Applications of Tree-ring Studies: Current Research in Dendrochronology and Related Subjects 9780860544289, 9781407346052

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Applications of Tree-ring Studies Current Research in Dendrochronology and Related Subjects

edited by

R. G. W. Ward

BAR International S-eries 333 1987

B.A.R. 5, Centremead, Osney Mead, Oxford OX2 ODQ, England.

GENERAL EDITORS A.R. Hands, B.Sc., M.A., D.Phil. D.R. Walker, M.A.

BAR -S333, 1987: 'Applications of Tree-ring Studies'

©

The Individual Authors,1987

The authors’ moral rights under the 1988 UK Copyright, Designs and Patents Act are hereby expressly asserted. All rights reserved. No part of this work may be copied, reproduced, stored, sold, distributed, scanned, saved in any form of digital format or transmitted in any form digitally, without the written permission of the Publisher. ISBN 9780860544289 paperback ISBN 9781407346052 e-book DOI https://doi.org/10.30861/9780860544289 A catalogue record for this book is available from the British Library This book is available at www.barpublishing.com

Contents

Page Nos.

Growth Rings in the Aerial and Root Xylem of some North Temperate Hardwoods

P. Gasson

The Palaeoclimatic Significance of Growth Rings in Late Jurassic/Early Cretaceous Fossil Wood from Southern England

J.E. Francis

21-36

The Contribution of Growth Ring Studies to the Reconstruction of Past Climates

G.T. Creber & W.G. Chaloner

37-67

Relationships between British Climate and the Radial Growth of Quercus Species an Empirical Approach to Climate Reconstruction

K.R. Briffa

69-90

Dendroclimatology of Pinus sylvestris L. in the Scottish Highlands

M.K. Hughes

91-106

· Dendroglaciological Investigations in Norway

1-19

J.L. Innes

107-120 -

Mediaeval Dendrochronology in Exeter and its Environs

J. Hillam & C.M. Mills

121-125

A 700 Year Dating Chronology for Northern France

J.R. Pilcher

127-139

Problems of Dating and Interpreting Results from Archaeological Timbers

J. Hillam

141-155

The Belfast CROS Program - Some Observations

J.R. Pilcher & M.G.L. Baillie

157-163

Sapwood Estimates and the Dating of Short Ring Sequences

J. Hillam, R.A. Morgan & I. Tyers

165-185

A Review of the Methodology for Calibrating Radiocarbon Dates into Historical Ages

T.C. Aitchison & 187-201 E.M. Scott

The Belfast 'Long Chronol_ogy' Project

M.G.L. Baillie & 203-214 J.R. Pilcher

Dendrochronological Studies of Bog Pine from the Rannoch Moor Area, Western Scotland

R.G.W. Ward, B.A. Haggart & M.C. Bridge

215-225

The Dendrochronological Study of Sub-Fossil Wood in New Zealand

M.C. Bridge

227-232

P reface I n M ay

1 984

a o ne d ay s ymposium

o n t he g eneral s ubject o f t ree-rings

w as h eld a t t he C ity o f L ondon P olytechnic .

T he p urpose o f t he m eeting w as

t o b ring t ogether p eople w orking o n s ome a spect o f t ree-rings, b e t hey i n a D epartment o f B otany ,

B iology ,

a nd

t he r ange o f d isciplines

s o o n .

I n

f act ,

w as w ider t han a nticipated,

F orestry ,

A rchaeology ,

E cology ,

M athematics

r epresented a t

t he m eeting

a t estimony t o t he g reat r ange o f a pplications

t hat h ave b een f ound f or t ree-ring s tudies . T his t he

c ollection o f a rticles

p apers

w ere

s pecifically

f or

p resented t his

a t

v olume .

r epresents t hose r eceived ,

i s t he

t he o utcome o f t hat m eeting : s ymposium

T he b alance o f

a nd

o thers

t hose

s ome o f

w ere

i ncluded

w ritten

h ere

s imply

a nd d oes n ot r eflect t he much g reater b alance o f

t he p apers p resented a t t he s ymposium, a t w hich s essions w ere d ivided f airly e qually a mongst t he b iologists, t he a rchaeologists, t he d endroclimatologists a nd t he s tatisticians . p roperties

o f

r ing-width

T he a bsence o f a rticles d escribing t he s eries

i n

t his v olume

i s

s tatistical

p articularly

r egretted .

N evertheless , h opefully t he p apers p resented h ere w ill c onvey s omething o f t he i ntention a nd s cope o f t he M ay m eeting , a nd d emonstrate o nce m ore t he p otential o f t ree-ring s tudies i n s o m any f ields . T he j ob o f e diting a c ollection s uch a s t his i s n ot e xactly p referable t o a h oliday c ruising r ound t he G reek I slands, ( or i ndeed, a nywhere e lse !) . H owever , i t h as b een m ade much e asier b y t he p atient c o-operation o f a ll t he c ontributors, t he r eviewers, t he p ublishers a nd t he c artographic s taff o f t he E nvironmental S tudies a nd G eography D epartment I nstitute . Ie xtend my t hanks t o a ll o f t hem .

o f t he R oehampton

R .G .W . W ard F eb.

1 987

G rowth R ings i n t he A erial a nd R oot X ylem o f S ome N orth T emperate H ardwoods P . G asson J odrell L aboratory R oyal B otanic G ardens K ew , R ichmond S urrey

A BSTRACT T he w oody p arts o f a t ree c an b e d ivided i nto a erial x ylem a nd r oot x ylem .

T runk a nd b ranch x ylem i n ag iven s pecies u sually h ave s imilar c ell

d istribution p atterns w hereas r oots a re o ften a natomically d ifferent .

I n

P edunculate O ak ( Quercus r obur L . ) t he a erial x ylem i s r ing p orous a nd t he r oots a re e ither r adial p orous o r d iffuse p orous . T he r oots o f a ll s pecies o f o ak u sually h ave x ylem w ith i ndistinct g rowth r ing b oundaries . I n c ontrast , i n E uropean B eech ( Fagus s ylvatica I . ) t he x ylem i n a ll p arts o f t he t ree i s d iffuse p orous o r s emi-ring p orous a nd h as w ell d efined g rowth r ing b oundaries . T he d ifferences b etween a erial a nd r oot x ylem i n o ak a nd b eech a re s hown w ith p articular r eference t o h ow w ell t he g rowth r ings a re d efined , a nd t wo o ther B ritish t ree s pecies w ith r ing p orous a erial x ylem, E uropean A sh ( Fraxinus e xcelsior

L .)

a nd

E nglish

E lm

( Ulmus p rocera

S alisb .)

a re

i llustrated f or c omparison . T he p oorly d efined g rowth r ings i n m any r oot s amples o f t hese a nd o ther s pecies ( particularly t hose w ith r ing p orous a erial x ylem) w ill o ften r esult i n d ifficulties w ith t heir d ating a nd a ging.

I NTRODUCTION •D endrochronology

r elies

o n

t he

r ecognition o f

a nnual

g rowth r ings

i n

w oods u nder i nvestigation . I n m ost n orth t emperate s pecies g rowth r ings a re r eadily i dentified i n a erial ( trunk a nd b ranch ) x ylem , b ut t hey m ay b e d ifficult o r i mpossible

t o d istinguish i n r oot x ylem o f

t he

s ame s pecies .

M ost t ree r ing s tudies a re c oncerned w ith a erial x ylem, a nd r oots a re u nlikely t o b e e ncountered e xcept i n b og d eposits o f n atural o rigin o r p erhaps a s f ragmentary m aterial f rom a rchaeological s ites . H owever , a k nowledge o f t he a natomical a ppearance o f t he r oots o f c ommon d endrochronological s ubjects i s c learly o f p ractical v alue w hen a ccurate i dentification o f s amples i s n ecessary , e ven i f t heir d ating a nd a ging i s n ot i ntended o r p ossible . T his p aper i llustrates t he a natomical d ifferences b etween a erial x ylem a nd r oot x ylem w ith r egard t o b oth v essel d istribution a nd t he o ccurrence, a ppearance a nd r elative d efinition o f g rowth r ings . T he s pecies s elected a re P edunculate O ak ( Quercus r obur L . ) , af requent s ubject o f d endrochronological i nvestigations, E uropean A sh ( Fraxinus e xcelsior L . ) a nd E nglish E lm ( Ulmus p rocera S alis W . W ith t he e xception o f b eech , i n a ll o f t hese s pecies t he r oot s tructure i s m arkedly d ifferent i n a ppearance f rom t hat o f t he a erial x ylem .

