailwater”). By constructing deep pits in playas, the typical surface area of the water is decreased, evaporation losses are thus minimized, and water pumping costs are lowered relative to the cost required to pump water from the regional aquifer. Pits also have been constructed to reduce the surface-water coverage of playas permitting easier cultivation of the basin or to provide more consistent water for livestock (fig. 1.8). Of the playas larger than 4 hectares (10 ac) in the Southern Great Plains, 69% have had pits constructed in them (Guthery and Bryant 1982). The percentage of playas with pits in central and eastern Nebraska (also called “dugouts” in Nebraska) is also very high (LaGrange 1997) (fig. 1.9). Guthery et al. (1981) noted that most of the construction of pits in the Southern Great Plains had occurred from the mid-1960s through the 1970s. With changes in irrigation strategies, from traditional furrow flooding to the current use of center pivots and underground drip irrigation, the rate of construction of new pits has been greatly reduced, and the consequent need for maintenance (e.g., dredging out sediments) of pits already constructed has diminished. These pits and trenches, as described in subsequent chapters, have a great influence on not only the hydrology but also the fauna and flora depending on playas. The hydrology of a wetland shapes the entire ecosystem.

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PLAYA SOILS

The majority of playas can be characterized by the presence of a specific hydric soil in their basin. In the Southern Great Plains most soils are Vertisol, with the Randall series occurring most frequently. However, the Lipan, Ness, Lofton, Stegall, and Pleasant series also indicate playa presence on county soil maps (Allen et al. 1972; Guthery and Bryant 1982; Nelson et al. 1983; Sabin and Holliday 1995; USDA 1996). These soils may not be substantially different from Randall but simply classified differently by different soil-survey teams. In Nebraska, the most common soils lining the basins of Todd Valley and Rainwater Basin playas are also nearly impervious clays in the Butler, Filmore, Scott, and Massie series (Gilbert 1989). The hydric soils of the Central Table and Southwest playas have been primarily listed as Scott and Filmore, although the taxonomy of some of these soils has been changed recently and are now listed in the Lodgepole series. Sometimes a playa exists on the landscape with the characteristic hydric soil but does not occur on the county soils map or occurs on the map without a soil designation. For example, in Baca County, Colorado playas often do not have a soils designation on the soils maps but are indicated merely as “intermittent lake.”

Figure 1.9 Modified playa in Rainwater Basin region of south-central Nebraska. (Photo by T. LaGrange, courtesy of Nebraska Game and Parks Commission.)

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The clay mineralogy of Southern Great Plains playa soils is similar to that existing in the surrounding watershed soils (Allen et al. 1972). The dominant clays of playa soils and those in the immediate watershed are montmorillonite and illite (Allen et al. 1972; Dudal and Eswaran 1988). The clay fraction of playa basin soils usually exceeds 50%, frequently 80% at the playa center. These clay soils often undergo gleying, indicating soil that has experienced frequent inundation (Haukos and Smith 1996). Gleying is a feature indicative of hydric playa soils. Redox concentrations, such as bodies of iron/manganese oxides, are also indicative of hydric soils (USDA 1996). These concentrations include soft masses, nodules, pore linings, and concretions formed under anaerobic conditions caused by flooding (USDA 1996, 26). Moreover, the clay of Southern Great Plains playa basins has a darker color easily distinguishable from soil in the surrounding uplands (Luo et al. 1999). The high clay content of playa soils makes them only very slowly permeable relative to the soils of the surrounding watershed. Playas, therefore, have an excellent water-holding capacity (Bruns 1974). This hydrologic feature is what makes them so important from a landscape and ecological standpoint. As playa soils dry, they form large cracks, and eroded soil from the surrounding watershed slumps into the cracks. When the soil becomes wet again, the sediments become mixed with the subsoil. For example, in an attempt to reconstruct vegetation in playas from the past 120 years, Rhodes and Smith (unpublished data) surmised that different depths of sediments from soil cores could be aged using Cesium137. The different-aged sediments could then be taken into a greenhouse and, because many upland and wetland plants have long-lived seeds (e.g., van der Valk and Davis 1979), the seeds could then be germinated. This would allow examination of recent historic changes in playa plant communities. However, the Cesium results were nonsensical. Because playa sediments mix so thoroughly during wind and flooding events, older sediments are not necessarily deeper than younger ones and there is no consistent pattern in the mixing. THE GREAT PLAINS PLAYA SET TING

To comprehend playas and their ecology, one must understand their setting in the Great Plains landscape. Prior to European settlement, the Great Plains or prairie regions of the midcontinent of North America existed from east of the Mississippi

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River to the front range of the Rocky Mountains and from Saskatchewan and Manitoba to Central Texas (fig. 1.2). Today, the grasslands of the Great Plains are some of the most endangered ecosystems in North America (Samson and Knopf 1996), because of their conversion to one of the most intensively cultivated regions in the world (Samson and Knopf 1994). Climate in the Plains where playas occur is temperate in the northeast, to semi-arid in the west, and dry steppe in the south. Given the great latitudinal variation in the range of playas in the Great Plains, it is to be expected that the climate varies accordingly (table 1.2). The major consistency throughout the entire region, however, is that precipitation comes mainly from thunderstorms, is typically highest in May and June, stays relatively high although variable through summer, and then drops off in October. Most often, therefore, playas usually fill with water in spring and summer. Average annual precipitation varies from a low in Midland, Texas, of 38 centimeters (15 in.) to a high in Grand Island, Nebraska, of 63 centimeters (25 in.), with amounts being higher in the eastern portions of the High Plains than in the west. However, in the Plains such “averages” are deceiving. Extremes in precipitation are the rule, average years uncommon, and droughts frequent in the Great Plains. Winters are relatively dry, although snow depths can often be substantial ( 25 cm; 1 ft). Potential evaporation rates vary from 284 centimeters (112 in.) in Midland, to 230 centimeters (90 in.) in Dodge City, to 165 centimeters (65 in.) in Grand Island. Evaporation and precipitation greatly influence the length of the playa hydroperiod, the native vegetation present, and the agricultural crops that are grown. PRAIRIE VEGETATION AND RECENT CHANGES

Prior to cultivation by European settlers, the Great Plains was a large continuous grassland, the largest vegetative province in North America (Sampson and Knopf 1994). Its vastness was interrupted with woody plants only rarely along stream and river courses (Weaver 1968). The region also was endowed with abundant wetlands (e.g., Stewart and Kantrud 1972; Prince 1997), including playas (Bolen et al. 1989). Ecologists, geologists, and economists have divided the Great Plains into three general grassland zones. From east to west is the tallgrass, mixed-grass, and short-grass prairie. Although many individuals have identified these three grassland zones, there is little general agreement on their actual boundaries and extent.

Nebraska

Kansas

North Platte

Grand Island

Month

C°

cm

C°

cm

C°

cm

C°

cm

C°

cm

C°

cm

C°

cm

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual

5.8 2.4 2.5 9.0 14.6 20.0 23.3 22.2 16.3 9.4 1.9 4.3 8.9

0.91 1.09 3.05 5.05 8.71 8.56 7.77 4.42 4.17 2.49 1.68 1.19 49.02

5.6 2.6 3.2 10.5 16.3 22.0 24.8 23.3 17.7 11.3 3.11 3.7 10.0

1.17 1.83 4.80 6.35 9.70 9.93 7.19 7.16 7.24 3.43 2.64 1.80 63.25

1.2 1.7 6.4 12.6 17.9 23.6 26.8 25.7 20.6 4.2 6.2 0.3 12.9

1.24 1.57 3.96 5.21 7.70 7.87 8.23 6.93 4.85 3.25 2.11 1.65 54.58

1.0 0.1 3.9 9.7 15.0 20.7 24.2 22.9 17.8 11.2 3.7 1.4 10.5

1.04 0.98 3.00 3.30 8.86 8.10 7.29 4.57 3.99 2.29 1.75 1.04 46.23

1.7 4.0 8.4 13.8 18.6 23.4 25.9 24.7 20.6 14.7 7.8 2.7 13.8

1.27 1.55 2.44 2.51 6.30 9.40 6.65 8.18 5.05 3.48 1.75 1.09 49.68

3.8 6.2 10.7 16.2 20.8 25.1 26.7 25.5 21.7 16.3 9.9 4.8 15.6

0.77 1.73 2.26 2.46 5.97 6.99 6.02 6.38 6.60 4.72 1.91 1.35 47.37

5.8 8.4 13.2 18.1 22.7 26.4 27.8 27.1 22.9 17.8 11.4 7.0 17.4

1.02 1.57 1.47 2.11 5.03 3.94 4.32 4.29 6.65 4.42 1.75 1.42 38.00

Source: National Oceanic and Atmospheric Administration 1999.

Dodge City

Texas Goodland

Amarillo

Lubbock

Midland

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Table 1.2 Mean daily temperature and mean monthly precipitation for selected locations throughout the Great Plains where playas occur, 1961–1990

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Researching various grassland publications for this description I examined at least 15 different vegetation maps for the Great Plains. All varied to some degree, especially in regards to the eastern and western boundaries of the tallgrass and short-grass prairies, respectively. For example, Steinauer and Collins (1996, 40) had the tallgrass prairie extending into Indiana but the short-grass prairie just barely touching eastern New Mexico, whereas Wright and Bailey (1982, 83) did not have the tallgrass prairie extending east of the Mississippi River but did include the short-grass prairie of eastern New Mexico. For purposes of this paper I have redrawn the three general grassland zones roughly following Küchler (1975) (fig. 1.2). The area containing playas from western Nebraska to the southern end of the Llano Estacado is considered short-grass prairie. The uncultivated uplands and playa watersheds in this region are dominated by grasses such as gramas (Bouteloua spp.) and buffalo grass (Buchloë dactyloides) with scattered occurrence of wheatgrass (Agropyron spp.), three-awns (Aristida spp.), yucca (Yucca spp.), prickly-pear (Opuntia spp.), and various other forbs (Küchler 1975). Where sandy soils predominate, mixed stands of sandsage (Artemisia filifolia), sand shinnery oak (Quercus havardii), bluestems (Andropogon spp.), grama grasses, and yucca occur. Percent declines in native short-grass prairie, largely due to cultivation, have ranged from 80% in Texas to 20% in Wyoming (Samson and Knopf 1994, 419). The majority of the Rainwater Basin playas of south-central Nebraska occur in the mixedgrass prairie with uplands historically dominated by bluestems, wheatgrass, and needle grass (Stipa spp.) (Küchler 1975). Native mixed-grass prairie has declined by 77% in Nebraska and 30% in Texas (no data were available for Oklahoma) (Samson and Knopf 1994, 419). The eastern edge of the Rainwater Basin playas and the Todd Valley playas are in tallgrass prairie that was historically dominated by bluestems, switchgrass (Panicum virgatum), and Indian-grass (Sorghastrum nutans) (Küchler 1975). Samson and Knopf (1994, 418) estimated that 82 –99% of the tallgrass prairie has been cultivated, a loss unsurpassed in any other major North American ecosystem. The prairie grasses of the Great Plains evolved with fire and herbivory. However, with changes in fire frequencies, altered historic grazing (herbivory) regimes, and intentional plantings by land managers, exotics and native woody plants have encroached into the remaining prairie. Although climate is likely the overriding factor in creating the Great Plains grasslands, fire has historically played an

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important role in preventing woody plant encroachment and rejuvenation of the grassland (Wright and Bailey 1982, 82). Wright and Bailey (1982, 81) suggested that a natural (nonhuman-caused) fire frequency of 5 to 10 years was reasonable throughout the Plains grasslands. Many of these fires burned the numerous scattered wetlands, including playas. Herbivory by native large and small mammals was also important in the maintenance of Plains grasslands (Bragg and Steuter 1996; Weaver et al. 1996). Bison (Bison bison) are commonly listed as one of the dominant grazing forces affecting historic grassland vegetation and rightly so. But herbivory by elk (Cervus elaphus), prairie dogs (Cynomys spp.), pronghorn (Antilocapra americana), and insects, among other native species, was also substantial and influential on the composition and structure of prairie vegetation. In the Southern High Plains, honey mesquite (Prosopis glandulosa) and junipers (Juniperus spp.) have moved into the grasslands. In the mixed- and tallgrass prairies, eastern red cedar (Juniperus virginiana), various oaks (Quercus spp.), and other woody plants have expanded into the prairie. Woody vegetation has also increased along riparian areas, which have had their flows drastically altered as a result of reservoir construction and withdrawals for human consumption and irrigation (Friedman et al. 1998). Native species such as cottonwood (Populus deltoides) and willow (Salix spp.) have often greatly expanded their coverage. But of much worse ecological consequence has been the spread of exotics like saltcedar (Tamarix pentandra) and Russian olive (Elaeagnus angustifolia). Although Russian olive is a serious threat to Plains ecosystem integrity (Olson and Knopf 1986), until recently it was included in shelterbelt and wildlife cover plantings by the U.S. Department of Agriculture and is still by some state forestry agencies. These same native and exotic species now also exist along the margins of many Great Plains wetlands where they were historically absent. Among the many exotic grasses and forbs (too many to list here) that have been introduced into the prairie are crested wheatgrass (Agropyron cristatum), Old World bluestem (Bothriochloa ischaemum), and Kentucky bluegrass (Poa pratensis). Many of these grasses were introduced to “improve” grazing by domestic livestock or to establish cover on previously farmed or eroded sites. The introduction and expansion of exotic grasses continues today, most recently through the Conservation Reserve Program (CRP) of the U.S. Department of Agriculture.

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CURRENT AGRICULTURE AND POPULATION TRENDS

The dominant agricultural crops cultivated in the southern portion of the Southern High Plains are cotton, grain sorghum, and winter wheat (TDA 2001). Corn, grain sorghum, and winter wheat are grown in the northern and central Southern High Plains, with the area of cotton decreasing (TDA 2001). In the High Plains of Colorado, Kansas, and Oklahoma wheat and grain sorghum are predominant but some corn and sunflowers are also grown (CDA 2001; KDA 2001; ODA 2001). In southwestern Nebraska wheat and sorghum are important crops, but in some counties (e.g., Perkins) areas of corn are increasing (NDA 2001). Corn and soybeans are dominant throughout the Todd Valley and Rainwater Basin of Nebraska (NDA 2001). Essentially all grasslands in the Great Plains not enrolled in the CRP are grazed by domestic livestock. As a result of Title XII of the 1985 Food Security Act, and its successors, a significant portion of the cultivated Great Plains was taken out of crop production for a period of at least 10 years through the CRP. In exchange for annual rental payments, landowners throughout the United States replaced annual crops on qualified highly erodible lands with perennial cover to reduce soil erosion and reduce surplus production in some areas. Because most of the Great Plains is highly erodible, from the forces of wind and water, these lands easily qualified for the CRP. Indeed, the highest density of CRP lands in the nation occur in the Great Plains (Licht 1997, 120). For example, over 900,000 hectares ( 2 mil. ac) alone were enrolled in the Southern High Plains of Texas (Berthelsen et al. 1989). On most of the enrolled land in the Great Plains annual crops were replaced with perennial grasses. As subsequent requirements of the CRP were modified and 10-year contracts expired, some of the previously enrolled land was put back into crop production. The loss of some of this perennial grass cover came as a result of fluctuations in the agricultural economy (e.g., rising crop prices). In addition, the same previously established exotic perennial grass cover no longer completely qualified for the CRP. Unfortunately, in most of the Great Plains exotic grasses had been allowed to be planted under the 1985 program (Licht 1997, 121). As the initial contracts expired after 10 years, the CRP required that a higher percentage of native grasses be planted in those original fields. By

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PLAYAS AND THEIR ENVIRONMENT

helping restore native species, this requirement was a positive revision to the program. In states, such as Kansas, that had the foresight to plant a higher percentage of native grasses to begin with, qualifying lands for reenrollment was not as great a problem as elsewhere in the Plains. Most of the crops in the western Great Plains, where playas occur, depend on irrigation for successful production because of unpredictable and scarce precipitation (Nall 1990). Irrigation is possible because of the existence of the world’s largest aquifer. The Ogallala Aquifer underlies much of the western Great Plains region from South Dakota to the Southern High Plains of Texas and New Mexico (Reeves and Reeves 1996). The presence of the aquifer, and, at least historically, inexpensive natural gas to allow pumping, encouraged much of the expanded cultivation of the Great Plains grasslands since the early 1900s (Nall 1990). Given the current rate of water use, however, much of the western Great Plains will likely see extreme shortages by the year 2020 potentially resulting in severe social and economic consequences (Luckey et al. 1988). For example, that portion of the Ogallala Aquifer south of the Canadian River in Texas and New Mexico receives little recharge. The aquifer is essentially being mined to sustain crop production. At average pumping depths ranging from 30 to 200 meters (100 – 600 ft) (Bolen et al. 1979), the Ogallala dropped by more than 15 meters (approx. 48 ft) in the Southern High Plains between 1930 and 1980 (Weeks 1986). The drought of the 1990s, the worst on record throughout much of the Southern High Plains, has resulted in continued high use of aquifer water. Between 1993 and 1997 there was an average decline of more than 2 meters ( 6 ft) throughout most of the region (Donnell 1998). In Lea County, New Mexico, the Associated Press (1998) reported that it would take 1,900 years to replace (recharge) the water pumped from the aquifer in the last half century. Even with widely hailed water-saving technological advances such as centerpivot and drip irrigation to replace traditional furrow flooding, and the enrollment of extensive CRP acreages, aquifer levels continue to decline. Indeed, center pivots have allowed irrigation to expand to sloping lands that did not allow traditional furrow irrigation, further decreasing aquifer levels (fig. 1.10). In some areas of the Southern High Plains, water levels have dropped so far that it is no longer economically feasible or hydrologically possible to irrigate. Landowners in such areas have had to revert to dryland farming, an economically

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Figure 1.10 Agriculture in the western Great Plains is dependent on irrigation (center-pivot in playa in this illustration) from the groundwater of the Ogallala Aquifer. Much of the southern portion of the aquifer recharges at a very slow rate and pumping can be equated with mining. (Photo by author.)

much riskier proposition than irrigated agriculture (Nall 1990). Numerous farms have gone out of business because of the effects of the declining aquifer (Associated Press 1998). In some areas north of the Southern High Plains, where the Ogallala historically has not been tapped at such a high rate, irrigated agriculture is expanding west. For example, in southwestern Nebraska the number of center pivot irrigation systems has been increasing. Irrigation permits the expansion of crops such as corn that require more water than do traditional crops like wheat. This agricultural shift is increasing soil erosion compared to dryland farming methods and causing a more highly fragmented prairie and playa environment. Along with the difficulties faced by agriculture such as declining aquifer levels, increased energy costs, and recent low commodity prices, there has been a decline in the human population throughout the Great Plains. “Depopulation” has occurred throughout much of the Great Plains since the early 1900s, although the demographic declines have gone largely unnoticed by the remainder of the country

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(Nickels and Day 1997). In exemplifying what has occurred throughout the rural Great Plains, Nickels and Day (1997) reported human population trends in 67 counties in the Texas portion of the Great Plains since the early 1900s. From 1900 to 1930 the number of farms in the Texas Great Plains increased along with population in 64 of the 67 counties. However, since the 1930s and the Dust Bowl, the population in most of the 67 counties declined, and 54 of the 67 counties lost population in the 1980s. This trend persisted into the 1990s but slowed. Of the 67 counties studied by Nickels and Day (1997), 43 lost population in the 1990s (U.S. Bureau of the Census 2002). Depopulation has been mirrored in other western Great Plains states such as Nebraska and Oklahoma, which lost population in 50 of 52 and 22 of 23 Plains counties, respectively, during the 1980s (Popper 1992). The depopulation of the Plains of Texas has been due to many interrelated factors including younger people leaving rural environments, the agricultural economy, and, in many areas, declining water availability for agriculture (Nickels and Day 1997). As a result of these changes, many of the Plains rural areas are becoming poverty-stricken because of fewer employment opportunities, aging populations, and subsequent declining tax bases (Davidson 1990). The declining tax base has resulted in fewer local governmental services. Fewer educational opportunities, reduced police and fire protection, and limited health care are notable. The declining population and human fortunes of the Great Plains has led some to propose reverting much of the private land to federal ownership and creating a large prairie-bison preserve (Matthews 1992; Popper 1992). This proposal has not been met with open arms by most of the private landowners in the western Great Plains (Licht 1997, 115), but, as suggested in the final chapter, modifications of the idea may offer some potentials for regional economies.