1

C HARACTERISTICS O F A ERIAL A ND R OOT X YLEM 1 : P edunculate O ak T he a erial x ylem a natomy o f t his a nd r elated o ak s pecies i s w ell k nown , a nd t here a re m any p ublished i llustrations a nd d escriptions o f " typical" t ransverse s ections o f t he w ood ( e .g . S chweingruber 1 978 ; B aillie 1 982) . T he a erial x ylem i s r ing p orous i .e . t he e arlywood v essels f ormed i n t he e arly p art o f t he g rowing s eason a re m uch w ider t han t he l atewood v essels i n a g iven g rowth r ing . T his a pplies t o a ll a erial x ylem f ormed f rom t he s econd y ear o nwards i n a s tem o r b ranch . H owever , i n t he f irst t wenty y ears o r m ore t he m ean d iameters o f t he e arlywood v essels i ncrease a nnually i n a l inear f ashion a nd t he s ize d ifference b etween e arlywood a nd l atewood v essels i s l ess m arked i n t he y oung t ree ( Gasson 1 984). G rowth

r ing

b oundaries

a re

w ell

d efined

i n t he a erial x ylem o f

t his

s pecies , a nd t he r ing p orous p attern o f v essel d istribution i s a pparent i n a ll b ut t he n arrowest r ings . T his p attern h as n ot d eveloped i n s ome r ings o f t he s ample s hown i n P late 1 :1, i n w hich t here h as b een a p eriod o f v ery s low r adial g rowth l asting f or a pproximately 8 y ears . T his s ample w ith u nusually n arrow g rowth r ings h as b een d eliberately c hosen t o s how a n e xtreme modification o f t he r ing p orous s tructure o f t he a erial x ylem . V ariation i n r ing w idth i n t his s pecies o f o ak i s c onsiderable , r anging f rom 2 00-8 ,000 u m ( 8 m m) i n m aterial e xamined b y t he a uthor ( Gasson 1 984) , w hilst i t h ardly e xceeds 4 mm i n t he M ediaeval a nd m odern t imbers d iscussed b y F letcher ( 1974) . I n t he p eriod o f v ery s low r adial g rowth s hown i n P late 1 :1, t he b oundaries o f t he g rowth r ings f ollow t he c ontours o f t he e arlywood v essels o f t he p revious r ing , a nd a ccurate r ing c ounting d ifficult .

t here i s v irtually n o l atewood , m aking A much l ess c ommon modification i n

a erial x ylem s tructure w as r eported b y F letcher ( 1975) , w ho f ound t hat 0 .4% o f 2 4 ,000 r ings h ad a bnormally n arrow e arlywood v essels a lthough t he g rowth r ings o f w hich

t hey w ere a p art w ere o f n ormal s ize .

T hese a bnormalities

were f ound i n w ood u sed f or p anel b oards b etween 1 450-1620 t o s upport p ortrait p aintings i n E ngland a nd F landers, a M ediaeval c upboard f rom O xford, a nd p arts o f a R oman q uay i n L ondon . F letcher s uggested t hat e xceptionally c old w inters a nd s prings c ould h ave c aused t his a bnormality b y i nterfering w ith c ambial a ctivity a nd c ell e nlargement d uring e arlywood f ormation . N o s uch a bnormalities w ere f ound i n m odern m aterial b y F letcher ( 1974, 1 975) o r b y t he p resent a uthor, e ven i n t he y ears f ollowing t he e xtreme w inters o f 1 946-7 a nd 1 962-63 . R oot s tructure d iffers i n a n umber o f r espects f rom t hat o f a erial x ylem . a re a ll

G rowth r ing b oundaries i n t he t hree e xamples s hown i n P lates 1 :2-4 l ess w ell d efined , a nd t he x ylem i s d iffuse p orous ( Plate 1 :2) o r

r adial p orous ( Plates 1 :3-4) . t han t hose o f a erial x ylem o f

I n m ost r oots t he v essels a re m uch w ider t he s ame c ambial a ge . I n P late 1 :2 w hich

s hows a r oot f rom a m ature t ree , w ith a c ambial a ge o f a bout 1 0 y ears t he v essels a re o f c omparable w idth t o t he e arlywood v essels o f m ost m ature a erial x ylem s amples, ( cf . p late 1 :1, w hich s hows a erial x ylem w ith a c ambial a ge o f a bout 1 00 y ears) . T he v essels a re c rowded a nd t he i ndistinct g rowth r ings a re n arrow . T he x ylem c an b e d escribed a s d iffuse p orous . I n P lates 1 :3 a nd 1 :4 a gain s howing r oots w ith a c ambial a ge o f a bout 1 0 y ears, t he v essels a re c onsiderably n arrower a nd a re a rranged i n u ndulating r adial l ines , h ence t he t erm r adial p orous . A v ery g radual r eduction i n v essel d iameter i s a pparent a cross t he g rowth r ings , w hich a re r easonably w ell d efined i n t he e xample i n P late 1 :3 , b ut i n 1 :4 , a t ap r oot, i t i s v irtually i mpossible t o d etect r ing b oundaries .

2

T he v essels i n t hese

t wo s amples m ay b e

n arrower t han t hose i n t he l ateral r oot s hown i n P late

1 :2 b ecause t hey a re f rom i mmature t rees . t rees h as c orresponding n arrower v essels .

T he a erial x ylem i n t hese y oung M any l ateral a nd t ap r oots h ave

l ess w ell d efined r ing b oundaries t han t hose s hown i n P lates 1 :2 a nd 1 :3 . T he a ppearance o f g rowth r ings a t r elatively l ow m agnifications h as b een d iscussed a bove . A t h igher magnification , i ndividual f ibres, a xial p arenchyma a nd r ay c ells a re m ore e asily d istinguished , a nd g rowth r ing b oundaries i n a erial x ylem a re l ess a pparent . P late 2 :4 s hows a r ing b oundary i n a erial x ylem ,

i n w hich a w ide e arlywood v essel o ccurs w ithin 3

c ells o f t he b oundary . T he l ast-formed l atewood c ells o f t he p receding g rowth r ing c onsist o f f ibres w hich h ave t he s ame t angential d iameter a s t he p revious a nd s ubsequent c ells i n t he s ame r adial f ile b ut h ave a r adially " flattened" a ppearance . I n t his e xample t he n arrow b and i s o nly 2 -3 c ells w ide, a nd c onsists o f n ormal f ibres, w hereas i n many a erial x ylem r ing b oundaries g elatinous f ibres m ay a lso b e p resent o r p redominant . I n r oots, r adial " flattening" o f f ibres i s l ess a pparent a t r ing b oundaries ,

a nd t he

l ack o f w ide v essels a t t he b eginning o f a y ear 's r adial i ncrement m eans t hat g rowth r ings a re l ess c learly d efined . G elatinous f ibres a re a bsent f rom m ost r oots. I n

s ummary ,

t he

g rowth

r ings

o f p edunculate o ak a re w ell

d efined

i n

a erial x ylem a nd l ess w ell d efined o r s ometimes i mpossible t o d istinguish i n r oots.

2 : E uropean B eech T his s pecies h as d iffuse p orous o r s emi-ring p orous x ylem t hroughout t he t ree . A t ypical s ection o f t runk x ylem i s s hown i n P late 2 :3 a t t he s ame m agnification a s P late 1 . T he v essels a re n arrower a nd m ore c rowded t han i n p edunculate o ak a nd g rowth r ings c an b e c learly d istinguished . P late 2 :1 s hows a n e nlarged p art o f P late 2 :3 i n w hich t he f ibres a t t he e nd o f t he r ing a re m ainly g elatinous. I n r oots ,

g rowth r ing b oundaries a re n ot q uite s o d istinct b ut t hey a re

g enerally much b etter d efined t han i n o ak ( Plate 2 :2) . A s w ith o ak , g elatinous f ibres w ere n ot o bserved i n r oots , a lthough t hey a re o f f requent o ccurrence i n a erial x ylem . T he v essels t end g radually t o d ecrease i n d iameter f rom e arlywood t o l atewood . H owever , v essel w idth i s n ot a s g ood a n i ndicator o f t he p osition o f r ing b oundaries a s i t i s i n t he a erial x ylem o f o ak . A b etter i ndicator i n b eech i s t hat r ays o ften b ulge t angentially a t r ing b oundaries , i . e . t hey a re n oded ( Plate 2 :2 a nd 2 :3) . I n s ummary ,

t he d ifferences b etween a erial a nd r oot x ylem i n b eech a re

r elatively s light a nd r ing b oundaries a re a lways c learly d iscernible . A ging a nd d ating o f b eech g rowth r ings i s t herefore u sually p ossible .

3 : E uropean A sh I n t his s pecies t he a erial x ylem i s r ing p orous a nd t he r oots a re m ore o r l ess d iffuse p orous . a re u sually d istinct, m agnification .

P lates 3 a nd 4 s how t hat g rowth r ing b oundaries a nd a s i n o ak t hey a re r eadily o bserved a t l ow

A s i n o ak a erial x ylem , n arrow g rowth r ings c onsist m ainly

o f e arlywood ( Plate 3 :2) . T he r ing b oundaries i n a erial x ylem a re c omposed o f f ibres a nd s ometimes v essels a nd a xial p arenchyma c ells , s o t hey a re m ore

3

c omplex t han t hose o f o ak . E ach b oundary i s u p t o a bout 4 c ells w ide ( radially ) a nd a s i n o ak a nd b eech t he c ells i n a g iven r adial f ile h ave s imilar t angential d iameters b ut t he b oundary l atewood c ells h ave s maller r adial d iameters g iving t hem a " flattened" a ppearance ( Plates 3 :1 a nd 3 :2) . O ccasionally a r adial p air o f v essels c onsists o f o ne l atewood v essel a nd o ne e arlywood v essel f rom t he f ollowing y ear . T his d oes n ot o ccur i n o ak w here t he b oundary c onsists o nly o f f ibres a nd r ay c ells . T he x ylem ,

f ibres

i n

r oots

a re

u sually

t hinner—walled

t han

t hose

o f

a erial

a nd i t i s u nclear i n t ransverse s ection w hether s ome c ells a re a xial

p arenchyma o r f ibres .

R ing b oundaries c an b e

s een i n a ll

t he

r oots

e xamined ( Plate 3 :3 a nd 3 :4, P late 4 ), b ut a t t he h igher magnification ( Plate 3 :4 i n p articular) t he c ells i n t he b oundaries a re n ot markedly d ifferent i n s hape f rom t hose f ormed p reviously a nd s ubsequently . T he r elationship b etween g rowth r ings a nd a nnual r adial g rowth i s n ot c lear i n t his s pecies w here s everal r oots ( Plate 4 ) h ave g rowth r ings w hich m erge o r d ivide . G rowth r ings t hroughout t he t ree a re u sually a ccentuated b y t he p resence o f e arlywood v essels v ery c lose t o t he b eginning o f ag iven r ing . T hese v essels a re u sually l arger t han l atewood v essels i n t he p revious r ing , b ut i n s ome r oots a ll v essels a re v ery n arrow a nd s parsely d istributed ( Plates 4 :1 a nd 4 :3). I n s ummary ,

g rowth

r ings

a re c learly d efined

i n b oth a erial a nd r oot

x ylem , b ut i n r oots t he c ells a t g rowth r ing b oundaries a re n ot m arkedly d ifferent f rom t hose i n t he r adially a djacen t l atewood a nd e arlywood , a nd g rowth r ings a re o ften d iscontinuous.

4 : E nglish E lm T he E nglish E lm i s o ne o f t hree B ritish e lm s pecies which a ll h ave s imilar x ylem a natomy . T he a erial x ylem i s r ing p orous d iffuse p orous . T he a ppearance o f t he a erial x ylem i s w ith g roups o f s mall l atewood v essels i n d iscontinuous w ith f ibre b locks ( Plate 5 :1) . H owever , t his p attern i s

a nd t he r oots a re v ery d istinctive , b ands a lternating l ess a pparent i n

r oots w here t he g rowth r ings a re l ess o bvious a nd t he l atewood v essels a re w ider t han t hose i n a erial x ylem o f c omparable c ambial a ge . T he g rowth r ing b oundaries o f r oots ( Plates 5 :2 a nd 5 :3) a re o ften p oorly d efined m aking i t d ifficult t o d istinguish b etween i ndividual g rowth r ings . P late 6 :1 s hows a r ing b oundary i n a erial x ylem i n w hich a " flattening" o f t he f ibres a t t he e nd o f t he l atewood i s n ot v ery m arked .

T he b oundary i s m ade

d istinct mainly b y t he p resence o f v ery w ide e arlywood v essels .

T he

v essels a t t he b eginning o f s ome g rowth r ings i n r oots a re o ften w ider t han t hose i n t he p revious l atewood w hich a ccentuates t he r ing b oundary , a nd i n P late 6 :2 t he b oundary i s v ery w ell d efined f or a r oot . A p robable r ing b oundary s hown i n P late 6 :3 i s a pparent o nly b ecause o f a c hange i n v essel d iameter. I n s ummary ,

g rowth r ings a re w ell d efined i n t he a erial x ylem a nd l ess

a pparent i n r oots.

D ISCUSSION T he a natomical d ifferences b etween a erial a nd r oot x ylem w ith r egard t o g rowth r ings a nd v essel d istribution a re t he t wo c haracters m ost u seful t o

4

d endrochronologists .

T hese a nd o ther c haracters l isted b y F ayle ( 1968) a nd

C utler ( 1976) i ndicate t hat t he a natomical d ifferences e xhibited b y a erial a nd r oot x ylem a re a r eflection o f t heir r espective e nvironments a nd p hysiological r oles . T he a erial x ylem s upports t he l eaves i n t he o ptimum p osition

f or

i ntercepting

l ight

a nd

s upplies

t hese

l eaves

w ith w ater a nd

n utrients f rom t he r oots . T he w hole a erial s ystem o f t he t ree i s s ubject t o v ariable w eather c onditions a nd f luctuations i n e nvironmental s timuli w hereas t he r oots a re p rotected a nd s upported b y t he s oil . T he r oots a ct a s a n a nchor a nd a s s torage o rgans a nd s upply w ater a nd n utrients f rom t he s oil t o t he a erial p arts o f t he t ree . R oots a re s ubject t o l ess e xtreme v ariations i n e nvironment a nd t end t o b e l ess h ardy i n c old c onditions ( Kramer a nd K ozlowski 1 979) . T he g reater u niformity o f t he r oot e nvironment m ay h elp t o e xplain t he p oorly d efined g rowth r ings i n m any r oots . C onditions s uitable f or r adial g rowth m ay c hange m ore g radually t han i n t he a erial e nvironment a nd a s a r esult t he r adially " flattened" c ells a t t he e nds o f g rowth r ings i n a erial x ylem a re l ess l ikely t o d evelop i n r oots . L ebedenko ( 1962) s tated t hat r adial g rowth i n r oots o ften e xtends w ell b eyond l eaf f all b y w hich t ime r adial g rowth i n t he s tem h as e nded f or t he s eason , a nd L ongman a nd C outts ( 1974) g ave t he s easonal p eriods o f r adial g rowth i n t he t wo B ritish O aks ( Q . r obur a nd Q . p etraea ) a s m id-April t o m id-September i n t he s tem a nd M ay t o O ctober i n r oots . I n a ddition t o t his e vidence b y i mplication , L ohr ( 1969) f ound t hat l ow l ight l evels i nduced o ak a nd a sh t o p roduce a erial x ylem w ith p oorly d efined g rowth r ings . S everal a uthors h ave a lso p rovided e vidence t hat r ing p orosity i n a erial x ylem c an b e m odified b y e nvironmental s timuli .