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CHAPTER 2

ORIGIN AND DEVELOPMENT

A

lthough most people from forested and mountainous areas outside of the Plains may find the western prairies aesthetically “boring,” geologists consider the High Plains one of the most interesting and challenging arenas in North America. This is especially true of the Southern High Plains, where most playas occur, and where the formation of this huge plateau and its underlying aquifers continue to be discussed. Similarly, unlike other wetlands such as estuaries or prairie potholes, which can be accurately aged in geologic terms and form through known fluvial and glacial events, respectively, the age of playas and the processes responsible for their origin and formation continue to be debated. As Reeves and Reeves (1996, 195) noted, some of the origin/formation confusion results from semantics. “Origin” is quite different from “development.” Thus, the origin of a playa, its starting point, may be different from how the playa subsequently developed (Wood and Osterkamp 1984). Similarly, the “playa basin” is often different than the actual “playa” as defined by its hydric soils. A playa and its associated “floor” are generally inclusive of the area defined by hydric soils whereas a playa basin includes the nonhydric soil immediately adjacent to hydric soil including the sloped watershed. Further, it is likely that the formative processes of playas in the northeastern areas, like the Rainwater Basin in Nebraska, differ from those in the west and south. This difference might be inferred from variations in shape alone. The almost perfectly circular playas in the western Plains vary from the irregularly shaped playas in the Rainwater Basin (fig. 2.1). Data concerning formation of Rainwater Basin playas are also sparse, but playa occurrence along some paleodrainage features suggests a different combination of processes may be involved in their formation than in High Plains playas (Starks 1984).

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Figure 2.1 Top photograph illustrates a typical playa in the Southern Great Plains in its near circular appearance, whereas the bottom photograph illustrates the typical oblong appearance of Rainwater Basin playas on a northwest-tosoutheast long axis. (Top photo by author; bottom photo by T. LaGrange, courtesy of the Nebraska Game and Parks Commission.)

REVIEW OF HYPOTHESES — SOUTHERN GREAT PLAINS

Nelson et al. noted, “Playa basin soils are predominately clay [primarily Randall clay] and strikingly similar regardless of location within the Southern Great Plains, reflecting similar formative processes” (1983, 45). Although soils may reflect similar playa formation, many agents have historically been implicated in the origin and formation of Southern Great Plains playas and

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few have been agreed upon. These agents include large mammals wallowing on the prairie soils (Gilbert 1895; Reeves 1966) to wind deflation (Evans and Meade 1945). Subsidence, primarily the collapse of carbonates in solution underlying the Plains or evaporites, also has been proposed (Johnson 1901; Baker 1915; Theis 1932; Finley and Gustavson 1981; Paine 1994). The associated importance of piping and the subsidence around geologic “pipes” (vertical linear features leading to the subsurface through basin soils) has further been implicated (Rubey 1928; Frye 1950; Reeves 1966). The possibility of playa origins from meteorites has even been suggested (Evans 1961). The debate among geologists continues, so I summarize the major opinions provided in the most recent publications of Osterkamp and Wood (1987), Wood and Osterkamp (1987), Gustavson et al. (1994, 1995), Sabin and Holliday (1995), and Reeves and Reeves (1996). DISSOLUTION PROCESSES

For the Southern Great Plains, Osterkamp and Wood suggested that “any depression on the High Plains surface that periodically stores and transmits water to the subsurface may develop into a playa basin” (1987, 217). They cited examples of initiation of several playas in the past 60 years. Once the water begins accumulating in a depression, other processes take over and lead to playa enlargement. Osterkamp and Wood (1987) concluded that the principal processes involved in this next aspect of playa development were dissolution of carbonates and movement of particulates (including organic matter) with the water percolating into the subsurface. Under this scenario, organic matter is carried below the basin soil surface by water that has been ponded in the depression. Organic matter is then oxidized, releasing carbon dioxide, which reacts with infiltrating water to form carbonic acid. The carbonic acid then causes dissolution of carbonates. Most of the Southern High Plains plateau topsoil is underlain by a calcium carbonate substance known as “caliche” (Reeves and Reeves 1996). Caliche is actually a thick, hard, calcium carbonate deposit that makes up what is locally termed the “caprock” (fig. 2.2). (“Caprock” is also a local synonym for the entire Southern High Plains plateau, or Llano Estacado. This hard limestonelike material, or caliche, is not rock at all but of sufficient form and texture that it is mined throughout the region for use as a roadbed material and base for building foundations.) Because organic matter from plants collect

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Figure 2.2 The northwestern escarpment, or “caprock,” of the Southern High Plains, north of Clovis, New Mexico. (Photo by author.)

in the basin, the process continues and there is progressive development of the playa in an outward fashion. Osterkamp and Wood noted numerous examples of carbonate (caliche) dissolution below playas. Accordingly, subsidence, the lowering of the land surface into these dissolution voids, occurs mainly at playa margins allowing the playa to enlarge. At the playa margins, therefore, are different landslopes defining the playa edge and floor. However, Reeves and Reeves (1996) noted, on the basis of drilling data, not all playa basins in the Southern High Plains indicate carbonate dissolution below them. It appears, therefore, that dissolution of carbonates is important in the continued formation of some playas, but not all. Wood and Osterkamp (1987) suggested that this type of formation process would lead to the near circular appearance of playas in the western Great Plains. Deflation, as a result of wind, they felt, would have led to playas with a more irregular oblong appearance because winds do not affect playas equally from all directions. Most winds in the Southern High Plains approach from the west and southwest. Wood and Osterkamp did acknowledge, however, “that many of the initial or ‘proto’ basins were of eolian origin . . .” (1987, 224). Origin here correctly refers to initiation of the playa depression, and formation to the subsequent development of the playa floor. Often the two processes of initiation and development are different, but not necessarily so. Wood and Osterkamp (1987) further discounted the importance of wind deflation in playa development by arguing that in addition to the circular shape of playas not being consistent with the theory of wind deflation, the existence of many playas in a linear arrangement along

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substrate fracture zones is not consistent with deflation hypotheses. Moreover, the apparent random occurrence of playas relative to topography and slope outside of these fracture zones, the lack of lunettes (dunes on the lee sides of playas) adjacent to most playas, the inadequate lunette volume relative to playa volume, the high clay content of lunettes (suggesting lunette formation after playa formation), and the movement of radioactive clay particles into the unsaturated zone at the edge of playas during aquifer recharge events all suggest that wind was not of primary importance in playa development. WIND FORCES

Reeves and Reeves (1996) and Sabin and Holliday (1995) argued, however, that the simple presence of lunettes on the east and south sides of some playas (about 5% in the Southern High Plains) indicates that wind deflation has been important in the development of some basins (fig. 2.3). Deflation of playa depressions is hypothesized to occur after playas have had sufficient water depths to have either prevented plant growth or enhanced decomposition of existing plants. The playas then dried, exposing unprotected basin soils to wind erosion. Anyone who has lived in this region for a reasonable length of time has seen this occur. Moreover, the above argument suggests wind is important in formation and maintenance but not necessarily origin. It is possible that wind simply maintains the existence of some playas by removing naturally collected sediments. Contrary to Osterkamp and Wood (1987), Sabin and Holliday (1995) proposed that wind erosion was the primary force enhancing playa development rather than dissolution and subsidence. They based their claim on a geographic analysis of playa frequency and size relative to soil texture. Comparing the frequency of playas and their size, as determined from topographic maps, to soil texture of the surrounding watershed, they noted that substrate texture was significantly related to the occurrence and distribution of playas. Further, soil texture was related to playa area, depth, and shape. Soil particle size is smallest in the north-northeastern portion of the Southern High Plains and largest in the south-southwest. The soils in the north are clay— clay loam, becoming sandy farther south with loamy sand—sandy loam soils occurring in the southern portions of the region. (However, precipitation also varies along the soil-texture gradient with precipitation being greatest in the northeastern portion of the Llano and lowest in the southwestern region. The importance of

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Figure 2.3 Sections of topographic maps from the vicinity of Petersburg, Texas. Lunettes are present on the southeast corners of several of the playas as indicated by contour lines. Smaller lunettes are present in the lower map section, and large lunettes exist in the upper section.

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soil texture versus precipitation would therefore appear difficult to distinguish.) Sabin and Holliday (1995) found the highest numbers of playas on topographic maps in the coarse-textured soils of the southwestern Southern High Plains. The next highest density of playas was found in the northeastern portion of the region. As revealed in Chapter 1, these results do not agree with counts by Guthery et al. (1981) who used soils maps to estimate playa density and occurrence (Nelson et al. 1983). Guthery et al. found highest densities of playas in medium- and fine-textured soils of the east-central Southern High Plains. Sabin and Holliday did not address this anomaly. Counting methodology, therefore, along with semantics, has also influenced interpretation of formative processes in playas. Sabin and Holliday (1995) also found that the greatest variability in individual playa basin area and the largest playas occurred in the northern areas of the Southern High Plains where fine-textured soils exist. Playas with the largest range in basin depth also occurred in the fine-textured soil zones, but the largest playas were not necessarily the deepest. Again, characterizing playas is not easy. Sabin and Holliday felt that wind deflation affected playa depth and that the depth of a playa was therefore related more to the depth and soil texture of the local substrate than to the surface area of a playa. Under this hypothesis, when winds hit resistant layers of the deeper substrate, wind force expands the playa’s surface area horizontally rather than deepening it vertically. The playas occurring on the finest-textured soils also were the most round relative to playas in other soil textures (Sabin and Holliday 1995). According to Sabin and Holliday (1995), the playas were apparently less round in coarse soils because erosion occurred more easily than in fine-textured soils. Roundness is greater in fine-textured soils because this less permeable soil promotes surface runoff of water, rather than infiltration in coarse soils, maintaining playa roundness. Therefore, according to their theory, wind erosion aids in deepening and expanding basins, but water erosion acts equally around the circumference of the basin making playas more round because the finesttextured soils are least permeable. These water-eroded soils that wash into the basin are further deflated by wind. Sabin and Holliday stated, “While the eluviation and dissolution hypothesis of Osterkamp and Wood (1987) suggests that playa area and depth should increase with increasing permeability, the reverse is actually the case” (1995, 300).

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Again, however, their measurements were made from topographic maps, not from the hydric soil–defined wetland, which may yield different results. Sabin and Holliday’s explanation of playa development and formation is essentially an expansion and elucidation of a slightly earlier theory proposed by Gustavson et al. (1994); however, the latter did not present any data. MULTIGENIC IDEAS

Reeves and Reeves (1996) suggested that no single course of events could be substantiated in the origin and development of all playas in the Southern High Plains. They proposed that playas originated wherever there were depositional (elevational) lows or “irregularities” in the surface of the Plains. Many of these low areas could have then collected water and attracted herds of large mammals such as bison (modern bison or prehistoric Bison antiquus) and other extinct species (mastodon, horse, etc.) (fig. 2.4). These herds of large mammals would be attracted not only to the water but also to mud for wallowing. They would then transport large amounts of basin soils out of the depression. Presumably, as noted earlier, during the times the basins did not contain water, the basin soils would be subject to wind erosion because either the water or large mammal activity (or both) did not allow vegetation to persist and prevent this erosion. Following Reeves and Reeves (1996), these “young” playas, classified by them as Type I basins, then generally developed through carbonate dissolution and gradual subsidence of underlying substrates (as suggested in Osterkamp and Wood 1987) into older and often larger Type II basins. Some wind deflation, piping and subsidence around pipes, and eluviation also contributed to Type II basin development (Reeves and Reeves 1996). The main differences between Reeves’s (1990) Type I and Type II basins are that the former generally do not have a lunette and have a minimum of lacustrine fill (i.e., ancient lake deposits). Some small basins in the Southern Great Plains have substantial lacustrine fill and are therefore actually older and considered Type II playas (Reeves and Reeves 1996). Therefore, size alone is not a sufficient distinguishing feature between Type I (younger) and Type II (older) basins (Reeves and Reeves 1996). Also, unlike the Type I basin, the hydric soil—in this case, Randall clay—is inset into the lacustrine fill in Type II basins. Reeves and Reeves (1996) found no evidence of steep sloped bottoms or semicircular playa margins, which

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Figure 2.4 Photo from 1904 in the eastern portion of the Texas Panhandle (Gould 1906). Gould refers to this as a “buffalo wallow.” However, note the almost circular shape. Although bison may have wallowed here, it is unlikely they gave it a circular shape.

would be indicative of solution and large-scale dome collapse of underlying material. Further, they did not find subsurface fluvial channels or deposits in Type II basins. Gustavson et al. (1994, 1995) also supported, for the most part, a multigenic theory of playa origin. They stated, “These landforms are the result of a series of intermittently active processes, including wind, fluvial erosion and lacustrine deposition, pedogenesis, dissolution of soil carbonate, salt dissolution and subsidence, and animal activities, that collectively produced the typically shallow and roughly circular playa basins on the High Plains” (Gustavson et al. 1994, 12). However, the emphasis they placed on factors that were most important in playa origin and formation varied from the other studies. Gustavson et al. (1995) noted that wind deflation was the primary force in playa formation with the other factors being less important. Some of the differences of opinion that currently exist among geologists relate to views of whether or not playas change as they age in geologic time. In other words, some small playas remain, in the terminology of Reeves’s (1990), Type I playas and do not progress to Type II

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(Gustavson et al. 1995; Hovorka 1995). Hovorka (1995), for example, concluded that playa basins in an area northeast of Amarillo, Texas, have changed little over geologic time. Reeves and Reeves (1996) have taken exception to this conclusion by noting that many of the basins in that area are still forming. Osterkamp and Wood (1987) also concluded that playa formation was continuing and that most basins were not in a static state of existence. (Indeed, if it were not for the fact that we humans are filling the playas in so rapidly, this continuing formation would be a comforting fact. But formation simply cannot keep up with human-induced sedimentation of the basin [Luo et al. 1999].) Moreover, Reeves and Reeves (1996) noted that many outwardly appearing Type I basins are actually older in geologic perspective and should be classified as Type II. As noted, surface area alone is not a good indicator of playa basin age. Based on data from drilling, they concluded that playas in the region studied by Hovorka decreased in surface area with playa depth. Finally, Reeves and Reeves (1996, 203 – 204) noted that for playas to remain in a “non-evolving” “steady state” throughout “much of Quaternary time” would have required consistent uniformity in climate (precipitation, wind, evaporation, temperature), sedimentation factors (rate, mineralogy, particle size, seasonality, environment), playa surface characteristics (permeability, piping, flora, hydroperiod, gilgai, etc.), and geologic factors (depths of Quaternary sands, caliche depth, Permian salt depth, fracture locations, etc.). The likelihood of these conditions remaining static is infinitesimal and contrary to existing geologic, ecologic, and hydrologic data. However, it therefore seems odd to be placing playas in two age categories (i.e., Type I and II). The process of formation is continuing and accordingly the ages would be continuous. In addition to the lunettes that exist adjacent to about 5% of the Southern High Plains playas, some playas also appear to have formed more straightened margins on their north, east, and south edges (Gustavson et al. 1995). This has been attributed to wave action initiated by wind (Reeves 1966; Price 1972). These observations are consistent with seasonal prevailing winds in the region. Northerly and westerly winds are common in fall and winter, while southerly winds are prevalent in late spring and summer (Gustavson et al. 1995). In particular, on the north side of some playas it is common to see eroded “scarps.” Scarps are simply steep eroded elevational changes, similar to a step, generally less than a half meter high. It is believed that these scarps exist more on the north sides of playas because southerly

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winds occur more frequently in summer when playas are more likely to contain water. This creates a more powerful erosional force than wind alone and encourages formation of a scarp. SOUTHERN GREAT PLAINS SUMMARY

Although there is no consensus by geologists on the relative importance of individual factors (i.e., wind, dissolution) in playa origin and formation in the Southern Great Plains, it appears there is now agreement that several forces are responsible for origin and formation of playa basins. So often scientists are looking for one factor as causing an event; seldom is the real world so simple. Geologists may never nail down the actual importance of individual factors in overall Southern High Plains playa origin and formation, especially given the anthropogenic changes that have occurred in the Great Plains landscape. For example, it is probably not possible to know what contribution large mammals made to the origin of playas given they are not likely to be present again in those numbers. However, it would be possible for geologists to date a large number of playas in a systematic fashion over a wide area using drilling data. Only this type of study would clarify much of the speculation about playa age and the relative importance of formation factors. OTHER PLAYAS IN THE WESTERN GREAT PLAINS

Other playas in the Central and Northern High Plains also often appear circular and possibly were formed under similar processes to those in the Southern High Plains (Frye 1950). But, in the Powder River Basin of Wyoming, Brough noted that the playas were mainly oval or elliptical in shape: “Elongation of many playas in the direction of the prevailing winds indicate that eolian processes contribute to both the geomorphic characteristics and formation of the playas in the Powder River Basin of Wyoming” (1996, 44). These playas, however, may have an aspect to their origin that is distinct from other playas in the Great Plains. This region of Wyoming is known for its coal deposits. Brough noted, “The location of the playas, disturbances in some of the soil horizons, and the presence of clinkers support the idea that these playas were formed by the ignition and burning of coal beds and the sequential subsidence in the overburden to form the topographical depressions” (1996, 91). His use of the word “form” here probably is similar to my earlier use of “ori-

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gin” in that some of the playa depressions being discussed here originated through coal ignition, and other processes such as wind led to their continued formation. Few other studies on playa origin and formation exist for other areas of the High Plains. R AINWATER BASINS OF NEBR ASKA

As noted earlier, the Rainwater Basin playas in south-central Nebraska, to the east of the High Plains, generally lack the circular appearance of other Great Plains playas, have irregular shapes, and may have formed through a combination of other processes because of different landforms and external forces (Kuzila 1994). Certainly less geologic data on their origin exists than for playas in the Southern High Plains. Kuzila (1994) noted that many of the smaller wetlands in the Rainwater Basin region were not generally discernible on topographic maps because they may only be 1 meter (3 ft) deep (similar to High Plains playas) relative to the surrounding upland. These wetlands, however, are easily documented on soil survey maps, as noted by the presence of Butler, Filmore, Scott, and Massie soil series (Gilbert 1989; Kuzila 1994). Moreover, unlike playas in the remainder of the Great Plains, some of the Rainwater Basin playas have been naturally “breached” (Starks 1984). In other words, they have some external drainage where the water may reach a certain depth and “spill” into riparian features (e.g., streams, creeks). Similar to other Great Plains playas, many Rainwater Basin wetlands also have lunettes on the south and east sides of the basin (fig. 2.5). Starks (1984) found that 51 of 120 wetland basins he surveyed had lunettes. This occurrence is a much higher frequency than that found for playas in the Southern High Plains (about 5%). The high frequency of lunettes, however, could be an actual occurrence or an artifact of sampling. Remember from Chapter 1 that only about 10% (or 400) of the Rainwater Basin playas remain. The others have been drained or filled. Recent data on the remaining playas may contain lunettes at a higher frequency than if historic data (prior to drainage) were used. The few studies on Rainwater Basin playa origin and formation have come from just a few depressions. Krueger (1986) studied one basin and 16 adjacent drill holes. He concluded the basin was originally formed during the Wisconsonian period and later modified by wind and wave action. Kuzila (1994) examined two basins and their stratig-

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Figure 2.5 Lunette on the southeast side of a Rainwater Basin playa. (Photo by R. Stutheit, courtesy of the Nebraska Game and Parks Commission.)

raphy. He found that the present landscape of the Rainwater Basin region was a result of loess (windblown silt) deposition tending to smooth out the more rugged paleolandscape. Indeed, almost the entire Rainwater Basin region falls within the area known as the Central Loess Plains. Low points in that paleolandscape, however, remain and collect precipitation runoff. These low points and shapes, or morphologies, of present-day playas therefore appear inherited from that historic landscape (Kuzila 1994). Similar to Krueger, who invoked wind and water in subsequent basin formation, Kuzila felt that the Rainwater Basin playas formed from water erosion and subsequent wind deflation of those eroded materials. Starks (1984) noted that the southern extent of glaciation did not reach his study area in the Rainwater Basin region and therefore glaciation was not likely involved in basin formation. Some of the Rainwater Basin playas appear to have a northwest to southeast orientation (Starks 1984). This may be related to paleodrainage patterns (Stutheit et al. 2001). If windblown silts (loess) were deposited after these drainage patterns developed, this could have closed the drainage off, allowing formation of this type of closed playa

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basin morphology. These processes of wind and water action appear related to formation of large ( 50 ha) basins, but it is unclear whether they are involved in smaller basin development (M. C. Gilbert, personal communication). The relative importance of wind and water processes may also vary on a geographic basis within the Rainwater Basin region. For example, the playa basins in the western portions of the Rainwater Basin appear smaller and shallower than those to the east, which are larger and deeper (M. C. Gilbert, personal communication).