K nowlson ( 1939) e xamined

s ome s essile o ak ( Q . p etraea) t runks t hat h ad b een b uried f or s everal y ears, d uring which p eriod t hey h ad f ormed r oot-like x ylem . C onversely , L ebedenko ( 1962) f ound t hat c hestnut ( Castanea s ativa ) d uring a p eriod o f e xposure t o t he a ir .

r oots

f ormed

s tem-like x ylem

T he m odification o f v essel d istribution i n r oots i s l inked w ith l oss o f g rowth r ing d istinctiveness . A ll t hree r ing p orous s pecies e xamined ( oak , a sh a nd e lm) h ave r oots w hich a re n ot r ing p orous, p robably a s a r esult o f t he m ore u niform e nvironment o f t he s oil . T his l oss o f r ing p orosity c orresponds w ith a n i ncrease i n v essel d iameter w ith r espect t o c ambial a ge . R oots a re c loser t o t he w ater s ource o f t he t ree t han t he a erial x ylem , a nd t here i s t herefore l ess r isk o f a ir e mbolisms i n r oot v essels i n a ll b ut t he w orst d rought c onditions. T he r easons f or a natomical d ifferences b etween a erial a nd r oot x ylem a re c omplex .

T hese d ifferences a re m ost a pparent i n r ing p orous s pecies a nd i n

c ontrast t he o nly d iffuse p orous s pecies s tudied , b eech , d isplays r elatively l ittle a natomical v ariation t hroughout t he t ree , g rowth r ing b oundaries b eing w ell d efined i n r oots a nd a erial x ylem .

T his i s a lso t rue o f s ome

o ther d iffuse p orous s pecies , b ut l ime ( Tilia a mericana) r oots e xamined b y F ayle ( 1968) w ere v ariable i n t his r egard . E ven i n b eech , t ransverse s ections a t h igh m agnification ( Plate 2 :2) s how t hat g rowth r ing b oundaries a re s lightly l ess w ell d efined i n r oots . D ifferences b etween a erial a nd r oot x ylem a re n ot r estricted t o g rowth r ings, v essel d istribution a nd v essel d iameter . T he s torage f unction o f r oots i s r eflected i n a h igher p arenchyma a nd c orrespondingly l ower f ibre c ontent i n m ost r oots , w hich w as o bserved i n a ll t he s pecies e xamined . a ddition ,

t he c ells i n r oots a re u sually w ider ,

5

l onger ,

I n

t hinner-walled a nd

l ess

l ignified ,

h eartwood a nd

t yloses a re r are a nd p ith

i s

a bsent

( Fayle

1 968 , C utler 1 976) . A s h as b een d emonstrated t hese d ifferences c ombine t o g ive t he r oots o f s ome s pecies a n e ntirely d ifferent a ppearance f rom t heir a erial x ylem . I n c onclusion , w ith a f ew e xceptions, s uch a s b eech, r oots a re l ess l ikely t o b e o f v alue i n d endrochronological

s tudies

t han t runks a nd

b ranches.

A cknowledgements I would

l ike

t o

t hank D r .

D .F .

C utler a nd D r .

c onstructive c riticism o f t he m anuscript . H arwood p rocessed t he p hotographs.

6

M r .

M .

P .

R udall

f or t heir

S vanderlik a nd

M r .

T .

R EFERENCES B aillie, M .G .L . 1 982 . C anberra : C room H elm . C utler,

D .F .

1 976 .

B otanical S eries, N o . 3 , F ayle,

D .C .F .

V ariation p p .

1 43-156 .

1 968 .

T iming , A natomy . T oronto.

T echn .

F letcher ,

1 974 .

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T he B ritish O ak

T ree-Ring D ating a nd A rchaeology .

( eds .

R oot

Wood

A natomy .

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N o .

9 .

F aculty

o f

F orestry ,

U niversity o f

A nnual R ings i n M odern a nd M edieval T imes .

t he

M .G .

a nd

B ritish

P erring ,

I sles

F .H .) .

b y E .W .

P ublished

C lassey

L td"

F letcher, J .M . 1 975 . R elation o f A bnormal E arlywood D endrochronology a nd C limatology . N ature 2 54 , p p . 5 06-507 . G asson ,

P .

1 984 .

K nowlson ,

H .

f or

I n : T he

F arringdon ,

i n O aks

t o

R ecognition o f C haracteristic F eatures o f S econdary

X ylem o f S elected H ardwoods . C ollege).

O ak .

L eiden

R adial G rowth i n T ree R oots: D istribution , R ep .

M orris ,

B otanical S ociety o f B erks . p p . 8 0-97 .

i n

L ondon a nd

1 939 .

P hD .

T hesis ,

U niversity o f L ondon ( Imperial

A L ong T erm E xperiment o n t he R adial G rowth o f t he

T he N aturalist ( April

1 ) ,

p p .

9 3-99 .

K ramer, P .J . a nd K ozlowski, T .T . 1 979 .

P hysiology o f Woody P lants .

N ew Y ork , S an F rancisco , L ondon : A cademic P ress . L ebedenko ,

L .A .

1 962 .

C omparative A natomical A nalysis

W ood o f R oots a nd S tems o f s ome Woody P lants . A kademiya n auk S SSR 5 1,

p p .

1 24-134

E .

1 969 .

K .A .

E ngland .

R TS 2 194) .

J ahresringverlust b ei L aubbau inen m it r ingporigen H olz .

A llgemeine F orst-und J agdzeitung 3 0/4599). L ongman ,

t he M ature

( Translation f rom R ussian b y N ational

L ending L ibrary f or S cience a nd T echnology , L ohr ,

o f

T rudy I nstituta l esa

a nd C outts ,

M .P .

1 40,

1 974 .

p p .

1 8-22

( Forestry

A bstracts

P hysiology o f t he O ak T ree .

I n :

T he B ritish O ak ( eds . M orris, M .G . a nd P erring, F .H .) P ublished f or t he B otanical S ociety o f t he B ritish I sles b y E .W . C lassey L td" F arringdon , B erks. S chweingruber,

F .H .

1 978 .

M icroscopic W ood A natomy . S tructural

V ariability o f S tems a nd T wigs i n R ecent a nd S ubfossil W oods f rom C entral E urope . S wiss F ederal I nstitute o f F orestry R esearch .

7

P late 2 4 .5 . 2 .

1 . P edunculate O ak . 1 .

L ateral

T runk , r oot

T ransverse s ections o f s econdary x ylem .

W istman ts W ood, f rom

a m ature

D artmoor .

t ree ,

C ambial

M iddlesex .

a ge

o ver

C ambial a ge

A ll x

1 00

l ess

y ears .

t han 1 0

y ears . 3 . L ateral r oot f rom a n i mmature t ree a bout 1 5 y ears o ld , S ilwood P ark , B erkshire . C ambial a ge l ess t han 1 0 y ears . 4 . T ap r oot f rom a n i mmature t ree a bout 1 0 y ears o ld , S ilwood P ark , B erkshire .

8

P late 2 .

E uropean B eech , ( 1-3) a nd P edunculate O ak ( 4-5) .

s ections o f s econdary x ylem .

T ransverse

1 , 2 , 4 & 5 s how a g rowth r ing b oundary ( x

2 40) , 3 s hows t wo g rowth r ings ( x 2 4 .5) . 1 . T runk . C ambial a ge o ver 1 00 y ears . 2 . L ateral r oot , v ertically o rientated . C ambial a ge l ess t han 1 0 y ears . 3 . T runk a t l ower m agnification s howing 1 979 a nd 1 980 g rowth r ings . 1 -3 f rom t he s ame b eech t ree a t K ew G ardens , S urrey . 4 . T runk , l ocality u nknown . C ambial a ge o ver 1 00 y ears . 5 . L ateral r oot , S ilwood P ark , B erkshire . C ambial a ge l ess t han 1 0 y ears .

1 0

m i n» M

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T ransverse s ections o f

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A ll x

7 9 . 1 . T runk , s ite u nknown . 2 . B ranch , K ew G ardens , S urrey . C ambial a ge 4 -10 y ears o ld . 3&4 . L ateral r oots , H endon , M iddlesex , b oth l ess t han 2 0 y ears o ld .

I t i s n ot k nown w hether t hey c ome f rom t he s ame t ree .

1 2

P late 4 .

E uropean A sh .

T ransverse s ections o f r oot s econdary x ylem .

x 2 4 .5 . 1 . H endon , M iddlesex . 2 . K ew G ardens, S urrey, f rom a y oung p lant. 4 . S ite u nknown .

1 4

S urrey .

3 .

A ll

A binger,

P late 5 .

E nglish E lm .

2 4 .5 . 1 . T runk , s ite L ateral r oot, S urrey .

T ransverse s ections o f s econdary x ylem . u nknown , P rinces R isborough 3 . L ateral r oot, E ssex .

1 6

c ollection

A ll x

P R385A .

2 .

1 . 7

P late

6 .

E nglish

E lm .

T ransverse

s ections

o f

s econdary

x ylem

s howing

g rowth r ing b oundaries . A ll x 7 9 . 1 . T runk , s ite u nknown , P rinces R isborough c ollection , P R385A . 2 . L ateral r oot, S urrey . 3 . L ateral r oot, E ssex .

1 8

• e t. 40 % •‘ 6 14j

i l l 2

1 9

.

e t ,"

T he P alaeoclimatic S ignificance o f G rowth R ings i n L ate J urassic/ E arly C retaceous F ossil W ood f rom S outhern E ngland J .E . F rancis D epartment o f G eology & G eophysics U niversity o f A delaide B ox 4 98 G PO A delaide S outh A ustralia 5 001

A BSTRACT F ossil c onifer w ood

i s p resent a s i n s itu f ossil f orests i n t he U pper

J urassic P urbeck F ormation i n D orset a nd i n d rifted p lant b eds i n t he L ower C retaceous

W ealden G roup

o n

t he

I sle

o f

W ight .

A nalysis

o f

t he g rowth

r ings i n t he w ood a ssemblages h ighlights a c limatic c hange f rom a s emi-arid e vaporitic e nvironment i n t he P urbeck t o ah umid t emperate W ealden c limate . T he P urbeck g rowth r ings

a re n arrow

a nd

h ighly v ariable

i n

w idth

i n

r esponse t o a s trongly s easonal c limate i n w hich t he e rratic a vailability o f w ater markedly a ffected t ree g rowth . A Mediterranean-type c limate, c haracterised b y a s easonal a lternation o f w arm , w et w inters a nd h ot, d ry s ummers, i s s uggested f rom b oth t ree-ring a nd s edimentary e vidence. W hile s till

r eflecting

s easonal g rowth t he W ealden r ings a re s lightly

w ider a nd m ore u niform s uggesting t hat t he w ater s upply was t hen l ess l imiting

t o

t he

g rowth

o f

t he

c onifers .

O ther p alaeobotanical

s edimentary f eatures a lso r eflect t his i ncrease i n r ainfall . f actors c ausing t his c limatic c hange a re d iscussed. .

a nd

T he p ossible

I NTRODUCTION F ossil w ood

c onstitutes a n i mportant p art o f m any f ossil f loras .

T he

g rowth r ings i n t he w ood a re a n i mportant s ource o f p alaeoclimatic i nformation a s t hey a re o ne o f t he f ew m eans o f r ecording c ontemporaneous t errestrial c limates a nd b iological p roductivity . T he r esolution o f s ome c limatic p arameters s uch a s a nnual c yclicity a nd s easonality c an b e m uch m ore r efined t han t hat p rovided b y i nterpretation o f c limate-sensitive s ediments s uch a s c oals a nd e vaporites . W hile i mportant

t he

i nterpretation o f g rowth

r ings

i n

f ossil

w ood h as

p rovided

i nformation a bout c hanging g eological c limates o n a b road

s cale

( Chaloner a nd C reber 1 973 ; C reber a nd C haloner 1 984a , 1 984b ) t he s tudy o f i ndividual a ssemblages o f f ossil w ood ( " fossil f orests ') w ithin t he c ontext o f t heir s edimentary e nvironments c an p roduce d etailed p alaeoclimatic i nformation e .g . t he C retaceous f orests o f A ntarctica ( Jefferson 1 982) a nd t he J urassic f orests o f s outhern E ngland ( Francis 1 984) . I n t his p aper g rowth r ing a nalyses o f t wo f ossil w ood a ssemblages , t he l ate J urassic P urbeck f ossil f orests o f D orset a nd t he e arly C retaceous W ealden w ood d eposits o n t he I sle o f W ight , a re u sed i n a ssociation w ith s edimentary

2 1

e vidence t o e valuate t he c hanging p alaeoclimate a t t hese t imes .

G eological S etting i : T he L ower P urbeck F ossil F orest D uring t he l ate J urassic ,

a pproximately

1 45 m illion y ears a go ( Harland

e t a l . 1 982), much o f s outhern E ngland was c overed b y a -shallow, h ighly s aline l agoon i n w hich e vaporites w ere d eposited ( Howitt 1 964 ; W est 1 979) . T he D orset a rea w as s ituated a cross t he i ntertidal z one a t t he m argins o f t he l agoon .

M ounds a nd m ats o f a lgal s ediment c overed t his a rea , m erging

l andward i nto r endzina-like s oils ( Francis 1 986) w hich s upported g ymnosperm f orests ( Francis 1 983) . T he l agoon w ater p eriodically f looded t he f orests c overing t he t ree s tumps w ith a lgal-bound s ediment . T hese s ediments a re n ow p reserved i n t he b asal p art o f t he P urbeck F ormation i n D orset ( Figure 1 ) a s a s equence o f a lgal s tromatolitic l imestones a nd l agoonal p elletoid s iltstones ( the ' Caps ') , i nterbedded w ith c arbonaceous c lays ( the ' D irt B eds) w hich r epresent

f ormer

f orest

s oils

( Arkell 1 947 ; W est 1 975 ; F rancis 1 983) . T he t rees a re p reserved i n t he p alaeosols a s s ilicified b ranches a nd t ree s trumps w hich a re s till r ooted i n t heir o riginal g rowth p ositions . T heir c ompressed f oliage i s p reserved w ithin t he p alaeosols a nd a djacent l imestones . T he f ossil r emains i ndicate t hat t he P urbeck f orests w ere c omposed m ainly o f o ne t ype o f c onifer w ith w ood a nd s cale-like l eaves s imilar i n a ppearance t o m embers o f t he m odern C upressaceae ( Francis 1 983) . H owever, t hese t rees b elonged t o t he C heirolepidiaceae , a n e xtinct f amily o f c onifers w hich d ominated M esozoic v egetation ( Vakhrameev 1 970 ; A lvin 1 982) .

T he g rowth r ings o f t hese f ossil

t rees were s tudied t o i nvestigate t he a nomalous a ssociation o f f orest v egetation a nd e vaporites w ithin t he P urbeck e nvironment .

I i:

T he W ealden ' Pine R afts'

F ollowing t he r elatively q uiet P urbeck l agoonal s edimentation , c oarse s ands a nd g ravels were washed i nto t he b asin a s a r esult o f u plift a nd e rosion i n t he s urrounding h ighlands ( south-west E ngland , B rittany a nd t he L ondon a rea ) ( Allen 1 981) . T he W ealden e nvironment o f c oastal m udplains a nd e xtensive r iver s ystems d eveloped, w ith s alinities r anging f rom f reshwater t o m arine ( Allen 1 98 1) . A m ore d iverse f lora o f c onifers, c ycadophytes a nd f erns c olonised t he d rained ' u pland ' a reas a nd r iver f loodp lains ( Batten 1 974; H ughes 1 975; O ldham 1 976). F ossil w ood i s p resent a s a c omponent o f t he B rook f lora ( Hughes 1 975) i n t he W essex F ormation ( Barremian s tage ,

a pproximately 1 20 m illion y ears

a go) a nd i s f ound i n a bundance o n t he west c oast o f t he I sle o f W ight ( Figure 1 ) . T he s ediments h ere c onsist o f c oarse s ands a nd g ravels r epresenting m eandering r iver c hannels , i nterbedded w ith t hick v ariegated m arls w hich w ere o riginally d eposited o n t he f lood-plains ( Stewart 1 981) . P lant m aterial c onsisting o f l ogs , l eaves , c ones a nd p ollen i s p reserved w ithin ' plant d ebris b eds ' , w hich r epresent f ormer s torm o r f lood c hannels w hich b ecame c hoked w ith p lant d ebris . F ossil l ogs a re f ound i n t hese p lant d ebris b eds a nd a lso a t t he b ases o f t he c hannel s andstones . T hey r ange i n s ize f rom s mall t wigs l ess t han l cm i n d iameter t o l arge l ogs o ver l m a cross .