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CHAPTER 3

FLOR A

W

hen most ecologists consider “flora” they usually think of vascular plants; sometimes, as an afterthought, algae come to mind. Fewer yet consider fungi, mosses, bacteria, and viruses even though their diversity may surpass that of the higher taxa (Wilson 1999). There are few studies investigating these latter groups in Great Plains playas and only one mention of lichens; Bartz (1997) noted the occurrence of one ground lichen (Xanthoparmelia wyomingensis) in playas of northeast Wyoming. Further, other than the known frequent occurrence of avian botulism (Clostridium botulinum) and avian cholera (Pasteurella multocida) in playas, the primary information regarding playa bacteria is associated with human health concerns in urban playas (Westerfield 1996; Warren 1998). These lakes bear little resemblance to their rural counterparts in that urban lakes remain inundated for years and receive much different forms of watershed runoff. Westerfield (1996) used microbiological media that would select for determination of bacteria harmful to humans—total coliforms, fecal coliforms, and enterococci—in Lubbock city playas. He found 11 species (Aeromonas hydrophila, Aeromonas trota, Aeromonas veronii, Enterobacter cloacae A, Enterococcus faecalis, Escherichia coli, Plesiomonas shigelloides, Pseudomonas aeruginosa, Pseudomonas mendocina, Pseudomonas stutzeri, and Serratia marcescens), many of which can cause serious human health problems through various methods of contact such as direct consumption, inhalation, or through a break in the skin. ALGAE

There have been few investigations into playa algae. One of the most extensive studies focused on the macroalgae Characeae in the Southern Great Plains. In surveys from mid-July to

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Table 3.1 Common playa algae and macroalgae in the Southern High Plains Algae (Price 1987) Ankistrodesmus falcatus Bracteacoccus minor Chlorella vulgaris Closterium sp. Euglena gracilis Gloeocystis ampla Golenkinia sp. Oocystis sp. Phacus pleuronectes Phormidium sp. Rhizoclonium sp. Scenedesmus basilensis Scenedesmus quadricauda Spirogyra rhizobrachalis

Macroalgae (Proctor 1990) Chara braunii Chara foliolosa Chara haitensis Chara hydropitys Nitella monodactyla Nitella bastinii Nitella acuminata Nitella axillaris Nitella clavata Nitella mucronata Tolypella prolifera

Staurastrum cristatum

late September, Proctor (1990, 78) found 11 species of charophytes from three genera in 64 playas (table 3.1). He noted that sedimentation, from eroded watershed topsoil (e.g., Luo et al. 1997), caused a loss in the diversity of Characeae and that Chara braunii was generally the only charophyte occurring in playas that had received substantial sediment loads. Sedimentation has drastically altered playa ecology, a theme that I will be repeating in this volume. Because most playas exist in intensively cultivated regions, the diversity of Characeae is being negatively affected by intensive agriculture throughout a large region of the Southern Great Plains. Proctor (1990) noted that C. braunii could be present as early as mid-April, depending on precipitation, and that plants remain fertile as late as December. He felt that the persistence of C. braunii was due to repeated germination of the species throughout the season rather than to the individual longevity of plants. Tolypella prolifera was the only other Characeae likely to be present earlier in the year than C. braunii although Tolypella was much less widespread. Because of C. braunii’s reproductive characteristics, producing zoospores earlier and more rapidly than most other Characeae, it can withstand envi-

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ronmental degradation and frequent natural environmental fluctuation, thus making it the most widespread species in the region (Proctor 1990). Even with 11 Characeae species occurring in playas, there were generally only two species present (with C. braunii usually as one) in any given playa (Proctor 1990). Characeae species that occurred in more permanent water bodies in the Southern High Plains did not occur in playas, and those species that occurred in playas did not occur in the more permanent water sites (Proctor 1990). Certainly this speaks further to the importance of playas to biodiversity in the region. Similarly, bulbil-forming Characeae did not exist in playas, presumably because bulbils are not drought resistant and cannot withstand the frequent dry circumstances found in playas (Proctor 1990). (Bulbils are small bulbs permitting vegetative reproduction.) Although bulbils are most commonly produced by dioecious species, there are only two dioecious charophyte (non-bulbil-producing) species in playas (Nitella bastinii, Nitella monodactyla). Moreover, though six species of Characeae in playas are Nitella, Chara species made up more than 90% of the annual charophyte biomass (Proctor 1990; unfortunately actual biomass estimates were not presented). Also few extensive studies have examined the noncharophyte algae in playas. In an herbicidal runoff study, Price (1987) collected water and sediment samples from playas in two counties (Castro, Lubbock) of the Southern High Plains. He found 16 species after preparing those samples under laboratory conditions (table 3.1). Most were green algae and are common elsewhere. Cladophora spp. also occurs in playas but was not seen in Price’s study. Certainly further research on algae groups is needed throughout Great Plains playas. VASCULAR PLANTS

The first survey of the vascular flora of playas was by E. L. Reed (1930) for the Southern High Plains of Texas (common and scientific names follow Haukos and Smith [1997] unless noted otherwise). Reed also was the first to remark on the importance of playa vegetation to the regional diversity of the prairie flora: “The vegetation of the playas differs remarkably both in genera and species from that of the rest of the Staked Plains. In fact the playas have a flora peculiar to themselves” (1930, 598). He noted that the common species of playas that held “water for any considerable period” were western water clover (Marsilea vestita), bur ragweed (Ambrosia grayi),

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prostrate knotweed (Polygonum aviculare), Pennsylvania smartweed (Polygonum pensylvanicum), arrowhead (Sagittaria cuneata), mousetail (Myosurus minimus), blue mud-plantain (Heteranthera limosa), and a small sedge (presumably a spikerush [Eleocharis spp.]). All are still fairly common today with the possible exception of mousetail (Haukos and Smith 1997). Reed (1930) noted that buffalo grass was present only in the “shallow” playas. However, Parker and Whitfield (1941) found that buffalo grass may occur in more playas but less frequently in the playa center. Many of the various opinions on the prevalence and dominance of different types of vegetation in playas is likely related to how playa boundaries are defined, immediate past precipitation/irrigation events, and condition of the immediate surrounding watershed. The botanist William Penfound was one of the first ecologists to provide observations of playa flora outside of the Southern High Plains (Penfound 1953). He studied aquatic flora throughout the Oklahoma Panhandle. As might be expected, he found fewer aquatic plants in the playas than in eastern Oklahoma water bodies and little similarity in the species between the regions. Following the initial descriptive studies of Reed (1930), Parker and Whitfield (1941), and Penfound (1953), a series of more comprehensive vegetation surveys were conducted in various areas of the Southern Great Plains. In the playas of Texas, Rowell (1971, 1981) found 69 plant species, with Haukos and Smith (1993a) adding 17 more to his list. Haukos and Smith (1997) considered surveys by Hoagland (1991) for playas in southeastern Colorado, lists from Kindscher and Lauver (1993) and Kindscher (1994) for western Kansas, a multistate survey by Curtis and Beierman (1980) in Texas, New Mexico, and Oklahoma, and local surveys in four to five playas northeast of Amarillo, Texas, by Cushing et al. (1993) and Johnston (1995) in arriving at their figure of 282 different vascular plant species. Following that, an extensive survey of 224 playas in the five-state Southern Great Plains found another 64 species, making the current published total for that region 346 (a complete species list can be found in Haukos and Smith 1997). The prairie ecologist J. E. Weaver was possibly the first to publish some botanical descriptions of Rainwater Basin playas (Weaver and Bruner 1954, 121). Referring to playas correctly as “depressed areas,” he and Bruner noted that the vegetation was adapted to frequent wet/ dry conditions and was very different from the surrounding upland. They noted at least 26 species. Subsequently, Erickson and Leslie

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(1987) studied vegetation in six playas of the Rainwater Basin region and identified 64 species. Their study was aimed at species-soil relationships for regulatory purposes and therefore did not examine plant community relationships or overall species prevalence. Following that investigation, Gilbert (1989) examined 47 playa wetlands comprising 13% of the Rainwater Basin playas remaining in 1989. He found 212 noncultivated vascular plants in those wetlands. Most (greater than 80%) of those species also occur in the Southern Great Plains (Haukos and Smith 1997). In 10 playas in eastern Wyoming, Holpp (1977, 112 –113) found 46 plant species. More recently, Bartz (1997, 24) found 35 plant species in the wetland zones of seven playas from northeastern Wyoming. The flora of these Wyoming playas does not overlap as much with that found in the Southern Great Plains or in the Rainwater Basin. These playas appear to contain more sedges and rushes (Carex spp., Juncus spp.) and are very distinct from the surrounding uplands. PLANT SURVEY TIMING AND CLASSIFICATION

The description of playa plant community composition in various studies is related to the season of sampling and recent precipitation events. In the survey of 224 playas in the Southern Great Plains, Smith and Haukos (2002) found that, on average, only 38% of the species were similar between early in the growing season and late season. Because the same playas were sampled in both seasonal surveys, the change can be directly attributed to varying seasonal plant life histories (e.g., those species such as little barley [Hordeum pusilum] that complete their life cycle within a certain time of year—in this case, early spring) or the changing hydroperiod in the playa. As in other prairie wetlands, the primary influence on species composition in playas is hydroperiod, as reflected through the seed bank (van der Valk 1981). A playa that is dry in early spring, for example, can have its entire flora turn over if the basin receives substantial rain and is inundated. In the spring the playa community may have been dominated more by plants typically occurring in uplands versus those that can only grow in submerged conditions and are considered aquatic. A plausible example of this is a playa dominated by western wheatgrass in early spring. The playa is then inundated, and after a few weeks the wheatgrass has been killed by the flooding and replaced with longbarb arrowhead (Sagittaria longiloba) and blue

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mud-plantain. The converse is also possible. A playa that contains several inches of water in early spring and contains longbarb arrowhead and blue mud-plantain may dry as a result of evaporation or a landowner pumping the water out of the playa for irrigation purposes. In this typical situation, species that require exposed, moist substrates to germinate, like barnyard grass (Echinochloa crusgalli) and smartweeds (Polygonum spp.) may become dominant within a very short period of time. Numerous variations of this changing hydroperiod occur constantly in playas, resulting in a dynamic flora. This very dynamism usually prevents the establishment of plants that have long-term aquatic requirements and predictable hydrologic needs in their life history. Obviously, stable and predictable hydrologic requirements are not provided in most prairie wetlands, and for this reason playas have few endangered species. Most threatened or endangered wetland plant species have specific and predictable hydrologic requirements to allow them to complete their life cycle (e.g., bog pitcher plants [Sarracenia spp.]). Plants such as annuals and short-lived perennials thrive in this environment of rapidly changing conditions. They have long-lived seed banks or hardy underground structures (e.g., corms, tubers, rhizomes) that can take advantage of the temporally and spatially variable precipitation in a relatively rapid time frame (van der Valk 1981; Smith and Kadlec 1983). An aquatic species that required a more predictable set of environmental conditions, such as most plant species of concern to conservationists (i.e., those species whose populations may be experiencing severe declines, be threatened, or be endangered), would not last long in a playa. That is probably why playa plant communities are so ubiquitous throughout the Great Plains region. Perhaps that is also why playas (lacking in “species of concern”) are “understudied.” The plants that can take advantage of the rapidly changing conditions in Nebraska are the same as those in Kansas and Texas. Wetland plants are also used, along with soils and hydrology, in the legal determination of what is a jurisdictional wetland and what can be legally done to that wetland (i.e., Section 404 of the Clean Water Act; U.S. Army Corps of Engineers 1987). Therefore, plants have been classified accordingly. Plants that occur in and around wetlands have been classified as to their relative affinity to exist in hydric conditions (Reed 1988). An understanding of this classification scheme is key to understanding some of the issues surrounding playa plant communities and issues presented later relative to threats to playa

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ecosystems. This plant classification list was developed and updated through the National Wetlands Inventory by representatives from the U.S. Fish and Wildlife Service, U.S. Army Corps of Engineers (the primary agency responsible for wetland enforcement under Section 404), the Environmental Protection Agency, and the Natural Resources Conservation Service (the U.S. Department of Agriculture agency currently responsible for delineating wetlands on agricultural lands, where most playas occur). Using this system, plants are initially classified into groups that are obligate (OBL) wetland species (which occur with a 99% probability in wetlands); facultative (FAC) species, which can occur in wetlands but are not restricted to wetlands (34 – 66% probability); and upland (UPL) species (99% probability of occurrence in nonwetlands). The FAC species can be further broken down into groups that are more closely associated with wetland environments as facultativewetlands (FACW; 67–99% probability of occurring in a wetland) or groups that are more closely associated with uplands, facultative-upland (FACU; 67–99% probability of occurring in nonwetlands). To achieve additional consensus among the individuals ranking the plants, further modifiers were added: a “” to indicate species with a greater affinity to wetlands, and a “” for those having a greater affinity to uplands. Species with insufficient information to assign them to a category were designated as no indicator (NI). Plants are listed according to these categories by region, and those regional lists may differ from each other. For example, playa plants in New Mexico are assigned to Region 7, whereas playa plants in Texas are assigned to Region 6. Knotgrass (Paspalum paspalodes) is classified FACW in Texas but as OBL in New Mexico (Haukos and Smith 1997). These regional differences in classification are thought to take into account ecotypic variation in species but also reflect the varying opinions of different individuals from agencies in the various regions. The relative dominance of OBL and FACW plants over the other categories can be used to define playa boundaries (NRC 1995). Along with hydric soil maps, agencies define the wetland boundary for regulatory and agricultural benefit purposes. (Some current USDA initiatives, such as the Swampbuster, link wetland protection and federal agricultural payments to landowners.) The location of these boundaries, however, also influences how ecologists view the contributions of playas to regional biodiversity and, at the community level, how plants are distributed within the playa. As noted earlier, the discrepancies in the various counts of the number of playas was,

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Figure 3.1 A typical Southern Great Plains playa showing two vegetation zones (playa edge on the left, level playa floor on the right) within the hydric soil–defined wetland. Upland vegetation in the background is above the hydric soil and outside the wetland. (Photo by author.)

in part, due to the manner in which playa boundaries were defined (e.g., topographic maps vs. soil maps). The same problem can influence how individuals view plant community composition in playas. If, for example, ecologists use topographic map slope position to define the playa, versus hydric soils, they will likely reach a much different conclusion about potential plant community structure (e.g., zonation) in playas. The change in topographic map slope position is typically higher in elevation than the hydric soil boundary in Great Plains playas. Therefore, relatively more species of UPL and FACU plants would appear in the flora of a playa defined from a topographic map versus a playa that was surveyed within its hydric soil boundary (fig. 3.1). ZONATION

Zonation refers to the different life-forms or communities of wetland plants that are adapted to different levels of inundation (e.g., FACW vs. OBL or submergent vs. emergent macro-

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phytes). Because playas in the western and Southern Great Plains display little elevational heterogeneity, they are generally thought to display relatively little plant zonation (Haukos and Smith 1994b; Smith and Haukos 2002). The distribution of seeds is related to this zonation (van der Valk 1981; van der Valk and Welling 1988). Seeds float or are windblown until they reach an obstacle. A change in elevation, or a band of vegetation is such an obstacle (Smith and Kadlec 1983) (fig. 3.2). In the flat bottom of a Southern Great Plains playa this elevational obstacle, or lack thereof, is similar across the hydric soil basin unless there has been some type of hydrologic modification causing an elevational change. Excavations in the playa cause an elevation change in the hydric soil and permit additional zones or life-forms to occur. Haukos and Smith (1994b) examined the seed bank of eight playas along their hydric soil elevational gradient. Seed density and species composition did not vary along the hydric soil–defined elevational

Figure 3.2 Windrow of seeds in a playa. Seeds float in the air or water until a barrier is reached and then they settle out. (Photo by author.)

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gradient ( 1 m). In the undisturbed hydric soil of these playas there were essentially just two zones, the playa floor and the narrow hydric soil edge that extends upslope for only a slight elevational increase (often  1 m). This relatively narrow zone not only creates an obstacle; it also has a different hydroperiod than the lower zone, resulting in a different plant community. Reed (1930, 60) also suggested the occurrence of two zones, the large playa floor and the relatively smaller playa edge. He noted that the plants occurring in those zones did not occur higher in the “plains.” Parker and Whitfield (1941) commented on the zones of playas in the Southern High Plains but noted a third upland zone above the edge. This upland zone is outside the legally defined (by hydric soils and hydric plants) playa basin and is dominated by obligate UPL and FACU plants. More recently Hoagland and Collins (1997) studied plant zonation in and around 40 playas in the Southern Great Plains. They described four vegetative zones in these playas. However, plants outside the playa basin as defined by its hydric soil (Randall clay) were included in the analysis of zonation. They classified the four zones as (1) interior vegetation, (2) interior-edge vegetation, (3) upland-edge vegetation, and (4) upland vegetation. Therefore, as defined by the hydric soils, Hoagland and Collins (1997) also found two zones within the playa itself. However, they attributed the disparities in conclusions about playa zonation between their study, which suggested four zones, and others suggesting fewer (e.g., Guthery et al. 1982; Haukos and Smith 1993a, 1994b) to anthropogenic influences on hydrology. While anthropogenic influences doubtlessly affect plant species occurrence and community composition (Smith and Haukos 2002), it is more likely the differences in the number of plant zones observed among these studies are simply an artifact of sampling procedure. The species that may have invaded or increased as a result of anthropogenic influence are affected by the same hydrologic factors in playas that have not had as much anthropogenic disturbance. If one samples plant communities outside (above on an elevational scale) the hydric soil–defined wetland in a playa watershed he or she will find additional vegetation zones even when the playa may have received anthropogenic hydrologic influences. The primary influence is the lack of elevational range within the hydric soil wetland (Smith and Haukos 2002). Unlike playas in the High Plains, the Rainwater Basin playas ap-

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pear to have more zones in the hydric soil–defined wetland (Gilbert 1989). Weaver and Bruner also suggested as much by noting that playa depth ranged from just a “foot or two [ 0.5 m] below the general soil level . . . “ to “10 to 15 feet [2 –3 m]” (1954, 121). This greater elevational range in the hydric soil basin allows more plant zones adapted to different levels of inundation to occupy these southcentral Nebraska playas than is the case in other playas of the Great Plains. Gilbert (1989, 21) identified four zones within the hydric soil– defined basin that were all correlated to their relative depth within the wetland. Finally, in eastern Wyoming Bartz (1997) studied plant community zonation in seven playas. He concluded that there could be up to three zones in the playa itself and a fourth if one considered the surrounding mixed-grass prairie of the upland. Not all playas had all three zones, and three of seven playas had only two wetland plant zones. COMMUNITY COMPOSITION

The particular plant community occupying a given playa at a given time in the Great Plains is related primarily to depth of inundation and length of inundation (hydroperiod) or lack thereof. The hydroperiod is primarily related to precipitation events (or irrigation pumping and runoff in some agricultural instances). The same size and depth playa in one area may possess a vastly different extant flora than a playa a short distance away simply because it has received more, or less, precipitation. The word “extant” is used here to indicate the existing aboveground flora at the time of a survey. Because playa flora can change so rapidly, one must consider the extant flora and the underlying seed bank to get a true picture of the potential flora. Although plant community composition in playas is primarily and ultimately influenced by hydroperiod, grazing and fire are important secondary influences. Unfortunately, few studies have examined the direct influence of these factors on plant community composition in playas, except to note structural habitat changes such as percentage of cover.

southern great plains studies Beyond the brief description of Southern High Plains playa plant community type by Reed (1930), Guthery et al. (1982, 519–520) described various “physiognomic” plant groupings in

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101 playas for the same region. (Taxonomy and common names here follow Guthery et al. [1982, 519–520]; see table 3.2 for consistency in interpretation.) During surveys from June through August 1980, in three counties (Castro, Lamb, Bailey) of Texas, they noted the presence or absence of 33 plant taxa. Of these 33 they used 24 in an analysis to define 14 physiognomic types (table 3.2). Obviously the “crop” type had little potential for native vegetation. Their study was conducted during a below average (33 – 66% of normal) precipitation year, and they found the open water vegetation type, dominated by pondweeds, only in playas that had been hydrologically modified through pit excavation. In the counties Guthery et al. (1982) studied, pits were most often excavated to receive irrigation tailwater. Landowners could then repump the water for irrigation onto the land at less cost than pumping additional water from the aquifer. Pits generally hold precipitation and irrigation runoff for a longer time than if that water had been spread out over the playa, because less surface area is available for evaporation. This allows plants that require longer hydroperiods to persist in the pits. In that study the very existence of pits suggested that the playa received irrigation runoff. The constant recycling of irrigation water through the playa also permitted plants that required frequent moist conditions to exist. Therefore, modification of playas through pit excavation was also associated with the increased prevalence of bulrush and cattail (narrow-leaved emergent type) and of woody species, primarily salt cedar and black willow (tree-shrub type) (table 3.2). The relative importance of pit excavation alone versus the reception of additional irrigation water alone in shaping playa plant communities is difficult to separate, but it is really of little consequence. Both are correlated and important in the alteration of playa plant communities. Guthery et al. (1982, 523) further found that in that drought year pondweed, water clover (Marsilea sp.), smartweed (Polygonum lapathifolium), arrowhead, bulrush, and cattail only occurred in playas receiving irrigation runoff (nomenclature here follows that in Guthery et al. 1982). The spoil-bank type they defined within a playa was simply where soil was borrowed from pit or ditch excavation. It created an elevational change and barrier thus allowing another habitat to be created that normally would not exist. This type was dominated by exotics. Beyond the implications of playas with pits receiving frequent

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Table 3.2 Vegetation classified into 14 physiognomic types from 101 playas in Bailey, Lamb, and Castro Counties, Texas, June–August 1980 Percent Physiognomic type (1) Open water (2) Broad-leaved emergent (3) Narrow-leaved emergent (4) Mesic forb

(5) Wet meadow

(6) Johnsongrass (7) Disturbed forb

(8) Cultivation (9) Mudflat (10) Spoilbank

(11) Midgrass

(12) Short-grass (13) Road-pit (14) Tree–shrub

Dominant taxa Pondweed (Potamogeton spp.) Smartweeds (Polygonum bicorne, P. lapathifolium) Cattail (Typha domingensis) Bulrush (Scirpus spp.) Devilweed (Aster spinosus) Gray ragweed (Ambrosia Grayii) Barnyard grass, Red sprangletop (Leptochloa filiformis) Johnsongrass (Sorghum halepense) Summer cypress (Kochia scoparia) Texas blueweed (Helianthus ciliaris) Crop Absence of vegetation Summer cypress Camphor-weed (Heterotheca spp.) Western wheatgrass (Agropyron smithii), Vine mesquite (Panicum obtusum) Buffalo grass (Buchloe dactyloides) Absence of vegetation Willow (Salix nigra) Salt cedar (Tamarix gallica)

Secondary taxa

occurrence

Arrowhead (Sagittaria longiloba) Barnyard grass (Echinochloa crusgalli) Spikerush (Eleocharis spp.)

28 36

13 Smartweeds, Barnyard grass, Spikerush

26

Smartweeds, Spikerush, Devilweed

41

17 Horseweed (Conyza canadensis) Wild lettuce (Lactuca spp.)

Barnyard grass, Water-hyssop (Bacopa rotundifolia) Tumbleweed (Salsola kali)

9

20 16 1

2

Gray ragweed

13 12 21

Siberian elm (Ulmus pumila) Source: Modified from Guthery et al. 1982; courtesy of the Transactions of the North American Wildlife and Natural Resources Conference. Note: Dominant taxa were most prevalent in terms of coverage; secondary taxa commonly occurred in a type, but had lower coverage than dominant taxa.

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irrigation runoff, pits can also concentrate naturally occurring precipitation, effectively reducing the hydroperiod for the remainder of the playa plant community. Some pits, especially north of the Southern Great Plains in the Rainwater Basin, were specifically constructed to concentrate water not for irrigation, but to reduce the hydroperiod. This allows greater cultivation of the wetland. Pits are also constructed for livestock watering, which reduces the hydroperiod for the entire wetland. Finally, at times the construction of pits breaches the clay layer of the playa resulting in more rapid water loss. As mentioned previously, Hoaglund and Collins (1997) studied plant communities in 40 playas in southeastern Colorado, northeastern New Mexico, the extreme northwestern Texas Panhandle, and the Oklahoma Panhandle. Most of the playas they studied occurred on USDA national grasslands (e.g., Comanche, Kiowa, Rita Blanca). Therefore, most of these playas occurred in a native short-grass prairie setting with little cropland agriculture but an extensive cattlegrazing history. Many of the playas on the national grasslands, although not modified hydrologically to receive irrigation runoff, have had pits, ditches, and islands constructed in them to retain water for livestock or, ostensibly, to improve the playa as habitat for wildlife (later modifications were thought to improve use by waterfowl, not other species). According to Hoaglund and Collins 1997, within the hydric soil– defined basin of these playas, three grass species appeared to dominate: western wheatgrass, buffalo grass, and vine mesquite (Panicum obtusum). They were also the most widespread species in terms of percent of playa occurrence. Bur ragweed, snow-on-the-mountain (Euphorbia marginata), and frog-fruit (Lippia nodiflora) were important forb species. Hoaglund and Collins noted that the dominance of western wheatgrass decreased as elevation increased to the uplands but that buffalo grass was important throughout the wetland and into the upland. Many other species (69) occurred but none were as widespread as those listed above. Apparently water did not exist in the playas during the time playas were sampled. However, from their paper it is not possible to determine when during the growing season the vegetation was surveyed and, therefore, whether the vegetation was compared among playas for similar seasonal circumstances and whether it varied seasonally. This can influence perceptions of extant community composition.

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In the five-state playa flora study that Smith and Haukos (2002) conducted in more than 200 playas, the overall community composition varied significantly among six subregions of this portion of the Southern Great Plains (unpublished data). Correspondence analyses were used followed by cluster analyses to delineate the six subregions (fig. 3.3). The dominant species in these communities were similar among the six subregions, though the occurrence of uncommon species within a subregion allowed groups to be delineated. These delineations could simply be the result of varying precipitation and growing season patterns causing some species to be common in one county and uncommon in another. Some of the more common widespread species ( 50% occurrence in playas) throughout the region were western wheatgrass, rough pigweed (Amaranthus retroflexus), saltmarsh aster (Aster subulatus), buffalo grass, lamb’s quarters (Chenopodium album), narrow-leaved goosefoot (Chenopodium leptophyllum), horse-weed (Conyza canadensis), barnyard grass, spikerush (Eleocharis macrostachya), curlytop gumweed (Grindelia squarrosa), annual sunflower (Helianthus annuus), Texas blueweed (Helianthus ciliaris), little barley, summer cypress, frog-fruit, cheeseweed (Malvella leprosa), spotted evening primrose (Oenothera canescens), vine mesquite, Pennsylvania smartweed, spreading yellow cress (Rorippa sinuata), curly dock (Rumex crispus), silver-leaf nightshade (Solanum elaeagnifolium), and prostrate vervain (Verbena bracteata). Most of these occurred throughout the six subregions, but their frequency varied by whether the watershed in which they occurred was dominated by grassland or cropland. Annuals occurred much more often in cropland playas than in grassland playas (Smith and Haukos 2002). For example, lamb’s quarters and rough pigweed occurred more than twice as often in playas with cropland watersheds. Furthermore, the number of annuals in cropland playas averaged 8.9, while those in grassland playas averaged 6.7 (Smith and Haukos 2002). The above species occurrence data were supported by horizontal coverage information of perennial versus annual species within a playa. Playas with cropland watersheds had an average coverage of annuals at 30% and perennials at 47%, while playas with grassland watersheds had average coverage of annuals at 12% and perennials at 63%. (The percentages do not total to 100 because of either open water that had no plant cover or bare soil with no plant cover.) Clearly

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Figure 3.3 Counties sampled for flora description in the Southern Great Plains (Smith and Haukos 2002) and counties grouped by floral-defined ordination groups. (Figure by D. Haukos, courtesy of U.S. Fish and Wildlife Service.)

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watershed cultivation and its associated disturbances has influenced playa plant communities. Interrelated disturbances include sedimentation, irrigation (runoff into playas and pumping water from playas for surrounding crops), and hydrologic modification through pit and ditch construction. The increased rate of disturbance in playas with cropland watersheds versus those with grassland watersheds is predisposing playas to being dominated more often by annual species (Smith and Haukos 2002). Cultivation of playa watersheds is further associated with the bioinvasion of native flora by exotics (Smith and Haukos 2002). On average, there were twice as many exotic species occurring in playas with cropland watersheds than in those with grassland watersheds (4.7 vs. 2.3, respectively). Coverage data supported the species frequency data (15.6% crop vs. 6.3% grassland). Cultivation, and its associated disturbance (e.g., hydrologic modification, irrigation), has caused increased and consistent disturbance to playas throughout the Great Plains such that it is difficult to find playas that have natural floral communities. Cultivation disturbances permitted the increase in bioinvasion by exotics likely by increasing the prevalence of bare soil and by altering moisture regimes, which change germination conditions. There are shorter playa hydroperiods because of sedimentation but also, at times, longer, because of increased moisture from irrigation runoff. Exotics have exploited this new niche in the altered playa environment. There were 65 common (occurred in greater than 5% of playas) species in Southern Great Plains playas (Smith and Haukos 2002). Ten were exotics, a substantial portion (15%) of the community. Furthermore, only one of the 10 was perennial, illustrating that exotics have played a role in shifting the balance from perennials to annuals. Moisture and bare soil disturbances are not the only factors promoting bioinvasions; increased nutrient loads can cause this as well (Hobbs and Atkins 1988; Burke and Grime 1996). As noted earlier, playas form the lowest elevation points for the vast majority of agricultural enterprises in the region. Therefore, playas often receive additional nutrient inputs from crop fertilizer and livestock fecal runoff (Irwin et al. 1996). Similar to playas with cropland watersheds, modification of playas in grassland environments, even national grasslands, has also allowed plant community composition to change. Many playas in native grassland environments have had pits dug in them for concentrating

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water, which, as noted, allows nonnative species that require deeper water to colonize and allows more upland-adapted species to move into the now dryer hydric soil. Wildlife managers have even created ditches, pits, and islands in playas in many national grasslands in the region with help from a nongovernmental wildlife organization. This produces similar results to irrigation and livestock watering pits. The effect of livestock grazing on playa plant communities, which occurs on many playas with cultivated watersheds and is likely on all playas with grassland watersheds, has not been documented. Obviously the prairie environment, including playas, evolved under the influence of large herbivores (e.g., bison, elk, pronghorn). How native herbivore grazing varies from domestic livestock is unknown. Plant selection and season/frequency of use are likely different. Guthery et al. (1982) suggested that grazing of playas by domestic livestock may promote buffalo grass and associated plants (e.g., gray ragweed) at the expense of western wheatgrass. However, these were correlative data, in that direct tests/experiments were not conducted. The buffalo grass vegetation type was simply associated with livestock grazing. Further, some species such as knotgrass, considered an obligate wetland plant throughout much of the region, have been promoted as a means to increase livestock grazing potential in playas (W. Wyatt, personal communication, High Plains Underground Water Conservation District, Lubbock, Tex.). Finally, though playas are frequently burned in the Southern Great Plains, few studies have examined the influence of burning on plant community composition. In cattail-dominated playas, a relatively rare playa vegetation type in the Southern Great Plains (Haukos and Smith 1997), fire had little influence on community composition and its effects on habitat structure lasted no more than 4 months (Smith 1989).

wyoming studies In eastern Wyoming, Holpp (1977) characterized plant communities in 10 playas. The dominant grasses in those playas were western wheatgrass and foxtail barley (Hordeum jubatum). Numerous common dandelions (Taraxacum officinale) were also present. The dominant sedges and rushes were common sedge (Carex filifolia) and Baltic rush (Juncus balticus). Holpp stated that “the intensity of the grazing pressure resulted in the decline of the perennial grass species and allowed the encroachment and establish-

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ment of the forb community. Thus, perennial forbs dominated the plant composition of all grazed sites” (1977, 104). Although these conclusions may be correct, similar to Guthery et al.’s (1982) study the data were collected through association not experimentation; the observations thus should be viewed as preliminary. As noted earlier, Bartz (1997) found up to three zonal plant communities in other playas from Wyoming. He termed them, from the lowest elevation in the playa to the highest: “spikerush,” “foxtail barley,” and “western wheatgrass.” As found in studies of playas elsewhere, Bartz found the most diverse (31 species) community, or zone, occurred on the edge of the upland and wetland. The five most abundant species in this western wheatgrass zone were its namesake, followed by prairie junegrass (Koelaria cristata), blue grama (Bouteloua gracilis), spikerush (Eleocharis acicularis), and salt grass (Distichlis spicata). The next two zones appeared to be distinguished based on the amount of another spikerush (Eleocharis palustris) that the playas contained, because the most abundant species in each zone was that spikerush. The foxtail barley zone also contained its namesake, and western wheatgrass, a spikerush (E. acicularis), and bur ragweed. The lowest, or spikerush, zone contained both spikerushes, bur ragweed, western wheatgrass, and shortawn foxtail (Alopercurus aequalis).

rainwater basin studies Gilbert (1989) has conducted the most extensive survey of Rainwater Basin flora from 47 wetlands (probably 10% of the wetlands remaining in the region). He delineated five major plant zones, four of which could be considered to contain some hydric plants (fig. 3.4; nomenclature follows Gilbert 1989 here). Beginning in the deepest or longest hydroperiod zone, Inner Marsh, he found all obligate wetland species, or as he classified them, “aquatic bed” and “drawdown” species. Aquatic bed species were dominant and required submerged conditions to survive, whereas drawdown species required moist exposed soils to germinate. The drawdown (shallow water) portion of this zone was dominated by two species of arrowhead (Sagittaria spp.), bur reed (Sparganium eurycarpum), water plantain (Alisma triviate), water-hyssop (Bacopa rotundifolia), and blue mud-plantain. The dominant aquatic bed species were duckweed (Lemna minor), water fern (Azola mexicana), and common bladderwort (Utricularia vulgaris). Occasionally a “reverse” zonation condition occurs here (T. LaGrange, personal communication). When the

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Figure 3.4 Vegetation zones in Rainwater Basin wetlands as defined by Gilbert (1989, 21). (Figure by M. Gilbert, courtesy of U.S. Army Corps of Engineers.)

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deep open water area dries occasionally, the area becomes dominated by Outer Marsh plants but is still surrounded by living persistent emergents. Moving up in elevation, the next zone was the Persistent Emergent zone, which was mostly dominated by robust herbaceous emergent perennials such as three species of cattail (Typha spp.) and three species of bulrush (Scirpus spp.). Again, all of these are considered obligate wetland plants. The Outer Marsh zone appeared more diverse, with dominants including five smartweed (Polygonum spp.) species, three spikerushes (Eleocharis spp.), Plains coreopsis (Coreopsis tinctoria), and western water clover (fig. 3.4). With the exception of coreopsis all are considered facultative wetland plants or obligate. Hydrophytic grass dominants in this zone included two barnyard grasses (Echinochloa spp.), reed canary grass (Phalaris arundinacea), rice cutgrass (Leersia oryzoides), bearded sprangletop (Leptochloa fascicularis), and bluejoint (Calamagrostis canadensis). At least six species of forbs were subdominants. The next zone upslope was termed Transition, with western wheatgrass, foxtail barley, Carolina foxtail (Alopecurus carolinianus), meadow foxtail (Alopecurus pratensis), ticklegrass (Agrostis hyemalis), and Virginia wild rye (Elymus virginicus) as stand dominants (fig. 3.4). Four sedges (Carex spp.), three ragweeds (Ambrosia spp.), dock (Rumex spp.), and horse-weed (Conyza canadensis) were among some of the other dominants (Gilbert 1989). The upland zone was dominated by either planted or native grassland typically grazed by livestock. An examination of the wetland species listed above by Gilbert (1989) also shows a relatively high percentage ( 10%) of exotic species. Given the highly altered state of Rainwater Basin playas, this is not surprising. They have been subjected to the same physical insults that other playas have with some additional threats such as drainage. These alterations have a higher occurrence in south-central Nebraska than in other playa locations.

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CHAPTER 4

FAUNA

A

s with the flora in playas, the animal life existing in these wetlands depends not only on the current playa hydroperiod but also on its historic hydroperiod. Similar to seeds, many invertebrates can remain dormant in playa sediments for decades reflecting past colonization events and hydric conditions (Hairston et al. 1995). Although seeds and invertebrates are generally considered in this light, many amphibians also may remain dormant in playa soils for years, their presence a result of past habitat conditions, with their future dependent on the proper moisture conditions to emerge. Beyond the influence of historic and current hydric conditions on playa fauna are the landscape factors that shape the composition of the existing animal communities. For example, the position of the playas in the Great Plains and North American landscape dictate which bird species can use playas during migration or breeding in dry or wet conditions. INVERTEBR ATES

Similar to other ecosystems, the understanding of invertebrate fauna in playas is far behind that for vertebrates. The simple groupings used in this chapter, with invertebrates included in one overall group versus the several groups of vertebrates, illustrates the bias and availability of faunal data for playas. This condition exists with full knowledge that invertebrate diversity and abundance far surpasses that of vertebrates, and invertebrates are key in all ecosystem processes (Wilson 1999). They are important food sources to most vertebrates inhabiting playas including amphibians, shorebirds, and waterfowl (Davis and Smith 1998a; Anderson et al. 1999a; Anderson et al. 2000), essential to playa nutrient cycling, and predators/parasites on other invertebrates as well as vertebrates.