M ost w ood i s p reserved a s c ompressed ,

2 2

c arbonised l ignite b ut

F igure 1 .

M ap s howing t he o utcrops o f t he P urbeck F ormation a nd t he W ealden

G roup i n D orset a nd o n t he I sle o f W ight, t he

a nd s ome l ocalities m entioned i n

t ext .

T able

1 . S ummary o f r esults o f g rowth r ing a nalyses o f a nd W ealden f ossil w ood.

F or i ndividual s pecimen d ata f or

t he W ealden w ood s ee b elow. F rancis

S ample

N o.

P URBECK W EALDEN

1 984,

o f

M ean r ing w idth ( mm)

9.

F or P urbeck t ree-ring d ata s ee

p .294, T able 2 .

r ing s eries

2 0

t he P urbeck

R ange o f r ing w idths ( mm)

M ean

R ange o f M S

S ensiii vity

v alues

1 .13

0 .52-2.28

0 .527

0 .290-0.778

2 .94

0 .41-8.90

0 .345

0 .246-0.443

W ealden t ree-ring r esults: N o. o f r ings S ample n o.& l ocality

W 1 51

M ean r ing w idth ( mm)

M ax. r ing w idth ( mm)

M in. r ing w idth ( mm)

M ean S ensiti vity

C ompton C hine

4 0

2 .27

4 .5

0 .6

0 .362

W 1 27 C ompton C hine

1 2

2 .17

3 .9

1 .4

0 .380

4 .89

6 .9

3 .4

0 .298

2 .44

5 .5

2 .0

0 .259

6 .96

8 .9

4 .2

0 .285

2 .69

6 .1

1 .1

0 .246

W 1 50 C ompton C hine W 1 48 C ompton C hine W 1 23 C hilton C hine W 1 43 H anover P oint W 1 01

8 . 1 8 3 5

C ompton C hine

3 7

1 .48

3 .3

0 .3

0 .397

W 1 00 C ompton C hine

2 3

1 .54

2 .8

0 .6

0 .439

W 1 02 C ompton C hine

2 2

2 .00

4 .3

0 .8

0 .443

2 3

s ome h as b een p ermineralised w ith s ilica o r c alcite a nd r etains i ts c ellular d etail . T he m ost w ell-known o f t hese w ood d eposits i s t he ' P ine R aft ' a t H anover P oint ( Figure 1 ) w here l arge c alcified t runks a re p reserved a s al og j am, e xposed o n t he f oreshore a t l ow t ide . A lthough t he t rees w ere t ransported a way f rom t heir g rowth s ites t he e xcellent p reservational s tate o f t he p lant m aterial ( Oldham 1 976 ; A lvin e t a l 1 981) a nd t he k nown p alaeogeography ( Allen 1 981; S tewart 1 981) i mplies t hat t he w ood h ad o nly d rifted a s hort d istance b efore b eing b uried . T he f ossil w ood f rom t he W essex F ormation b elongs t o s everal f orm-genera o f f ossil c onifers ( Alvin e t a l 1 981) . O ne W ealden c onifer P seudofrenelopsis w as r econstructed b y A lvin ( 1983) a s a t all f orest t ree w ith s cale-like f oliage , s imilar t o t he P urbeck t ree ( Francis 1 983 , f ig . 3 ) a nd a lso a m ember o f t he C heirolepidiaceae .

G ROWTH R ING A NALYSIS T he g rowth r ings w ere m easured f rom p olished s labs o f f ossil w ood i n t ransverse s ection u sing a b inocular m icroscope , f rom a cetate p eels o f a cide tched s urfaces a nd f rom p etrographic t hin-sections . T he r ings w ere m easured a long a r adial l ine t o o btain a s l ong a s eries a s p reservation p ermitted , a lthough r eadings o ften h ad t o b e m ade a long a djacent r adii t o a void p atches o f p oor c ellular p reservation . O ften o nly a p ortion o f t he o riginal d iameter o f t he l og w as p resent s o t he r adii o f c urvature o f t he r ings w ere n oted t o e stimate w hether t he w ood s pecimen c ame f rom a l arge t runk o r s mall b ranch . S ome o f t he s tandard s tatistical p arameters u sed t o d escribe m odern r ing c haracteristics ( Fritts 1 976 ; C reber a nd C haloner 1 984b ) w ere u sed f or t his s tudy . H owever , s ome a daptions h ad t o b e m ade f or t he a dditional p roblems a ssociated w ith f ossil w ood s uch a s t he u se o f o nly f ragments o f w ood w ith a l imited n umber o f r ings, a n u nknown p osition w ithin t he t ree a nd v ery l imited k nowledge o f l ocal s ite f actors w hich h ad a ffected g rowth ( Creber a nd C haloner , t his v olume ) . T he p resence o r a bsence o f r ings w as n oted a nd t he a verage r ing w idth c alculated a s a n i ndication o f s easonal g rowth r ates . T he v ariability o f t he r ing w idths w as t he m ost i nformative p arameter , i llustrating t he s ensitivity o f t ree g rowth t o c limatic i nfluence . T his w as e xpressed i n t erms o f M ean S ensitivity ( MS) a nd c alculated u sing t he f ormula :

w here X i s t he r ing w idth , ni s t he n umber o f r ings a nd t i s t he y ear n umber o f t he r ing ( Fritts 1 976) . T he M S v alues r ange f rom 0 w here t here i s n o v ariation f rom y ear t o y ear , t o at heoretical m aximum o f 2r epresentative o f t he g reatest v ariation . A n a rbitrary v alue o f 0 .3 i s t aken t o s eparate " complacent" t rees w hich h ave h ad l ittle c limatic i nfluence ( MS 0 .3 ) f rom t hose w hich w ere " sensitive" t o c limatic v ariation ( MS 0 .3) ( Fritts 1 976) . T he A nnual S ensitivity ( AS), t he d ifference i n w idth b etween a p air o f c onsecutive r ings d ivided b y t heir a verage w idth ( Creber 1 977) , w as a lso u sed t o d etermine t he n ature o f t he a nnual v ariation . F or s elected r ings w ith g ood c ellular p reservation i ndividual c ell

2 4

d iameters a long r adial f iles w ere m easured i n o rder t o c lassify t he r ings a ccording t o t he s cheme p roposed f or f ossil w ood b y C reber a nd C haloner ( 1984b , p . 3 71) . T he c umulative a lgebraic s um o f e ach c ell 's d eviation f rom t he m ean o f t he r adial c ell d iameters w as c alculated a nd t he g raphs i n F igure 2 d rawn .

R esults R ing s eries c onsisting

o f

8 t o

1 05

r ings w ere o btained,

a lthough

c ontinuous s eries o f 2 0 -3 0 r ings w ere m ost f requent . T he r esults a re s ummarised i n T able 1 . C onspicuous g rowth r ings w ere p resent i n b oth t he P urbeck a nd W ealden wood, a nd i n a ll t rees t he r ings c onsisted o f a r elatively w ide z one o f l arge , t hin-walled e arlywood c ells , f ollowed b y a v ery n arrow z one o f o nly 4 - 5 t hick-walled l atewood c ells ( Figure 2 ) . T his p attern i s c haracteristic o f t he r ing t ypes D a nd E i llustrated b y C reber a nd C haloner ( 1984b, p . 3 73), a nd i ndicates a r elatively u niform g rowing s eason f ollowed b y am arked t erminal e vent r epresenting a c essation o r r etardation o f c ambial a ctivity . T he P urbeck g rowth r ings w ere m uch n arrower ( average 1 . 13mm) t han t hose o f t he W ealden ( average 2 .94mm) ; t he l argest P urbeck r ing ( 2 .28mm) b eing c onsiderably s maller t han t he m aximum s ize o f t he W ealden r ings ( 8 .90 =) . H owever, t he most s triking f eature o f t he P urbeck r ings i s t heir g reat v ariation i n w idth f rom y ear t o y ear ( Figure 3 ) .