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The very diversity and inherent variability in occurrence and numbers of invertebrates has often discouraged scientists from tackling invertebrate studies in wetlands (Murkin et al. 1994). Identification to even the family level of classification can be difficult, while estimating population size, generation time, and turnover is daunting. Although these difficulties have not encouraged widespread study of invertebrates in wetlands, it is these large gaps in our knowledge that should foster future studies. The chance to make important and immediate contributions to playa ecology is extraordinary. SCOPE AND SAMPLING LIMITATIONS

Almost all work on playa invertebrates has focused on playas that have water in them at the time of investigation. Lists of invertebrates occurring in playas (e.g., Sublette and Sublette 1967) typically ignore the terrestrial/dry phase of playas. However, the importance of this invertebrate community to the structure and function of playa ecosystems is undoubtedly substantial. Although some might expect invertebrate communities of dry playas to closely mirror the surrounding uplands, this assumption has not been tested. Because playa flora varies along the gradient from upland to hydric soil basin (see Chapter 3), it is probable that many plant-specific, and indeed soil-specific, invertebrates also vary along that gradient. Certainly studies are warranted on invertebrate communities during the dry phase in the life history of a playa. Further, many so-called terrestrial species inhabit the emergent vegetation, above the water, or are incorporated into aquatic food-web processes as the playa fills with water (Anderson and Smith 2000). This forms an especially unique community of aquatic and terrestrial species occurring together, with undescribed new species (J. Anderson 1997). Moreover, the majority of invertebrate studies in playas have focused on “macro-invertebrates” (fig. 4.1). “Macro” is a somewhat arbitrary designation related to a specific screen size through which sampled invertebrates will not pass (usually 500 – 600 µm) (e.g., Anderson and Smith 1996). Oftentimes the term macro is used without definition, leaving the reader to guess which groups may have been excluded from the community. The “microinvertebrate” fauna (e.g., rotifers) of playas is relatively unexplored, but its importance to playa ecology cannot be dismissed. The findings of the few studies on aquatic invertebrates in playas often have little in common, which is likely due to variable sampling

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Figure 4.1 Common invertebrates sampled in playas, including midge larvae, leech, and dragonfly larvae. (Photo by author.)

methodology, geographic area of sampling, and season of sampling. However, much of this variability is inherent in playas on a local basis (Sublette and Sublette 1967; Merickel and Wangberg 1981; Thompson 1985; Neck and Schramm 1992; Anderson and Smith 1998), which can be related to the proximity of other aquatic habitats (influencing colonization events) and those species that were able to remain dormant in playa sediments (J. Anderson 1997; Hall et al. 1999). Furthermore, the amount and type of vegetation, which regulates the density and species composition of invertebrates (e.g., Krecker 1939; Krull 1970; Downing 1981), also varies greatly among playas on a local basis (Smith and Haukos 2002). Finally, the length and timing of the playa hydroperiod has a tremendous influence on the community composition and abundance of aquatic invertebrates (Anderson and Smith 1998). COMMUNITY COMPOSITION

Even with limitations and problems with the level of taxonomic resolution of playa invertebrates, it is apparent the invertebrate fauna of playas is especially diverse and underestimated. Hall et al. (1999) summarized the data from Sublette and Sublette (1967), Parks (1975), Merickel and Wangberg (1981), Neck and Schramm (1992), Horne (1996), and Hall (1997) and identified 124 macroinvertebrate taxa, at various taxonomic levels. When data from

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Davis (1996) and J. Anderson (1997) are included, the number of taxa exceeds 170 (see appendix). Some of the taxa (e.g., arachnids) recorded by J. Anderson (1997) include unique invertebrates inhabiting vegetation protruding above the water’s surface. Hall et al. (1999) noted that the aquatic insect community of playas was especially diverse when compared to the noninsect community (84 versus 40 taxa, respectively, in the taxa they examined). However, as they found, this is in part due to the state of our taxonomic understanding. Knowledge of aquatic insect taxonomy in playas appears more advanced than that for noninsects. Hall et al. (1999) noted that the number of annelid and ectoproct (bryozoans) species is unknown because of limited collections and improper preservation of specimens. The molluscs also have been a neglected group in playa studies, with only 8 species identified (appendix). Of the noninsects, arthropods appear most diverse, but resolution of copepods and branchiopod cladocerans has only been made to the class or ordinal level. A bright spot, the nonclacloceran branchiopods (clam, fairy, and tadpole shrimp) and ostracods (seed shrimp) are relatively more well known (Moore and Young 1964; Ferguson 1967; Horne 1993, 1996). A word of caution is warranted relative to some of the aquatic invertebrate studies in so-called Great Plains playas. Some of the species listed may not occur in playas but rather in groundwater-connected salt lakes (e.g., Sissom 1976). It is often difficult to determine actual collection location of species described in some taxonomic studies. Moreover, the majority of published studies on playa invertebrates have been conducted in the central Southern High Plains and invertebrate studies for the more northern playas are rare. For the Rainwater Basin region of Nebraska there has only been one published study on aquatic invertebrates. Gordon et al. (1990) examined eight playas during spring and summer and identified 39 taxa all at the family level or higher. Extrapolating results of invertebrate studies conducted on the Southern High Plains to playas outside that region is ill-advised for a variety of reasons, not the least of which are regional speciation and climatic differences (King et al. 1996). INFLUENCES ON COMMUNITY COMPOSITION

The immediate response of invertebrates to a playa filling with water can be astounding. A couple of weeks after basins have filled, the water’s surface can literally bubble with activity. After this typical initial flush of high abundance and low diversity

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of individuals emerging from dormancy within the playa, the community composition begins to shift and diversity generally begins to increase. The shifts in community assemblage are related to additional intraplaya emergence and colonization from outside of the playa by other invertebrates (J. Anderson 1997; Moorhead et al. 1998). However, these colonization and persistence (e.g., dormancy, diapause) events are not necessarily mutually exclusive life-history strategies employed by aquatic invertebrates. J. Anderson (1997) found that 58% of the taxa persisting in playas through diapause also actively colonized playas as a primary life-history strategy of inhabitation. After some threshold, however, increased length of inundation (hydroperiod) can result in decreased invertebrate abundance. Seasonally flooded wetlands often have higher invertebrate abundances than those flooded for longer periods such as semipermanent marshes (Neckles et al. 1990; Batzer and Resh 1992). This length of inundation threshold, resulting in lower abundance and/or diversity, is likely due to biotic interactions. As the hydroperiod lengthens, competition and predation play an increasingly important role in determination of aquatic invertebrate assemblages and abundance. Sublette and Sublette (1967), Merickel and Wangberg (1981), and Moorhead et al. (1998) found that following playa flooding in summer crustaceans were initially dominant and then insects possessing active dispersal capabilities become more prevalent. Many of these later-emerging and -colonizing species are predaceous. Subsequent competition and predation not only by invertebrates but also by amphibians and birds may decrease abundance of initially emerging species. For example, Davis and Smith (1998a) found that shorebirds reduced the abundance of invertebrates in some playas in the spring. Also, although overall invertebrate densities were higher in early spring, biomass and species diversity were greater by late summer and early fall in playas that had little emergent vegetation. Difficult to separate from the biotic, abiotic factors such as water and air temperature and day length also have an important effect on playa invertebrate communities. Playas can fill with water during any month, but given Great Plains precipitation patterns (see Chapter 1) Southern Great Plains playas typically fill in late spring and summer when day lengths and temperatures are at their annual highs. In the Rainwater Basin, filling also happens in spring as snow melts. Warmer temperatures and longer photoperiods generally permit greater growth and biomass of aquatic invertebrates (Ward and Stan-

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ford 1982). Studies of aquatic playa invertebrates outside the spring and summer months are rare. Anderson and Smith (2000) found that, in general, invertebrate abundance and diversity were greater in the September-flooded playas than those filled in November. This should be expected as a result of both abiotic (e.g., higher temperatures) and biotic (e.g., colonization opportunities) influences. Because of these biotic and abiotic relationships, altering the hydrology of playas has likely had a major influence on Great Plains invertebrate community composition. The construction of pits, trenches, or dugouts has created a more permanent water source in some playas by concentrating water in a smaller area reducing evaporative surface area. This has created a habitat that allows invertebrates, and other biota that require longer hydroperiods to reproduce, to invade and survive in playas thus changing biotic relationships. For example, Horne (1996) noted that Neck and Schramm (1992) found in playas with pits some species of molluscs not normally found in ephemeral pools. Further, there are reports of crayfish (Cambaridae) existing in a few playas of the Southern High Plains and north in Kansas and Nebraska (Flowers 1996; Nebraska Game and Parks Commission unpublished observations), which might be related to playa modification. Conversely, Rhodes and Garcia (1981) reported that playas with pits had fewer individuals and species of insects than playas without pits. Gray (1986) found that invertebrate abundance in playa pits peaked in late summer and early fall, coinciding with peak submergent plant biomass, a rare plant community outside of the pits. On the other hand, playa sedimentation, which has greatly reduced the volume and occurrence of playas throughout the Great Plains, also has likely had a profound effect on playa invertebrate communities by altering hydroperiods, plant communities, and benthic substrates. The implications of these hydrological influences on playa invertebrate evolution and extinction is unknown. Excellent reviews on the state of our knowledge of playa invertebrates have been done by Hall (1997) and J. Anderson (1997). FISHES

Due to the natural, ephemeral character of these wetlands, there were no known native fishes in Great Plains playas prior to human intervention. With dugouts and pits excavated in playas from Nebraska to New Mexico, playas that have fish are now scattered throughout the region. Most of these introduced species

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have been stocked by landowners on private lands for sportfishing. However, some have also been placed in playas on federal lands (e.g., Comanche National Grasslands, Colo.) and municipal properties (e.g., Lubbock, Tex.). Some have been stocked with fish for mosquito control (Nelson et al. 1983). Still other modified playas have fish such as black bullheads (Ictalurus melas) in them that apparently were not actively stocked by humans. Their means of dispersal to these playas is unknown (Bolen et al. 1989). Some of the more common fishes introduced into modified and urban playas include yellow bullhead (Ictalurus natalis), channel catfish (Ictalurus punctatus), sunfishes (Lepomis spp.), largemouth bass (Micropterus salmoides), common carp (Cyprinus carpio), golden shiner (Notemigonus crysoleucas), and fathead (Pimephales promelas) and brassy (Hybognathus hankinsoni) minnows (Curtis and Beierman 1980). Even rainbow trout (Oncorhynchus mykiss) have been stocked into urban playas for “put and take” fisheries (e.g., by the Texas Parks and Wildlife Department). These urban playas, which now have permanent sources of water, contain all sorts of exotic fauna and flora (Schramm et al. 1992). There are no studies addressing the influence of fish on native playa taxa. AMPHIBIANS

Aside from invertebrates, less is known about amphibians inhabiting Great Plains playas than any other major faunal group (A. Anderson 1997; Smith 1998). There have only been a few published ecological studies on amphibians in playas (Rose and Armentrout 1974, 1976; Anderson et al. 1999a,b). Although amphibians are key components of Great Plains wetlands, including serving as predator and prey, they have a relatively simple fauna compared to the other vertebrates. Six families of amphibians occur in Great Plains playas, with only one, but wide-ranging, species of salamander (fig. 4.2; table 4.1). The anuran (toads, frogs) species with the widest geographic range in the playas are the true toads (Bufonidae) and spadefoots (Pelobatidae), probably because of their ability to withstand rapidly fluctuating water levels. A Plains spadefoot can hatch in as little as 20 hours, in warm weather, and metamorphose from tadpole to terrestrial toad in less than 13 days (King 1960; Justus et al. 1977). Rapid reproduction and development is a prudent evolutionary strategy for aquatic species in playas. The Hylidae (tree, chorus, and cricket frogs) and

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Figure 4.2 Three of the most common amphibians in Great Plains playas. Top— nektonic tiger salamander; middle— Great Plains toad; bottom— New Mexico spadefoot toad. (Photos by author.)

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Table 4.1 Amphibians inhabiting playas of the Great Plains Family Ambystomatidae Bufonidae

Hylidae

Microhylidae Pelobatidae

Ranidae

Common name

Scientific name

tiger salamander Great Plains toad green toad Texas toad Woodhouse’s toad northern cricket frog spotted chorus frog western chorus frog Great Plains narrow-mouth toad Couch’s spadefoot toad Plains spadefoot toad New Mexico spadefoot toad Plains leopard frog

Ambystoma tigrinum Bufo cognatus Bufo debilis Bufo speciosus Bufo woodhousii Acris crepitans Pseudacris clarkii Pseudacris triseriata Gastrophryne olivacea Scaphiopus couchii Spea bombifrons Spea multiplicata Rana blairi

bullfrog

Rana catesbeiana

Source: From Curtis and Beierman 1980; A. Anderson 1997; Nebraska Game and Parks Commission 2002, M. Fritz.

Ranidae (true frogs) generally require more frequent wet conditions than toads or spadefoots. However, scientists know relatively little about the environmental conditions that stimulate amphibians to emerge and breed in playas, and as little about their hibernation/estivation patterns. What playa amphibians lack in diversity they make up for in abundance. Their numbers can easily overwhelm all other vertebrates. Further, the biomass of the aquatic amphibians in playas often surpasses all fauna, especially in summer. Tens of thousands of larval tiger salamanders can occur in a playa covering just a couple of hectares in area or the same magnitude of toadlets (recently metamorphosed tadpoles) can emerge from the same size playa in a single night (Gray 2002). A tiger salamander may lay more than 5,000 eggs in a clutch (Rose and Armentrout 1976), but this is relatively small compared to some anurans. A single Great Plains toad female can lay more than 40,000 eggs in one clutch and can have two clutches a year (Krupa 1986, 1988, 1994). The sheer magnitude of amphibian biomass and abundance is key to the survival of their predators such as larval invertebrates, American avocets (Recurvirostra americana),

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and black-crowned night herons (Nycticorax nycticorax) as well as their prey, invertebrates and fellow amphibians. Many amphibians are cannibals (Degenhardt et al. 1996). This relatively high abundance and low diversity of amphibians, in limited aquatic habitat, is exactly what makes them unique and valuable to the Great Plains landscape. Similar to their effects on other biota, pits, trenches, or other modifications to the playa hydrology also influence occurrence of amphibian species that require more permanent sources of water. Anderson et al. (1999b) found that spotted chorus frogs in the central Southern High Plains were found more frequently in playas with irrigation pits than those without. For those species that can survive more xeric conditions (Great Plains toad, Great Plains narrow-mouth toad, Plains spadefoot toad, New Mexico spadefoot toad) there were no differences in their occurrence between playas with and without pits. The occurrence of bullfrogs is also likely related to hydrologic alteration of playas and through direct stocking by landowners. They are probably not native to Great Plains playas. The amount of vegetative cover in the playa basin also affects amphibian occurrence. The five species noted in the previous paragraph showed a positive relationship between their occurrence and increasing amounts of vegetation (Anderson et al. 1999b). This was especially apparent for the Great Plains narrow-mouth toad and the spotted chorus frog. Anderson et al. (1999b) further compared the occurrence of these five species between playas with grassland watersheds to those with cropland watersheds. Cultivation in the watershed is an indication of disturbance, such as in the amount of sedimentation, that has influenced the playa (Luo et al. 1997), which can affect not only burrowing conditions for estivating amphibians but also the water quality, length of the playa hydroperiod, and flora. Anderson et al. (1999b) did not demonstrate a surrounding land-use effect on anuran species richness or frequency, but they found considerable annual variation in anuran richness and occurrence within the same playa. Frog and toad community composition changed in all 18 playas that were surveyed in two consecutive years. This led Anderson et al. (1999b) to suggest that anurans may forgo breeding in some years even when playas contain water. Therefore, recently initiated national surveys using annual auditory trends of breeding amphibians would yield results that are difficult to interpret in playas because of variable annual breeding effort rather than fluctuations in either species occurrence or individual species population size.

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According to Degenhardt et al. (1986), tiger salamander larvae will eat almost any animal matter they catch (Collins 1982; Hammerson 1982). Indeed, they form cannibal morphs, and as the name suggests they consume large numbers of conspecific larvae (Holomuzki and Collins 1987). Cannibal morphs are not common (Rose and Armentrout 1976), but occurrence may be stimulated by high density of conspecific larvae (Collins and Cheek 1983). Cannibalism can be an adaptive life-history strategy allowing more rapid growth and earlier metamorphosis (Lannoo and Bachmann 1984). Larval anurans may also become predator/cannibals, but data in natural environments are scarce. Anderson et al. (1999a) studied the diets of newly emerged, breeding Great Plains toads, New Mexico spadefoot toads, and Plains spadefoot toads in playas of the central Southern High Plains. Great Plains toads consumed 43 invertebrate taxa, and New Mexico and Plains spadefoot toads consumed 20 and 12 invertebrate taxa, respectively, even though they found individual spadefoot diets were more diverse than Great Plains toads. Interestingly the highest dietary overlap occurred between the largest species, Great Plains toad, and the smallest, New Mexico spadefoot. The most important food for all three species was Carabidae beetles. These beetles are mostly nocturnal like the three anurans. Moreover, based on the occurrence of food in stomachs, it appears Great Plains toads feed soon upon emergence and during breeding, whereas spadefoots feed more after breeding. A. Anderson (1997) also found that several of the top invertebrate taxa consumed by these toads were considered agricultural pests. Given the potential high density of toads, this fact may indicate economic benefits to the agricultural economy. The diet of larval anurans in playas remains to be studied. Most invertebrates consumed by adult anurans were “terrestrial,” not “aquatic” (Anderson et al. 1999a). Adult anurans were feeding on the edge of the playa or in the immediate watershed. This further illustrates the need to study terrestrial invertebrates in playas and the close links between the terrestrial and aquatic systems. Given the life history (aquatic larval and terrestrial adult) of playa amphibians, they would be ideal models for these types of investigation. Moreover, amphibians are believed to be declining worldwide with no single factor being consistently identified as the cause of decline across all regions (e.g., Blaustein and Wake 1990; Beebee 1996; Corn and Peterson 1996). The general lack of knowledge concerning Great Plains amphibians,

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their value as environmental indicators, and their potential decline should spur research on amphibians throughout the playas. REPTILES

There are few native reptiles occurring in the inundated portions of playas. The yellow mud turtle (Kinosternon flavescens) is the main exception, with a distribution coinciding with the distribution of most playas. Overall, however, most of its time is likely spent on dry land while the majority of its foraging, drinking, and mating occur in water (Degenhardt et al. 1996). Further, apparently the yellow mud turtle can estivate for up to two years (Rose 1980), which accounts for its widespread occurrence in playas. The turtle does not mate until it is 6 –16 years old and has a clutch of 3 – 10 eggs (Degenhardt et al. 1996). The reptile community in dry playas or on the interface of the water and associated watershed can be roughly as diverse as the amphibians, though they are not nearly as abundant. Probably the most numerous group is the garter snake, which feed on the invertebrates and amphibians inhabiting playas. They include the red-sided garter (Thamnophis sirtalis), western ribbon (Thamnophis proximis), wandering garter (Thamnophis elegans), western plains garter (Thamnophis radix), and checkered garter (Thamnophis marcianus) snake (Holpp 1977; Curtis and Beierman 1980; LaGrange 1997). Prairie rattlesnakes (Crotalus viridis), western diamondbacks (Crotalus atrox), and, in some areas, massasaugas (Sistrurus catenatus) also occur in the watershed and dry playas. Plains hognose snake (Heterodon nasicus), bull snake (Pituophis melanoleucas), and western smooth green snake (Liochlorophis vernalis) are commonly observed in the Southern Great Plains (Curtis and Beierman 1980; Haukos and Smith 1994a). The glossy snake (Arizona elegans) and common kingsnake (Lampropeltis getulus) are also known to inhabit playas (M. Fritz, personal communication, Nebraska Game and Parks Commission). The Texas horned lizard (Phrynosoma cornutum), short-horned lizard (Phrynosoma douglassii), and collared lizard (Crotaphytis collaris) are some of the more common lizards occurring in dry playas or their watersheds. The six-lined race runner (Cnemidophorus sexlineatus) and northern prairie lizard (Sceloporus undulatus) also likely forage in dry basins and their watersheds (M. Fritz, personal communication, Nebraska Game and Parks Commission). While the above-mentioned species and many other snakes and lizards doubtlessly occur in dry playas

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and associated watersheds, their occurrence has not been directly tied to the existence of the playa wetland. The ornate box turtle (Terrapene ornata) is common along playa margins. There are also occasional records of snapping (Chelydra serpentina) and painted turtles (Chrysemys picta) in playas. These occurrences probably result from release by humans into modified and urban playas. There are no published studies on the ecology of reptiles in dry or wet playas. BIRDS

More studies have been conducted on birds inhabiting playas than any other Great Plains faunal group. The majority of these studies, however, have been conducted in the Southern High Plains and the Rainwater Basin. Moreover, most of the research conducted on playa birds has focused on waterfowl and gallinaceous birds. Because of their importance to hunters and the agencies in charge of providing hunter opportunity and monitoring harvest (e.g., state fish and wildlife agencies, U.S. Fish and Wildlife Service), historically more funds have been made available to study these groups of birds than other groups such as passerines or shorebirds. Moreover, similar to invertebrates, the avifauna of playas has primarily been studied from an aquatic perspective. Knowledge of bird use of dry playas is limited, although use can be substantial (Smith and Haukos 1995). As should be expected, the greatest diversity and abundance of birds in Great Plains playas generally occur during spring and fall migration. This is because of the sheer number of species passing through the area, the seasonal nature of water and food availability in playas, and the importance of these wetlands in a semiarid landscape in which few other wetlands are available. The location and size of the geographic area containing playas dictates its importance to the vast majority of migratory birds in the midsection of North America. In the 17 counties of the Rainwater Basin region alone, 257 species have been documented, 176 of which use playa basins. The vast majority are migrant rather than resident birds (Gersib et al. 1990a; LaGrange 1997). For the Southern High Plains, Simpson and Bolen (1981) noted that 108 nonwaterfowl species could be found in playa basins, 63 of which were spring and fall migrants. Fischer et al. (1982) also counted nonwaterfowl species in 100 playas in the central Southern High Plains and found 116 species in 32 families, at least 90 of which were migrants. When waterfowl species and data from a local National Wildlife Refuge (Buffalo Lake) are considered, 185 species

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in 41 families could inhabit playas of the Southern High Plains (Haukos and Smith 1994a). Again the overwhelming majority of these are spring and fall migrants. Flowers (1996) found 168 species of birds associated with playas in Meade County, Kansas, most of which were present during migration. The numbers of bird species nesting in playas is less than during migration but still is impressive. Seyffert (2001, 3) listed 151 avian species nesting in the Panhandle of Texas. As noted for most other bird lists for the Great Plains region, the objective of his book was not to provide specific nesting habitat data, so it is understandably difficult sometimes to determine from Seyffert’s observations actual species nesting within playas. Moreover, playas can be dry for months, or even years, with typical upland species nesting in the basins. But then it rains, wetland-dependent species begin nesting in large numbers, and the entire avian community changes within a few days. This phoenixlike event can be seen in most prairie wetlands but is particularly striking in playas. Finally, the number of species using playas during winter is also less than during migration; but densities, especially for waterfowl, in wet playas can be substantial. Because most of the wet playas north of Texas freeze in late November or December and do not thaw until late February or March, their value to aquatic birds in winter is limited. However, this does not suggest that frozen playas with emergent vegetation or dry playas do not provide important winter habitat for other migrant and resident birds. Again, biologists simply have not studied playas from this perspective. These data are needed to fully appreciate the importance of playas from an annual and landscape perspective. Because birds have typically been studied in taxonomic groups, they are separated here by common groups and discussed as they proceed through the annual cycle. Although the terms “spring migration” and “fall migration” are used throughout this chapter from an avian annual cycle perspective, the actual migration dates may occur in human designated winter and summer seasons. SHOREBIRDS

migration Thirty species of migrating shorebirds have been documented using playas in the Southern High Plains of Texas (Davis and Smith 1998a), while Flowers (1996) found 26 species in Kansas

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playas (table 4.2). In the Rainwater Basin playas, 32 species have been recorded (Gersib et al. 1990a). In Texas the playas most commonly used by shorebirds lacked dense stands of emergent vegetation ( 25% wetland cover), and the edges were dominated by mudflats (Davis and Smith 1998a). Migrating shorebirds in these playas spent most of their time feeding (Davis and Smith 1998b). As has been demonstrated for migrating shorebirds in other Great Plains wetlands, such as in the glacier-formed Prairie Potholes Region, wetlands are key sites for feeding shorebirds during migration (Wishart and Sealy 1980; DeLeon and Smith 1999). Shorebirds that may migrate 12,000 kilometers (arctic breeding, South American wintering) must feed almost constantly in playas to regain and store nutrient reserves for continued migration and/or breeding needs (Davis and Smith 1998b). The diets of American avocets, long-billed dowitchers, least sandpipers, and western sandpipers (species spanning the range in body size and foraging guilds of most playa shorebirds) are dominated by invertebrates in the Southern High Plains (Baldassarre and Fischer 1984; Davis and Smith 1998a). However, seeds of wetland plants can also be important, composing 6 –25% of these shorebirds’ diets (Davis and Smith 1998a). Female migratory birds often feed on more protein-rich foods than males do to meet the demands of egg production (Blem 1990). Interestingly, unlike many other migratory birds such as waterfowl, diets of migrating shorebirds did not vary between the sexes during spring. The constraints imposed by variable prairie habitat conditions and a short migration period in spring may not allow female shorebirds to discriminate among food types at this stage of migration (Davis and Smith 2001). Diets of Southern High Plains shorebirds varied between spring and fall (Davis and Smith 1998a). The diets of all four species listed above were dominated by chironomids in spring, whereas in the fall diets were more diverse and not dominated by any one particular item. Most of these seasonal diet differences were related to food availability (Davis 1996). Sometimes early in spring migration, chironomids were the only macroinvertebrate species found in a playa, whereas in autumn many more invertebrate species were present. Furthermore, within the physical size constraints of individual shorebird species and the types of foraging they use (American avocets are scythers, while least sandpipers are pecking/probers), it appears shorebirds consume invertebrates relative to their availability (Davis and Smith 2001).

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Table 4.2 Shorebirds associated with playas at selected sites in the Great Plains during spring and fall migration

Species American avocet American woodcock Baird’s sandpiper Black-bellied plover Black-necked stilt Buff-breasted sandpiper Common snipe Dunlin Greater yellowlegs Hudsonian godwit Killdeer Least sandpiper Lesser golden-plover Lesser yellowlegs Long-billed curlew Long-billed dowitcher Marbled godwit Mountain plover Pectoral sandpiper Red knot Red-necked phalarope Ruddy turnstone Sanderling Semipalmated plover Semipalmated sandpiper Short-billed dowitcher Snowy plover Solitary sandpiper Spotted sandpiper Stilt sandpiper Western sandpiper Whimbrel White-rumped sandpiper Willet Wilson’s phalarope Upland sandpiper a

Davis and Smith 1998a. Flowers 1996. c Gersib et al. 1990a. b

Scientific name Recurvirostra americana Scolopax minor Calidris bairdii Pluvialis squatarola Himantopus mexicanus Tryngites subruficollis Gallinago gallinago Calidris alpina Tringa melanoleuca Limosa haemastica Charadrius vociferus Calidris minutilla Pluvialis dominica Tringa flavipes Numenius americanus Limnodromus scolopaceus Limosa fedoa Charadrius montanus Calidris melanotos Calidris canutus Phalaropus lobatus Arenaria interpres Calidris alba Charadrius semipalmatus Calidris pusilla Limnodromus griseus Charadrius alexandrinus Tringa solitaria Actitis macularia Calidris himantopus Calidris mauri Numenius phaeopus Calidris fuscicollis Catoptrophorus semipalmatus Phalaropus tricolor Bartramia longicauda

Southern High Plains Texas a

Meade County Kansas b

Rainwater Basin Nebraskac

x x x x

x x x x x

x x x x

x

x

x x x x x x x x x x x x x

x

x x x x x x x x x x x x

x x x x x x

x

x x x x x x x x x x x x

x x x x x x x x x x x x x x x x x x x x x x x x x x x x

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It is difficult to estimate numbers or percentages of a certain species population migrating through and using the playas. Because of the inaccuracies associated with knowing the actual continental population size of a species to begin with, estimating percentages of a population is problematic. Moreover, estimating “residency time”— the amount of time a particular shorebird uses a wetland before continuing migration—in particular playas is also very difficult (Skagen and Knopf 1994). Knowledge of residency time is necessary to determine how many birds are passing through the area. Further, playas are mostly scattered small wetlands with great regional variation in water availability. Although shorebird densities in an individual playa may be very high, extrapolating that density over a particular region is difficult unless you know the number of wet playas and the range in shorebird densities over a sample of those playas. This type of problem does not exist in large, more continuous tracts of wetlands such as Cheyenne Bottoms in central Kansas (Helmers 1991) or Delaware Bay (Clark et al. 1993) where several hundred thousand birds have been estimated. In the Great Plains, shorebirds may perceive the regional landscape of playas as if it is a large individual wetland (Farmer and Parent 1997). With these restrictions in mind, and the high density of playas in much of the region (as noted in Chapter 1), shorebird numbers have been extrapolated to the availability of wet playas, resulting in estimates of several million shorebirds using the Southern High Plains during migration (Davis and Smith 1998a). In the Southern High Plains, the migration of most shorebird species was much more protracted in summer/fall (5 –10 weeks) than spring (2 – 4 weeks) (Davis and Smith 1998a). This pattern likely exists for playas throughout the Great Plains. The longer duration of migration in the fall could be related to different migration chronologies of the different age/sex classes (Morrison 1984). In many sandpipers (Scolopacidae), adults migrate before juveniles, and adult males migrate after adult females. Shorter spring migration also may be related to the urgency of breeding, especially for arctic breeding species, which only have a narrow window in which to complete nesting (Paulson 1983; Smith et al. 1991; O’Reilly and Wingfield 1995). Although overall Southern High Plains shorebird abundance was higher in fall than spring, the spring migration was shorter and peak weekly abundances of shorebirds were much higher than in fall

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(Davis and Smith 1998a). This along with the relatively high numbers of species using the same habitat begs the question as to how these various shorebird species coexist (Davis and Smith 2001). Shorebirds have often been placed into different foraging guilds such as “small probers-gleaners,” which are represented by species like western and least sandpipers, and “gleaner-sweepers” like the American avocet (Helmers 1991). Among different foraging guilds, shorebirds typically used different habitats (e.g., mudflats vs. shallow water), separating out spatially (Davis and Smith 1998a). During spring, however, shorebirds within the same foraging guild used similar habitats but exhibited little temporal overlap in their weekly occurrence. The different species migrated at different times. Because peak weekly densities of shorebirds are high in spring and invertebrate biomass is less than in summer/fall, it may be beneficial, from a fitness standpoint, for shorebirds within the same foraging guild to migrate through the playas at different times (Davis and Smith 2001). Among other influences, factors such as migration distance, weather, foraging efficiency, and flight characteristics also affect timing of playa use in spring. Because Helmers (1991) and Skagen and Knopf (1994) witnessed similar separation of migration chronologies within the same foraging guilds in Kansas, it is likely this pattern is typical for shorebirds using playas throughout the Great Plains.

nesting There are several species of shorebirds breeding in the immediate vicinity of playas (fig. 4.3). In the Southern High Plains, American avocets, killdeer, black-necked stilts, and snowy plovers are most numerous although the stilts and snowy plovers become more rare to the north and east playa regions (Smith and Haukos 1995; Conway 2001). Most of these shorebirds nest along sparsely vegetated shorelines and avoid densely vegetated playas (Conway 2001). Conway (2001, 30) estimated that if hydrologic conditions were suitable, 258,000 avocet and 30,000 killdeer nests could be found in playas of the Southern High Plains. There are also a few records of spotted sandpiper and Wilson’s phalarope nesting in playas of the Great Plains (Flowers 1996; Seyffert 2001). Long-billed curlew and mountain plover nest regionally but are not necessarily directly associated with playas since they commonly nest above the immediate playa watershed in short-grass environments.

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Figure 4.3 Playas throughout the Great Plains serve as important migration habitat for more than 30 species of shorebirds. American avocets nest in and migrate through the playas. (Photo by W. Meinzer, courtesy of the U.S. Fish and Wildlife Service.)

winter Because most playas north of the Llano are frozen during winter, shorebird use is minimal there. In Southern High Plains playas there are generally only a few species seen in small numbers. Most winter much farther south. The one exception is the long-billed curlew, which consistently spends (roosting and feeding) the entire winter on playas of the Southern High Plains. Winter population estimates do not exist for the species here, but numbers can be substantial ( 2,000). WATERFOWL

migration Similar to shorebirds, the numbers and diversity of waterfowl using playas during migration is spectacular. At least 25 species of waterfowl use playas from Nebraska to Texas and New Mexico (table 4.3). During spring, in the Southern High Plains and other western playas, the dabbling ducks (Tribe Anatinae), lesser

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Table 4.3 Waterfowl species observed migrating through the playa lakes Tribe, Common name Cygnini Trumpeter swan Tundra swan Anserini Canada goose Greater white-fronted goose Ross’ goose Snow goose Cairinini Wood duck Anatini Mallard Gadwall Northern pintail Green-winged teal Blue-winged teal Cinnamon teal American wigeon Northern shoveler Aythyini Redhead Ring-necked duck Canvasback Greater scaup Lesser scaup Mergini Hooded merganser Common merganser Common goldeneye Bufflehead Oxyurini Ruddy duck Source: Curtis and Beierman 1980; Gersib et al. 1990a; Flowers 1996.

Scientific name

Cygnus buccinator Cygnus columbianus Branta canadensis Anser albifrons Chen rossii Chen caerulescens Aix sponsa Anas platyrhynchos Anas strepera Anas acuta Anas crecca Anas discors Anas cyanoptera Anas americana Anas clypeata Aythya americana Aythya collaris Aythya valisineria Athya marila Aythya affinis Lophodytes cucullatus Mergus merganser Bucephala clangula Bucephala albeola Oxyura jamaicensis

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Figure 4.4 Generalized shape of migration corridor for many waterfowl species in the Central Flyway during spring. This illustrates the importance of the Rainwater Basin region of Nebraska. (Figure courtesy of the Nebraska Game and Parks Commission.)

snow geese, and Canada geese are most common. In the Rainwater Basin white-fronted geese should also be added to the list (Krapu et al. 1995). Indeed, Rainwater Basin playas are believed to be critical spring migration habitat for waterfowl in the Central Flyway that have wintered in Texas and Mexico and are returning to the northern prairies and Arctic to nest (fig. 4.4) (Gersib et al. 1989). However, concurrent with the high loss of playas in the Rainwater Basin and the high concentration of waterfowl, there have been the annual outbreaks of avian cholera during spring. Although this disease primarily affects waterfowl during winter in the Southern High Plains, it is most destructive during spring migration in the Rain-

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water Basin. Several hundred thousand ducks and geese have died in Nebraska from the disease since 1975, the first year in which cholera was reported there (Stutheit 1988). Little is known about the factors associated with avian cholera outbreaks even though it has been a priority research and fiscal focus of the Department of Interior for the past 20 years (Windingstad et al. 1984). Smith and Higgins (1990) found that cholera epizootics were inversely related to semipermanent wetland densities in the Rainwater Basin. Smith et al. (1990) also noted that there was a positive relationship between snow amounts in the 60-day period before midMarch and the number of waterfowl dying from avian cholera. They hypothesized that the greater amounts of snow made grain less available to field-feeding waterfowl, which in turn caused more stress on the birds. Further, Smith et al. (1990) suggested these weather conditions may increase mold and mycotoxins on the grain that is fed upon by waterfowl, thus weakening their immune system and predisposing them to avian cholera. Migration of waterfowl through the Southern High Plains in spring begins in mid-February and continues through May. Canada geese, snow geese, northern pintails, and mallards are the first to migrate (many of which have also wintered in the region), while in late May blue-winged teal, gadwall, and northern shoveler are some of the last to continue north (fig. 4.5). Geese begin arriving in the Rainwater Basin in late February, often leaving by mid- to late March or early April, depending on weather conditions. Most ducks come a bit later roughly following the same chronology as those in the Southern High Plains (Gersib et al. 1990a). As with shorebirds, the autumn migration of waterfowl is more extended and generally a reversal of the spring. Of course, this is all mediated not only by immediate seasonal weather but by past weather conditions. Past weather influences future water availability in the wetlands as well as plant and invertebrate foods available to waterfowl. Agricultural crops in these migration stopovers also affect the species and numbers of waterfowl using a particular area. Further, the occurrence and abundance of certain waterfowl species may affect the occurrence and abundance of others. With the rapid rise in the lesser snow goose population in North America over the last few decades (Batt 1997), the numbers migrating through the playas also have greatly increased. In 1974 only 15,000 snow geese were counted during spring migration in the Rainwater Basin, but by 1989 there were more than 450,000 (Gersib et al. 1990a);

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Figure 4.5 Dabbling ducks such as these northern pintails and green-winged teal reach tremendous densities on playas during spring and fall migration. (Photo courtesy of J. Steiert.)

and in 2001 well over a million were counted (T. LaGrange, personal communication, Nebraska Game and Parks Commission). It has been hypothesized that in the Rainwater Basin, the mere presence of large numbers of snow geese, directly and indirectly, precludes use of wetlands and agricultural fields by other waterfowl (Smith 1998). The same problems with estimating abundance of shorebirds migrating through the playas exists for waterfowl. Although biologists have a better handle on the continental population size of most waterfowl species (Strickland et al. 1994) than that of shorebirds, data concerning waterfowl “residency time” in different playa regions are lacking so that abundance of a species present over a particular time is extremely difficult to determine. Regardless, biologists know that millions, if not tens of millions, of waterfowl use the playas during spring and fall (USFWS 1988; Gersib et al. 1990a). Benning (1987) estimated that 90% of the midcontinent white-fronted geese used the playas of the Rainwater Basin, while the U.S. Fish and Wildlife Service and the Nebraska Game and Parks Commission (1986) estimated that 50% of the midcontinent population of mallards and 30% of the continental population of northern pintails used this area during

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spring. No such published estimates exist for other playa regions in the Great Plains. Similar to shorebirds, waterfowl spend a large proportion of their time feeding during spring migration (Gersib et al. 1990b). However, waterfowl also are engaged in courtship activities and generally spend more time resting. Also, unlike shorebirds and ducks, the geese feed almost exclusively in the surrounding agricultural fields rather than in playas (Krapu et al. 1995). Expansion of agricultural production, especially corn, throughout the Great Plains has been implicated in the rapidly increasing snow goose population (Alisauskas and Ankney 1992). Corn varieties are being planted farther north, and snow geese can now stage and store nutrient reserves closer to their arctic breeding grounds. Migrating ducks feed on seeds and invertebrates produced in the playas as well as on agricultural grains. Female ducks generally spend more time feeding than males during spring migration (Krapu and Swanson 1977; LaGrange and Dinsmore 1989). Ducks are actively courting and/or have formed pair bonds during this period, requiring the males to spend more time defending their mates. This allows females to acquire specific nutrients for egg formation. Nutrients stored by waterfowl feeding in wetlands during migration are important to their subsequent breeding success (Krapu and Reinecke 1992).