T his i s r eflected i n t he

h igh M ean S ensitivity v alues, w hich o ften e xceeded 0 .7 . T hese t rees w ere c learly h ighly s ensitive t o c limatic s tress a nd i rregular g rowth c onditions . S uch h igh v alues a re c haracteristic o f t rees f rom s emi-arid f orests ( Schulman 1 956) a nd p articularly f rom t he l ower b orders o f s emi-arid f orests where water a vailability i s al imiting f actor t o g rowth ( Fritts e t a l . 1 965) . T he e rratic a vailability o f w ater i s a lso t he m ost o bvious c ause o f i rregular t ree g rowth i n t he e vaporitic P urbeck e nvironment . C onsistent w ith t his i s t he f requent p resence o f f alse , o r i ntra-annual r ings . T hese a re b ands o f s mall , d ense c ells w ithin t he z one o f l arge e arlywood c ells ( Francis 1 984 , F igure 5 d) , v ery s imilar t o d rought r ings ( Glerum 1 970) . T he W ealden t rees a lso h ave a ' sensitive ' f actors

r esponse

t o l imiting g rowth

( the a verage f igure f or M ean S ensitivity i s 0 .345) .

H owever,

F igure 4 i llustrates t hat a h igh p roportion o f t rees s how ' complacent ' r ing s equences a s a r esult o f more u niform g rowth . F alse r ings a re l ess f requent i n t he W ealden wood . T he h istograms o f A nnual S ensitivities ( Figure 5 ) r eflect t he m uch g reater d egree o f a nnual v ariation i n g rowth o f t he P urbeck t rees t han t he W ealden .

I n t he W ealden t rees 5 2 .5% o f t he

A nnual S ensitivity v alues a re ' sensitive ' , c ompared t o 6 3 .9% i n t he P urbeck t rees, a gain r eflecting t he m ore u niform g rowth i n t he W ealden t rees .

I nterpretation o f t he g rowth r ings T he p resence o f g rowth r ings i n t hese M esozoic w oods i s e vidence t hat t he c limate i n s outhern E ngland a t t hat t ime h ad w ell-defined s easons ,

a nd

w as u nlikely t o h ave b een a s easonless, t ropical e nvironment . T he n arrowness o f t he P urbeck r ings i mplies t hat t hese t rees g rew r ather s lowly w hereas t he W ealden t rees h ad m ore r apid g rowth r ates .

B oth s ets o f t rees

w ere i nfluenced b y r ather f luctuating c limates , t he m ost i mportant f actor p robably b eing t he a vailability o f w ater w hich s uggests v ariable r ainfall . T he P urbeck t rees a ppear t o h ave s uffered m ore f rom w ater s tress ,

2 5

n ot o nly

4 0

4 0

1

3 0-

3 0

2 0-

2 0

1 0

1 0

0

2

0

4

6

6

1 ' 0

1 2

1 ' 4

1 ' 6

1 8

-8 0

C e l l r ad ia l d iame te r , um

4 0-

-6 0

3 0\

/

-4 0

2 0-

WEALDEN WI02

1 0

-2 0

EW

' .0 —L W 0

0 0

2

4

6

6

1 0

1 2

1 4

1' 6

1 1 8

2 0

2 2

2 4

2 6

_______ C umu la t ive s um o f d ev ia t ions f r om mean r ad ia l d iame te r , um

V

2 8

C e l l n umbe r i n r i ng

F igure 2 . G raphs s howing t he c hange i n c ell r adial d iameter a cross s elected g rowth r ings i n P urbeck a nd W ealden w ood . T he e arlywood ( EW )/latewood ( LW ) b oundary i s c hosen a s t he p oint a t w hich t he c umulative s um f inally r eturns t o z ero . A m arked d ecrease i n c ell d iameter i n t he e arlywood z one o f t he P urbeck r ing m arks t he p osition o f a f alse r ing ( FR) .

T hese r ings a re

c haracteristic o f t he t ypes D a nd E o f C reber a nd C haloner 1 984b .

2 6

F igure 3 .

( a) G rowth r ings i n P urbeck f ossil w ood . P B 2 4 , P ortland . T hin-section o f s ilicified w ood . N ote t he v ery v ariable w idths o f t he r ings f rom y ear t o y ear . ( b ) G rowth r ings i n W ealden f ossil w ood . W 123 , B rook f lora , I sle o f W ight . T hin-section o f s ilicified w ood . T hese r ings a re m uch w ider a nd m ore u niform i n w idth . E ach r ing c onsists o f ab road z one o f e arlywood c ells t erminated b y an arrow r ing o f o nly 4 -5 l atewood c ells . S cale b ars r epresent 5 m m .

2 7

d ue

t o

l ack o f water a t

t he e nd o f

i ntermittent d rought d uring f alse r ings .

t he g rowing s eason b ut a lso f rom

t he g rowing

s eason i tself,

a s

i mplied b y t he

C omparison w ith t he g rowth p atterns o f l iving t rees o f k nown c limatic i nfluence h as s hown t hat t rees f rom m arkedly s easonal M editerranean-type c limatic z ones ( with w arm , w et w inters a nd h ot , d ry s ummers) h ave r ing c haracteristics m ost s imilar t o t he P urbeck r ing s eries ( Francis 1 984) . I n p articular t hey s how t he s ame s low g rowth r ates a nd t he s ame k ind o f v ariability o f a nnual g rowth ( and t hus h igh M ean S ensitivities) , a nd t hese c haracteristics r ainfall

c an

b e

w ithin t he

r elated

w et

t o

s eason .

t he

e rratic o ccurrence a nd

T he

W ealden g rowth

r ings

q uantity o f c ontinued

t o

e xhibit t his i rregular g rowth b ut t o al esser d egree . T he g reater w idth o f t he r ings a nd t he f ewer f alse r ings s uggests t hat a t t his t ime s easonality d id n ot h ave s uch a s evere e ffect o n t hese t rees, p ossibly b ecause t he r ainfall w as h igher.

S EDIMENTARY E VIDENCE E NVIRONMENTS

O F

S EASONAL

C LIMATES

I N

T HE

P URBECK A ND

W EALDEN

P urbeck S upport

f or

t hese

c onclusions

i s

p rovided

b y

s everal

a spects

o f

t he

s ediments a nd f ossils i n t he b asal p art o f t he P urbeck F ormation w hich a lso s uggest t he i nfluence o f a s easonal c limate . F irstly , c harophytes

a r ather a nomalous ( Barker e t a l .

n odules o f s ilicified

c ollection o f

s ilicified

w ood ,

s eeds

a nd

1 975) o f f reshwater a ffinities, a long w ith

e vaporite p seudomorphs o f h ypersaline

o rigin c an b e

f ound i n t he w estern p art o f t he P urbeck o utcrop ( West 1 975) . S imilar a ssemblages c an b e f ound t oday i n c oastal l agoons i n M editerranean c limatic z ones o f s outhern A ustralia ( Burne e t a l . 1 980) , a nd a re a ble t o c o-exist b ecause t he s easonal a lternation o f r ainfall a nd d rought c hanges t he s alinity o f t he l agoon w ater f rom f resh i n w inter t o h ypersaline i n t he s ummer . T his A ustralian e nvironment i s t hus c onsidered a nalogous t o t hat o f t he P urbeck ( West 1 975 ; B urne e t a l . 1 980) . • S econdly , t he q uartz t hat i s p resent i n t he s ilicified P urbeck w ood i s a t ype o f

c halcedony

e nvironments .