nesting Waterfowl production in the various Great Plains playa regions can be substantial. In the traditionally defined five-state playa region, it was hypothesized that up to a quarter million waterfowl could be produced in “wet” years (USFWS 1988). Production is likely closer to 50,000 –100,000 in average precipitation years. Historically, waterfowl production also was substantial in the Rainwater Basin playas (Evans and Wolfe 1967), but due to wetland loss and degradation, and conversion of grassland to cropland, production has been estimated at 10,000 ducklings in a normal year (U.S. Fish and Wildlife Service and Nebraska Game and Parks Commission 1986). In the playas, particularly in the Southern High Plains, the nesting period of waterfowl can extend from April through August. This extended breeding season makes it difficult to estimate waterfowl production, as typically accomplished in more northern areas. In those annually sampled areas, waterfowl production is determined with

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two aerial surveys, one in May, the other in July (Strickland et al. 1994). Pair data from May is incorporated with July brood data to estimate production. Using this approach, ducklings produced in the Southern High Plains might be grossly underestimated. Class I ducklings (no fully quilled feathers present) can be seen from April through September. If a playa receives substantial enough rain to fill in July, a duck pair will likely be seen courting there within a few days. It might be difficult to even distinguish the male from the female because the male has often molted from full breeding plumage. In the western Great Plains, the most common waterfowl species nesting in or immediately adjacent to playas are mallard, blue-winged teal, and cinnamon teal; but redhead, ruddy duck, northern pintail, northern shoveler, green-winged teal, canvasback, and gadwall nests also can be found (Traweek 1978; Rhodes 1978, 1979; Smith and Haukos 1995; Flowers 1996; Seyffert 2001). In the Rainwater Basin playas, the most common nesting species are mallard, blue-winged teal, American wigeon, gadwall, and northern shoveler (Gersib et al. 1990a). The dabbling ducks generally nest in dense vegetation of dry playas or in the upland immediately surrounding the playa (Berthelsen et al. 1989; Smith and Haukos 1995), while diving ducks such as redhead and ruddy duck nest in vegetation heaped up in the water. Although not studied in playas, it is likely that ducklings and attending hens feed predominantly on aquatic invertebrates (Krapu and Swanson 1977). With the success of stocking programs of many state conservation agencies, giant Canada geese are breeding throughout the United States, occasionally nesting in the Rainwater Basin, and now also in some urban playas in the Southern High Plains. Breeding geese are not native to playas and, from a native species perspective, hopefully will not become widespread in rural playas. The frequent drying of playas might discourage geese from reproducing over large areas.

winter Commonly cited estimates of the numbers of waterfowl wintering in the Southern High Plains, the major wintering area in the Great Plains, range from 500,000 to 2.8 million ducks (all species) and 100,000 to 750,000 geese (mainly Canadas and snow geese) (e.g., Simpson and Bolen 1981; USFWS 1988). These counts were generally made from surveys termed the “December goose count” and the “midwinter waterfowl survey.” However, estimates

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generated from these surveys may not be particularly reliable because there was seldom consistency in the area covered by the surveys among years and because they lack a sound statistical sampling basis (Strickland et al. 1994). Historically some pilot biologists flew to major regional reservoirs, and their vicinities, and then counted the waterfowl. This could lead to biased results because in dry years, when playas have less water, many ducks congregate on reservoirs whereas in wet years when playas have more water, fewer ducks may have been counted. In reality, then, there may have been more ducks in the wet year when fewer were counted. Regardless of past survey shortcomings (transects are now used), the playas of the Southern High Plains can be the most important wintering area in the Central Flyway for many species of waterfowl, especially mallards, the most numerous duck species in North America (Bellrose 1980; Bergan and Smith 1993). The duck species cited as being most common in winter are the dabbling ducks, especially mallard, northern pintail, American wigeon, and green-winged teal although northern shoveler and gadwall can also be numerous (e.g., Simpson and Bolen 1981). Because most playas are shallow palustrine basins, fewer diving ducks winter in the region. But where suitable habitat exists, ring-necked duck, lesser scaup, redhead, and canvasback can be locally abundant (table 4.2). Dabbling ducks use playas for feeding, roosting, and courting (Quinlan and Baldassarre 1984; Lee 1985; Sheeley 1988; Bergan 1990). Many species form their annual pair bonds during winter (fig. 4.6) (Weller 1965). Most of the dabbling ducks that winter in playas are produced in the Prairie Potholes Region of North and South Dakota, Manitoba, Saskatchewan, and Alberta (Bellrose 1980), although some also come from the Arctic or are, as stated earlier, hatched locally. Rhodes et al. (1993, 1995) examined genetic characteristics of American wigeons and mallards throughout winter in the Southern High Plains. Although these species are often considered to come from one widespread breeding population, and thus represent one wintering population, genetic data indicate otherwise (Rhodes et al. 1991; Rhodes and Smith 1993; Rhodes et al. 1996). Wintering populations of wigeon and mallard in playas represent mixtures of genetically heterogeneous breeding populations. The hundreds of thousands of the small race of Canada geese (B. c. parvipes, B. c. hutchinsii; Bellrose 1980) that winter in playas are

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Figure 4.6 Many dabbling duck species like these mallards form pair bonds while wintering in playas. (Photo courtesy of J. Steiert.)

difficult to separate into populations from a genetic standpoint (Cathey 1997). At least three wintering populations of Canada geese have been defined as occurring in the Southern Great Plains (Arctic Goose Joint Venture 1991). The large-bodied group (Hi Line Population) breeds in Montana and southern Alberta (and is not at question) while the two small-bodied populations (Shortgrass Prairie, Tallgrass Prairie) breed in the Arctic from the Mackenzie River in the western Northwest Territories east to Baffin Island in Nunavut (Bellrose 1980). The Shortgrass Prairie Population is thought to nest primarily in the western portion of this region, while the Tallgrass Prairie Population nests in the eastern Arctic. Shortgrass birds are thought to winter primarily in the playas, while Tallgrass geese are considered to winter to the east of the playas and along the Texas Gulf Coast, even though marked birds breeding in the different areas occurred in both wintering areas (Cathey 1997). Cathey et al. (1998) examined genetic subdivision of Canada geese breeding from the Mackenzie River to Baffin Island. Although there were genetic differences between east-

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ern and western populations of small Canada geese nesting in the Arctic, there was no consistent basis for separating Shortgrass and Tallgrass Prairie Populations wintering in playas (Cathey 1997). Beginning in the late 1960s and early 1970s several studies were initiated to examine wintering and migrating dabbling ducks in the central Southern High Plains (e.g., Rollo and Bolen 1969; Soutierre et al. 1972). Studies examined many aspects of waterfowl ecology including, but not limited to, nutrient reserves in wintering mallards (Whyte and Bolen 1984a,b) and field feeding by dabbling ducks (Baldassarre et al. 1983; Baldassarre and Bolen 1984). These studies showed how dabbling duck nutrient reserves (lipids, proteins) fluctuated over the winter and how field feeding and grain availability varied throughout the winter. Other studies examined waterfowl behavior (Quinlan and Baldassarre 1984; Lee 1985). Following those studies, investigations were initiated on environmental influences on nutrient reserves and body condition in dabbling ducks as well as the relationship between condition and overwinter survival. Initially, northern pintail diet and body condition were examined (Sheeley and Smith 1989; Smith and Sheeley 1993a,b). One of the first findings was that the diet observed in pintails depended upon how birds were collected. Birds used in most wintering waterfowl diet studies were obtained from hunters (a logical choice to increase sample sizes and minimize field work) or using methods that hunters typically employ (e.g., use of decoys). Results from these studies emphasized the importance of agricultural grain, especially corn, in the dabbling duck diet (e.g., Sell 1979; Moore 1980). When northern pintails were collected after they were observed feeding, they had consumed much more nonagricultural food, such as invertebrates and annual seeds from wetland plants, than birds collected using typical hunting techniques (table 4.4). The relative importance of grain in waterfowl diets had been overstated. The reason this is important is that waterfowl diets dominated solely by agricultural grains are thought to be nutritionally inferior to those containing a variety of wetland-produced foods (Haukos and Smith 1995). This can affect body condition, which is assumed to be related to survival. In wet years, more wetlands, and therefore more wetland food, should be available to ducks, thus improving their condition. Not only might this improved condition be related to the quality of food consumed (wetland foods vs. agricultural grains) but also to food availability. Greater food availability decreases energetic costs

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Table 4.4 Esophageal foods from hunter-killed northern pintails and those ducks collected (observed) while feeding in playas Aggregate % dry mass Hunter-killed (n  21)

Observed (n  26)

Corn Green matter Barnyard grass (Echinochloa crusgalli) seeds Smartweed (Polygonum spp.) seeds Dock (Rumex spp.) seeds

90.05* 4.11 0.25* 0.01* Tr a*

27.04 2.89 10.39 10.99

Little barley (Hordeum pussilum) seeds Pigweed (Amaranthus spp.) seeds Spikerush (Eleocharis spp.) seeds Goosefoot (Chenopodium spp.) seeds Other seeds Nonagricultural seeds Plant total Gastropoda Diptera Coleoptera Other animal Animal total

Tr* Tr* 0.01* Tr* 0.69* 0.96* 95.12* 0.09 4.78 Tr* 0.01 4.88*

Food

9.75 4.86 2.29 1.85 0.67 8.76 49.57 79.50 12.25 6.20 0.26 1.79 20.50

Source: Sheeley and Smith 1989; courtesy of Journal of Wildlife Management. a Trace ≤ 0.01%. *P  0.05; hunter-killed differs from observed.

of finding and consuming foods, thus improving body condition. In the pintail study, data indicated that in the wet year body condition was better, pintails did not field feed until late winter, they molted earlier, and formed pair bonds earlier than in the dry year (Smith and Sheeley 1993a,b). Because the environmental conditions (e.g., increased wetland availability) associated with wet and dry years were not experimentally manipulated, one could not be certain that the population attributes observed were not due to chance alone. But with similar conclusions reached elsewhere for waterfowl in wet versus dry years (e.g., in Miller 1986 for the Central Valley of California), it did lend confidence to conclusions and pointed to the need for future studies.

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Other studies were suggesting that as more winter habitat was available in wet years, more ducks were surviving the winter (Heitmeyer and Fredrickson 1981; Kaminski and Gluesing 1987; Raveling and Heitmeyer 1989). Bergan and Smith (1993) followed the pintail study with a radio telemetry investigation examining survival of female mallards during winter (November through February). Average survival over the winter period was about 78%. Precipitation, and therefore wetland availability, varied by year. As with pintails, body condition also varied by year with hen mallards in the poorest condition occurring during the driest year. And, as hypothesized, females in poorest body condition also had the highest mortality rates similar to black ducks (Anas rubripes) (Conroy et al. 1989) and canvasbacks (Haramis et al. 1986) in the eastern United States. Moreover, winter survival of mallards in playas was higher than that observed in the lower Mississippi Valley (94% vs. 82%) when playa data were restricted to the 70-day period examined in the Mississippi Valley (Reinecke et al. 1987). Factors associated with causes of mortality were quite different in Mississippi and western Texas. Hunting caused less mortality than did natural causes (e.g., predation, disease) in the Southern High Plains, whereas the reverse was true in the Mississippi Valley. There was little hunting pressure in the Southern High Plains and natural mortality was highest after the hunting season, in late January, which coincided with times of severe weather and avian cholera outbreaks. The next step was to examine why wet years produced conditions that had higher waterfowl condition and survival. Initially plant community response to wet- and normal-year precipitation was examined. By applying water in spring and midsummer, production of annual seed-producing plants was stimulated (Haukos and Smith 1993b). Those playas that were managed had production almost 10 times that of unmanaged playas. Waterfowl appeared to respond to the increased food availability, but daytime counts were confounded by differing availabilities of water among playas. Previous studies also showed that waterfowl spent less than 10% of their feeding time during the winter daylight hours (Quinlan and Baldassarre 1984; Lee 1985). Since feeding rates by waterfowl appeared relatively low in those previous studies and waterfowl count data were equivocal, other factors were examined that possibly influenced body condition and waterfowl use of playas with abundant seed foods. The next study examined numbers of waterbirds in playas that had

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variable levels of seed foods but similar water levels (Anderson and Smith 1999). It has generally been assumed that seed production was primarily driving the diet and use of wetlands by wintering dabbling ducks even though invertebrates are important components of wintering waterfowl diets (Fredrickson and Taylor 1982; Reid et al. 1989; Haukos and Smith 1993b). However, Anderson and Smith (1999) found there was no difference in daytime duck densities between playas with abundant seeds and those with little seed. Because the water during those surveys was murky and contained numerous molted feathers, it was thought that waterfowl might be using playas at other times. With the aid of a night-vision scope it became apparent that primary use of managed playas by dabbling ducks was occurring at night (Anderson and Smith 1999). Green-winged teal densities were 84 times higher at night than during the day. The majority of ducks in playas with dense annual seed-producing plants were feeding throughout the night. Ducks were using other wetlands during daylight hours for activities such as resting and/or courtship (Quinlan and Baldassarre 1984; Lee 1985). Diurnal sampling of waterfowl had been biasing perceptions of waterfowl ecology and use of playas (Tamisier 1976; Bergan et al. 1989; McNeil et al. 1992; Henson and Cooper 1994). Although waterfowl were selecting playas dominated by annual vegetation to feed in at night, it was unknown whether seed availability was influencing selection of managed playas. Subsequently, Anderson et al. (2000) found that green-winged teal were preferentially selecting invertebrates even though seeds were up to 10 times more abundant than invertebrates. Dabbling ducks, such as greenwinged teal, are undergoing prealternate molt during winter. They likely must meet the increased nutritional demands of molt not endogenously by using body reserves but exogenously through feeding preferentially on invertebrates. Indeed, in the year when aquatic invertebrates were more abundant, teal completed their molt earlier in the season than in the year when these foods were less abundant (Anderson et al. 2000). The reason ducks feed more at night than during the day in relatively more densely vegetated playas is a bit of a mystery. McNeil et al. (1992) proposed that waterfowl may feed at night more than during the day because night may provide the most profitable or safest opportunity to feed, or that nocturnal foraging occurs only when di-

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urnal feeding has not been sufficient to meet nutritional needs. As noted earlier, previous studies have shown that dabbling ducks in playas do not feed at high rates during the day. Therefore, the latter hypothesis does not hold in playas, and it is likely ducks feed more at night because obtaining food is more profitable or it is safer to forage at night (Anderson and Smith 1999). Availability of invertebrate prey, hunting pressure, and predation all could cause waterfowl to feed more at night (Tamisier 1976; McNeil et al. 1995; Anderson and Smith 1999). Hunting pressure on ducks in the Southern High Plains is low (Carney et al. 1983). However, it is possible that this is a relict behavior left over from hunting pressure experienced farther north within the year or even from previous generations. Invertebrates also could be more available to foraging waterfowl at night by moving higher in the water column (McNeil et al. 1995). But in the playas the water was only 10 –20 centimeters (4 – 8 in.) deep, making the entire water column available to foraging ducks (Anderson and Smith 1999). However, there is a relatively high potential density of avian predators occurring around playas (see next section). Most of these are diurnal predators. Of the three owls, the great horned (Bubo virginianus) is the only species capable of preying on waterfowl at night. The high abundance of raptors may influence ducks to choose more open playas during the day (J. Anderson 1997). Northern harriers (Circus cyaneus) and other diurnal raptors are consistently observed making predation attempts on waterfowl in playas. Ducks can detect avian predators during the day in relatively open playas but not in densely vegetated playas (Anderson and Smith 1999). OTHER AVIAN TAXA

migration Although sandhill cranes (Grus canadensis) use salt lakes in the Southern High Plains and riverine habitat in Nebraska for roosting, several hundred thousand use playas during migration for feeding, roosting, and a source of fresh water (Iverson et al. 1985). Endangered whooping cranes (Grus americana) also use the Central Table and Rainwater Basin playas of Nebraska during spring after returning north from coastal Texas (LaGrange 1997). They are only seen infrequently in the playas of the Southern High Plains. It is much more difficult to describe use of playas by other birds

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such as songbirds or raptors during migration. Few studies have been conducted on use of playas by these other birds, but also species lists for the various playa regions often do not separate migratory species versus residents or whether they are occurring in the vicinity of a playa versus the actual basin (Gersib et al. 1990a; Haukos and Smith 1994a). This lack of knowledge does not make their migrations through the playas any less spectacular than the more well-studied species. For example, several hundred bank swallows (Riparia riparia) may feed on insects emerging from a single Nebraska playa, while dozens of Swainson’s hawks (Buteo swainsoni) may sit in the wheat stubble surrounding a single Texas playa. To fully understand the importance of playas to all birds, and the avian role in the trophic structure of playa communities, future studies should examine use of playas during migration by these “terrestrial” species. Many species of concern also use playas during migration, including the whooping crane, the piping plover (Charadrius melodus), bald eagle (Haliaeetus leucocephalus), mountain plover, interior least tern (Sterna antillarum), and peregrine falcon (Falco peregrinus).

nesting Besides shorebirds and waterfowl, other aquatic birds nesting in wet playas include American coot (Fulica americana), pied-billed grebe (Podilymbus podiceps), eared grebe (Podiceps nigricallis), and white-faced ibis (Plegadis chihi) (fig. 4.7) (Smith and Haukos 1995; Flowers 1996; Seyffert 2001). These species generally nest on floating mats of vegetation or in emergent vegetation on the water’s surface. Aside from the American coot, it is likely that four other species of rail nest in playas—the black (Laterallus jamaicensus), king (Rallus elegans), sora (Porzana carolina), and Virginia (Rallus limicola)—although nesting records are scarce (Flowers 1996; Seyffert 2001). Least (Ixobrychus exilus) and American (Botaurus lentiginosus) bitterns also likely nest in emergent vegetation of playas, especially in the more northern areas. With playa modifications, and the associated increased prevalence of trees and emergent vegetation (as noted in Chapter 3), there has been an increase in the occurrence of rookeries. Black-crowned night heron and snowy (Egretta thula) and cattle (Bubulcus ibis) egrets now nest in playas. Occasionally yellow-crowned night heron (Nycticorax

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Figure 4.7 Grasshopper sparrows (top) and mallards (middle) commonly nest in dry playa basins and their watersheds while American coots (bottom) nest in wet playas. (Photos by author.)

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violaceus) are found nesting in trees associated with playas. Nightherons and egrets are commonly observed feeding on the invertebrates and amphibians occurring in playas. The occurrence of trees in these wetlands also has allowed nesting by songbird species not historically breeding in playas, or, for that matter, in the prairie environment throughout the Great Plains. Some of these bird species are exotic not only to wetlands and prairies but also to the North American continent. In the Southern High Plains, scissor-tailed flycatcher (Muscivora forficata), ash-throated flycatcher (Myiarchus cinerascens), western kingbird (Tyrannus verticalus), and house sparrow (Passer domesticus) have nested in trees occurring in playa basins (Smith and Haukos 1995). In trees, and on the ground, mourning dove (Zenaida macroura) nest densities have been reported as high as 8 nests/hectare (3 nests/ac) (Nelson et al. 1983). Numerous other species also likely nest in the exotic trees associated with playas (Nelson et al. 1983; Gersib et al. 1990a; Flowers 1996). If trees continue to expand into the prairies, and their associated wetlands, these numbers will increase and grassland species numbers will likely decrease. Depending on the structure of the herbaceous vegetation, the avian community nesting in dry playas varies. Those playas that had been moist the previous autumn or early that spring may be dominated by dense annual or short-lived perennial vegetation. Redwinged blackbirds (Agelaius phoeniceus) prefer this type of vegetative community, and select curly dock as their primary nest substrate in the Southern High Plains (Simpson and Bolen 1981). Nest densities in curly dock can often exceed 13 nests/hectare (5 nests/ac) (Smith and Haukos 1995). Yellow-headed blackbirds (Xanthocephalus xanthocephalus) also nest in playas with dense emergent vegetation (Fischer and Bolen 1981; Flowers 1996). Although some confusion exists over their taxonomic classification, eastern (Sturnella magna) and western (Sturnella neglecta) meadowlarks nest in dry playa basins, especially those that have stands of midheight grasses such as western wheatgrass and knotgrass (Smith and Haukos 1995; Flowers 1996). Northern bobwhite (Colinus virginianus) nest in playas with midheight grasses (Smith and Haukos 1995), and it is likely that Cassin’s sparrow (Aimophila cassinii) and grasshopper sparrow (Ammodramus savannarum) nest in the shorter to midheight grasses in the Southern High Plains (Berthelsen and Smith 1995). Other ground-

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nesting songbirds nest in playas, but no studies have investigated their ecology. In the late 1970s and early 1980s considerable research effort was expended on the importance of Southern High Plains playas to ringnecked pheasant (Phasianus colchicus). This research was spurred by the economic value of this exotic to individual landowners and regional economies (Bolen and Guthery 1982). Prior to the Conservation Reserve Program (see Chapter 1), playas provided most of the nesting habitat for all birds, including pheasants, even though playas occupy only 2% of the Southern High Plains landscape (Haukos and Smith 1994a). Nest densities of pheasants in playas was high, averaging 2.2 nests/hectare (almost 1 nest/ac) (Taylor 1980). However, because rainfall generally peaks in May, when pheasant and most other nesting birds are incubating eggs in dry playas, these nests are subject to loss from flooding. Predation on nests in playas also can be high in this restricted habitat surrounded by agricultural fields. With the advent of the Conservation Reserve Program and the planting of large areas of grasses, pheasant nest densities exceeding those in playas were recorded: 5 nests/hectare (2 nests/ac) (Berthelsen et al. 1990). The taller, denser grasses produced the highest pheasant nest densities. The presence of playas is key to the existence of pheasant populations in the Rainwater Basin, because there is little dense herbaceous upland habitat in this region.

winter Considerable research was also conducted on ring-necked pheasants in winter. Playas with dense emergent vegetation provide key wintering habitat for pheasants, mostly from a cover perspective and to a lesser extent from a food perspective (Whiteside and Guthery 1983). Winter pheasant densities in playas as high as 11/hectare (4.4/ac) have been recorded in the Southern High Plains (Guthery and Whiteside 1984). Native bird densities in frozen or dry playas also can be high, but actual estimates are limited (Smith and Haukos 1995). Barn (Tyto alba) and short-eared (Asio flammeus) owls often roost and feed in dry playas of the Southern High Plains. Their densities in emergent vegetated playas often exceed 1 owl/4 hectares (10 ac) (unpublished data). These owls are likely feeding on the abundant small mammals that exist in playa basins. Many songbird species also roost in the

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emergent vegetation of playas and feed on the seeds produced by annual plants. These playas may be dry or the seed that is being fed upon is protruding above the water’s surface. Hundreds of songbirds such as McCown’s longspur (Calcaius mccownii), Harris’s sparrow (Zonotrichia querula), and lark bunting (Calamospiza melanocorys) can be found feeding and/or roosting in a single playa (Smith and Haukos 1995). Many different raptor species (other than the two owl species mentioned above) prey upon the more than 30 species of birds and other fauna attracted to playas during winter. The raptors include prairie falcon (Falco mexicanus), peregrine falcon, bald eagle, golden eagle (Aquila chrysaetos), great-horned owl, Swainson’s hawk, roughlegged hawk (Buteo lagopus), red-tailed hawk (Buteo jamaicensis), ferriginous hawk (Buteo regalis), Cooper’s hawk (Accipiter cooperii), and, the most common, the northern harrier (Nelson et al. 1983; Smith and Haukos 1995). Several of these raptors exist around playas because of the presence of trees and fenceposts used as perch sites. Densities of raptors can often be relatively high. Curtis and Beierman (1980) counted 58 bald and golden eagles on a single playa. Nine-tenths of the midwinter population (about 400,000) of sandhill cranes winters in the Southern High Plains (Iverson et al. 1985). Although their focal roost points are in several salt lakes, a majority of the cranes spend at least some time roosting, feeding, and/or drinking water in playas. Indeed, some playas serve as major crane roosts with tens of thousands of birds. M AMM ALS

Throughout most of the playas there are no true aquatic mammals, although there can be muskrat (Ondatra zibethica) in modified Rainwater Basin playas. The lack of aquatic mammals in most present-day playas is related to their erratic and undependable hydroperiod. However, that fact aside, the mammals associated with the margins of wet playas or dry playas can be quite diverse and abundant (table 4.5). More species probably exist in the Rainwater Basin and Todd Valley playas, but lists have not been completed for those regions. With the extermination of the millions of bison and elk in the Great Plains, most mammals are now considered to be residents. However, although seldom considered, various migratory bat (Chiroptera) species feed on insects emerging from playas. Bats feed on emerging insects in playas from Nebraska to Texas, but

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unfortunately few studies have examined their composition and ecology relative to playas. Big brown (Eptesicus fuscus), red (Lasiurus borealis), and hoary (Lasiurus cinereus) bats are known to feed and water in Nebraska playas (M. Fritz, personal communication, Nebraska Game and Parks Commission). White-tailed deer are most common in playas in the northern and eastern portions of the playa range, but they are beginning to expand their range into playas farther west and south. Mule deer and pronghorn are found using playas in the western Great Plains where crop agriculture has not taken over vast tracts of uplands. Cropland agriculture has had a negative effect on pronghorn abundance and occurrence in playas (Leftwich and Simpson 1977, 1978). Not only do these ungulates use playas as a water source, they also forage and bed down in the basins. Recently exotic feral hogs have also been found in playas in the Southern High Plains. Their presence is not welcome because they destroy bird nests and root up native vegetation. Because playas often form the only habitat in such an intensive agricultural environment, some mammal populations reach relatively high densities in dry/moist playas. In the Southern High Plains, Scribner (1982, 11) found eastern cottontail densities in playas exceeding 20 individuals/hectare (8/ac). Moreover, due to the isolation of individual playas, population characteristics of eastern cottontails were playa specific (Scribner and Warren 1990). Desert cottontails also are present in Southern High Plains playas, but generally occur more often in playas in grassland settings with less dense annual vegetation than do eastern cottontails (Simpson and Bolen 1981). Native mouse and rat populations also can reach very high densities, but sadly specific studies on their ecology in playas are lacking. Scribner (1982) hypothesized that the cotton rat was the only species to exceed eastern cottontail populations in Southern High Plains playas. The existence of most mammalian predators in this highly agriculturalized environment is also tied to playas. Simpson and Bolen (1981) noted that, compared to the surrounding habitat, coyote, raccoon, striped skunk, and opossum preferred playa habitat. In the central Southern High Plains, Whiteside and Guthery (1980) found that 40% of the playas had coyotes. When one considers that the density of playas in that region can exceed 1 per 1.6 square kilometer (1/sq mi), densities are obviously high. The presence of coyotes in the Southern High Plains likely discourages the presence of red fox

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Table 4.5 Mammal species likely associated with playas in the Great Plains Common name Virginia opossum Short-tailed shrew Least shrew Desert shrew Eastern mole Desert cottontail Eastern cottontail Black-tailed jackrabbit White-tailed jackrabbit Thirteen-lined ground squirrel Franklin’s ground squirrel Black-tailed prairie dog Plains pocket gopher Jones’ pocket gopher Yellow-faced pocket gopher Plains pocket mouse Merriam’s pocket mouse Hispid pocket mouse Ord’s kangaroo rat Western harvest mouse Plains harvest mouse White-footed mouse Deer mouse Northern pygmy mouse Northern grasshopper mouse House mouse Meadow jumping mouse Hispid cotton rat Meadow vole Southern bog lemming Southern Plains woodrat Muskrat Porcupine Coyote Red fox Swift fox Gray fox Bobcat Raccoon

Scientific name

Source a

Didelphis virginiana Blarina brevicauda Cryptotis parva Notiosorex crawfordi Scalopus aquaticus Sylvilagus audubonii Sylvilagus floridanus Lepus californicus Lepus townsendii Spermophilus tridecemlineatus Spermophilus franklinii Cynomys ludovicianus Geomys bursarius Geomys knoxjonesi Cratogeomys castanops Perognathus flavescens Perognathus merriami Chaetodipus hispidus Dipodomys ordii Reithrodontomys megalotis Reithrodontomys montanus Peromyscus leucopus Peromyscus maniculatus Baiomys taylori Onychomys leucogaster Mus musculus Zapus hudsonius Sigmodon hispidus Microtus ochrogaster Synaptomys cooperi Neotoma micropus Ondatra zibethicus Erethizon dorsatum Canis latrans Vulpes fulvus Vulpes velox Urocyon cinereoargenteus Felis rufus Procyon lotor

A, C, E F A A F A, B, E A, B, E A, B, E F A, B, E F A, B, E A, B A, E A, E A, E A, E A, E A, E A A A A, C, D, E A A, D A, C, D F A, D F F A, B, E F A A, B, E F A, B, E A, B B A, B, E, F

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Table 4.5 (Continued ) Common name

Source a

Scientific name

Long-tailed weasel Mink Badger Striped skunk Pallid bat Big brown bat Red bat Hoary bat Feral hog Pronghorn Mule deer

Mustela frenata Mustela vison Taxidea taxus Mephitis mephitis Antrozous pallidus Eptesicus fuscus Lasiurus borealis Lasiurus cinereus Sus scrofa Antilocapra americana Odocoileus hemionus

A, C, D, F F A, B, E A, B, E B F F F G A, B B

White-tailed deer

Odocoileus virginianus

F

Source: Modified from Haukos and Smith 1994; courtesy of Landscape and Urban Planning, with permission from Elsevier Science. a A: Choate 1991. B: Curtis and Beierman 1980. C: Nelson et al. 1983. D: Guthery 1981. E: Simpson and Bolen 1981. F: data from Nebraska Game and Parks Commission. G: personal observation.

(Sovada et al. 1995). Because coyotes are not as detrimental to groundnesting birds as are red fox, the presence of coyotes likely positively influences the population dynamics of dabbling ducks and ringnecked pheasant. Red fox are more common, and coyotes less so, in central and eastern Nebraska playas. Raccoons are also common in playas. They especially prefer playas with dense emergent vegetation for denning (Juen 1981). Their occurrence is likely increased by the presence of playa modifications (such as pits), which increase the prevalence of water and denning structures. Many of the predator, rodent, and rabbit species inhabiting playas venture out of the basins for additional forage, but others can complete their entire life history in a single playa basin. Several endangered or threatened mammal species potentially occur in playas and their immediate watersheds. The recent (1998) petition by the National Wildlife Federation to the U.S. Fish and Wildlife Service to list black-tailed prairie dog as threatened or endangered has been particularly controversial throughout the Great

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Figure 4.8 Hispid pocket mice (top) and black-tailed prairie dogs (bottom) are common mammals in the playa watershed and in dry playas. (Photos courtesy of M. Wallace.)

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Plains (fig. 4.8). Black-tailed prairie dogs are common in playa watersheds, and in dry times dog towns expand into the basin. When playas flood from precipitation and their burrows flood, it is common to see prairie dogs in areas quite a distance from the nearest dog town. A declining species, swift fox are also found in playa watersheds and are often associated with prairie dog towns. Their populations have been declining throughout the Great Plains. Interestingly, one of the major causes of mortality documented thus far has been predation by coyotes. Numerous studies are now under way examining landscape and local biotic influences on swift fox populations. Possibly the most highly publicized endangered species potentially occurring in the Great Plains is the black-footed ferret (Mustela nigripes). These ferrets are closely associated with prairie dog towns, and Bolen et al. stated that “the possibility of its [black-footed ferret] existence in association with playa lakes remains real” (1979, 28). Choate (1991) noted that the last report of a black-footed ferret in the Southern High Plains was in 1963.

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CHAPTER 5

STRUCTURE, FUNCTION, AND DIVERSITY

T

he entire structure of a playa and its many associated functions can change within a few days. If a playa has been dry for more than a year, often only a thin layer of grass and spindly forbs remain in the basin. Then in early May thunderstorms can occur over the same playa and, more important, over its surrounding watershed, resulting in the basin being covered with a half meter (2 ft) of water. Within days there is an explosion of emerging aquatic invertebrates such as clam shrimp, obligate wetland plants begin germinating, toads and frogs emerge and begin calling in earnest, shorebirds arrive to feed on the invertebrates, and duck pairs start courting and will subsequently nest on the playa margin. These developments are typical in the life history of a playa and occur in thousands of playas each year. The climate of the Great Plains dictates this condition. Playas need this hydrologic (flooding/drying) disturbance to remain productive and be the key sites of biodiversity that they are. Playa fauna and flora evolved under these circumstances. Indeed, wetlands are wetlands because they dry out periodically. Among other things, this enhances decomposition and allows other plant and animal communities to emerge and/or recolonize and coexist with the species that required a more aquatic or a more terrestrial condition. The general public, many ecologists, and natural resource managers assume the aquatic condition is more beneficial probably because it appears to be more transient and full of life than the drier condition. However, this drying and flooding allows two vastly different communities and all intermediate permutations to exist on the same site increasing the diversity of the wetland. Further, among other ecosystem processes, the cycling of nutrients provided by the hydrologic disturbance is necessary for the various biotic communities to exist. Although it is unwise to generalize about playas, they will be char-

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acterized here in just a few different moisture phases: dry, moist, and inundated. These generalizations are necessary because so few basic studies on playa ecosystem properties have been conducted. Seasonal differences in structure and function will also be highlighted because ecosystem properties may vary substantially on this basis. Finally, some of the attributes associated with playas will be related to the biotic diversity among playas. WATER VARIABLES

However, before one considers these system properties, the water environment in which playas occasionally exist must be examined because water variables can have a large effect on the attendant biota. For example, J. Anderson (1997, 252) noted that most aquatic playa invertebrate taxa increased in abundance as dissolved oxygen, water depth, and temperature increased and as pH and electrical conductivity decreased. When playas are wet, the variation in water variables among playas is large and related to length of inundation. Because each playa exists within its own unique watershed (i.e., each watershed has been influenced differently, and has potentially different soils and vegetation), the amount and quality of the water entering each playa varies. General water variables in playas are considered here, and the “quality” and “contaminant” issues are considered later along with threats to playa ecosystems (see Chapter 7). As for other issues related to playa ecosystem properties, most data on water characteristics exist for the Southern High Plains, relative to that existing for the more northern playas. The first published limnological study on playa water characteristics in the Great Plains was for eastern New Mexico and adjacent western Texas by Sublette and Sublette (1967). Subsequent studies were conducted in West Texas by Parks (1975), Thompson (1985), Hall (1997), and A. Anderson (1997) among others. Although these studies report specific levels of chemical and physical water variables, seldom has the influence of water level fluctuations on these variables been studied. Moreover, because these studies span about 30 years, the techniques used to determine the amount, or even the units, of a particular water variable have changed, making comparisons among different studies difficult. Obviously one of the major influences on water variables is the surrounding watershed. Whether the watershed is cultivated or not is paramount. Cultivation will certainly affect many variables, espe-

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cially nutrient levels and water clarity. Not only will the watershed have a significant influence on the water variables, but the timing of sampling will also affect these observations. As evaporation and infiltration occur, the remaining water changes in character. Hall et al. (1999) summarized some of the general water data available for eastern New Mexico and western Texas playas (Sublette and Sublette 1967; Parks 1975; Hall 1997) (table 5.1). Playa water turbidity was highly variable, and the water could at times be very turbid. Readings with a Secchi disk (simply a round black-and-white visibility disk) of 2 centimeters ( 1 in.) were not uncommon (i.e., after a disk was submerged greater than 2 cm, a person could no longer see it). Hall et al. (1999) felt that the turbid conditions in playas were the result of organic and inorganic matter being held in suspension by persistent windy conditions. Similarly, Sublette and Sublette (1967) noted that the turbidity in a playa was related to the amount of vegetation in the basin—the more vegetation the less turbidity as a result of the vegetation mediating the effects of waves and wind. The high amounts of solids found in water of some of the playas for which Hall et al. (1999) summarized data (table 5.1) were believed to be related to erosion. It was not stated whether the sediments resulted from wind or water actions. However, from Luo et al.’s (1999) soil particle size analysis it is known that the sediments deposited in playas are primarily waterborne (eroded from the watershed) with wind deposition accounting for substantially smaller amounts. Hall et al. (1999) noted that most playas were “nutrient-rich,” thus fulfilling criteria to be classified as eutrophic. This likely depends on the time since the last runoff event and the length of time the playa has been inundated. Certainly this is true of most wetlands in the Plains. Being the endpoint of the watershed, they not only receive the sediments but substantial allochthonous inputs of nutrients as well. Because most playas are shallow and exposed to relatively high wind velocities, the dissolved oxygen ranges are similar for the surface water and the sediment-water interface (table 5.1). Hall et al. (1999) found that the majority of playas they studied in the Southern High Plains of Texas had chemical oxygen demands and biochemical oxygen demands (i.e., oxygen needs to carry out chemical and biochemical processes in wetlands) both greater than 8 mg/L dissolved oxygen. The 8 mg/L is the oxygen saturation level at the elevation of Lubbock, Texas (1,000 m; ⬃ 3,200 ft). Because biochemical and chemical oxygen demands typically exceeded saturation levels, anaerobic situations can occur; but, as noted above, winds and shallow water depths probably

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Table 5.1 Range of water variables found in playas of the Southern High Plains of Texas and New Mexico

Variable Physical properties

Nutrients

Cations, anions, and metals

Dissolved oxygen and oxygen demands

Measure Turbidity (NTU) Turbidity (ppm SiO2) Secchi depth (cm) Total solids (mg/L) Total suspended solids (mg/L) Total dissolved solids (mg/L) Total volatile suspended solids (mg/L) Specific conductance ( mhos) Hardness (mg/L) Hardness, total (mg/L CaCO3) Hardness, calcium (mg/L CaCO3) Alkalinity, methyl-orange (mg/L CaCO3) Total organic carbon (mg/L) Total inorganic carbon (mg/L) Total carbon (mg/L) Total Kjeldahl nitrogen (mg/L) Ammonia-nitrogen (mg/L) NO2/NO3-nitrogen (mg/L) Nitrate nitrogen (mg/L) Total phosphorus (mg/L) ortho-phosphate phosphorus (mg/L) Silica (mg/L) pH Calcium (mg/L) Magnesium (mg/L) Sodium (mg/L) Potassium (mg/L) Chloride (mg/L) Sulfate (mg/L) Arsenic ( g/L) Copper ( g/L) Dissolved oxygen, surface (% saturated) Dissolved oxygen, surface (mg/L) Dissolved oxygen, bottom (% saturated) Dissolved oxygen, bottom (mg/L) Biochemical oxygen demand (mg/L) Chemical oxygen demand (mg/L)

Range Minimum Maximum 20 45 2.0 195 10 120