( quartzine) o ften a ssociated

w ith h ighly a lkaline

I t i s f requently u sed a s ap alaeoclimatic i ndicator o f s emi-

a rid c limates ( Folk a nd P ittman 1 971) . T he s ource o f t he s ilica s eems m ost l ikely t o h ave b een a s a n i norganic p recipitate i nfluenced b y a lternating c hanges i n t he l agoon water p H ; a modern m odel r elated t o s easonality c hanges f or t his m echanism c an a gain ( Peterson a nd V on D er B orch 1 965). T hirdly ,

b e

f ound

i n

t he

A ustralian

l agoons

a t t he b ase o f t he r endzina-like f ossil s oil o f t he G reat D irt

B ed t here a re i rregularly l aminated n odules a nd c rusts o f f ine-grained c arbonate , c ommonly k nown a s c alcrete ( Francis, 1 986) . A s easonal s emia rid c limate w ith p eriods o f r ainfall f ollowed b y p eriods o f i ntense e vaporation i s a lways n ecessary f or t he f ormation o f t his p edogenic c alcrete ( Harrison

1 977) .

F inally , w ithin t he L ower D irt B ed i n e ast D orset, w hich h ere r epresents am arginal l agoonal d eposit, a ssemblages o f s mall c onchostracan b ranchiopod c rustacea

( fossil

c lam

s hrimps)

c an b e

2 8

f ound

o n l amina

s urfaces

( Francis

P u rbeck

Wea lden MS 0 .345

4 0-

3 0-

%

2 0-

1 0-

0 0 .2

0 .4

0 .6

0

0 :4

Mean S ens it iv ity

Mean S ens it iv ity

F igure 4 .

0 :2

H istogram o f M ean S ensitivity v alues f or t he P urbeck a nd W ealden

f ossil wood . O ver 9 5% o f t he P urbeck t rees h ave ' sensitive ' mean s ensitivity v alues ( MS > 0 .3) c ompared t o o nly 5 6% o f t he W ealden t rees . 2 0

Wea lden

1 5

MS 0 .527

9 61 0 ,1

1 1 . 1 M 0 1 1 1 1. 1 . 1.

5 -

0 .2

0 .4

0 .6

0 :8

1 :0

1 . 4

1 . 6

Annua l S ens it iv ity C lasses MS 0 .345

P u rbeck

0 :2

F igure 5 .

0 .4

1 1 .2 I ncreas ing V ar iat ion f r om Y ear t o Y ear 0 .8

1 .0

1 .4

1 . 6

H istograms o f A nnual S ensitivity v alues f or t he P urbeck a nd

W ealden t rees .

T he r ange i n a nnual v ariation i s m uch g reater i n t he P urbeck

t rees t han i n t he W ealden , m ean

0 :6

i n r esponse t o am ore i rregular c limate .

s ensitivity ) .

2 9

( MS =

1 984) . p ools

T heir m odern r epresentatives l ive i n s mall b rackish o r f reshwater which

o ften

d esiccate

d uring

t he

d ry

s eason ,

l eaving

' death

a ssemblages ' o n t he s ediment ( Tasch a nd Z immerman 1 960) . T he n umber o f c oncentric m oult l ines o n t he c arapaces c an b e u sed t o d etermine t he l ife s pan o f t he c rustacea ( Tasch

1 969) .

B y a nalogy t he P urbeck c rustacea m ay

h ave l ived f or a bout 5 m onths , w hich m ay a lso r epresent t he d uration o f t he w et s eason .

W ealden A lternative p eriod .

e vidence e xists

t o s upport t he n otion o f a w etter W ealden

T he i ncreased p roportion o f r iver, s torm a nd f lood s ediments

i ndicates t hat r iver s ystems b ecame a d ominant f actor o f t he W ealden l andscape , i mplying t hat u plift a nd e rosion o f t he a djacent h ighlands a nd a s ubsequent i ncrease i n r ainfall h ad o ccurred s ince t he P urbeck t imes . F urther , t he c hange t o am ore h umid c limate w as e nvisaged b y B atten ( 1974) a nd H ughes ( 1975) t o a ccount f or a more d iverse f lora w ith a h igher p roportion o f f erns t han t he P urbeck . T he m arked d ecrease i n t he a mount o f C lassopollis p ollen ( yielded b y m embers o f t he C heirolepidiaceae c onifer f amily ) , w hich i s a bundant i n t he P urbeck s ediments a nd a ssociated w ith s emi-arid, e vaporitic M esozoic e nvironments w orldwide ( Vakhrameev 1 970 ; S rivastava 1 976), a lso s ignifies a w etter c limate ( Batten 1 982). H owever t he s ediments i ndicate t hat t he r ainfall w as n ot c onstant t hroughout p eriodically

t he

y ear .

A llen

( 1981,

w et" c limate t o a ccount f or

p .

3 79)

e nvisaged

a " warm

a nd

t he a bundance o f s torm a nd f lood

s ediments i nduced b y p eriods o f h eavy r ainfall a nd t o maintain l akes i nhabited b y f reshwater i nvertebrates . T he f luctuating d ischarge o f t he r ivers w as a ttributed t o a n i rregular r ainfall p attern . A lthough i ntermittently v ery w et, d esiccation f eatures s uch a s mudcracks w ithin c hannel b ase s ediments a re e vidence t hat t he w ater l evels p eriodically b ecame v ery l ow d uring t he d ry s eason ( Stewart 1 983) . D uring t hese t imes t he v egetation b ecame s ufficiently d ry t o f uel f orest f ires, s o p roducing t he c harcoal w hich i s c ommon i n W ealden s ediments . M ost o f t he c harcoal i s f ormed o f c onifer w ood ( Alvin e t a l . 1 981) o r b urnt f ern s tems ( Harris 1 981) . T he f erns p robably g rew o n t he r iver f lood-plains b ut w ere r apidly b uried a fter t he f ire, e nsuring t heir p reservation . H arris ( 1981) c onsidered t hat t he c limate p robably c onsisted o f al ong d ry s eason f ollowed b y a s hort b ut s tormy w et s eason . D rought c onditions w ere n ot o f s ufficient i ntensity o r d uration , h owever , t o a llow t he p recipitation o f e vaporites w hich a re n otably a bsent i n t he W ealden s ediments . O xygeni sotope a nalyses o n m ollusc s hells i ndicate t hat t he W ealden t emperatures w ere c omparable t o t hose o f m odern t emperate/tropical c limates ( Allen e t a l . 1 973).

D ISCUSSION T he s ediments a nd f ossils r ecord a c hange f rom a s emi-arid , e vaporitic e nvironment d uring t he P urbeck t o am ore h umid t emperate W ealden s etting i n t he e arly C retaceous . T he f loras a lso r eflect t his t ransition , t he a rid P urbeck c onifer f orests w hich g rew i n w ell-drained s oils ( Francis 1 984) b eing r eplaced b y t he f ern a nd l ycopod t ype v egetation o f t he W ealden f loodp lains ( Batten 1 974) .

F urther d etails o f t his c hange w ere o btained b y

S laden ( 1983) a nd S laden a nd B atten ( 1984) f rom p alynofacies d ata a nd c laym ineral s uites .

T he p urbeck b eds c ontain a h igh

3 0

p roportion o f i llite a nd

m ixed-layer c lay m inerals w ith o nly a l ittle k aolinite , as uite f ound t oday i n a lkaline , p oorly-leached s oils o f s emi-arid a reas . T hroughout t he e arly C retaceous k aolinite i ncreases i n a dundance, r eflecting t he i ncreasing h umidity a nd l eaching i n m ore a cid c onditions . B y c omparison w ith t he c limatic p arameters g overning t he d istribution o f m odern c lay m ineral s uites S laden a nd B atten ( 1984) p roposed t hat t he P urbeck c limate w as a rid/semia rid w ith m ean t emperatures o f 2 0-25 °C a nd r ainfall l ess t han 5 00mm p er y ear . L ikewise t he W ealden ( Barremian ) c limate w as l ikely t o h ave b een w arm h umid-temperate w ith s imilar t emperatures b ut r ainfall p robably o ver 1 000mm p er y ear . T he g rowth p atterns i n t he f ossil w ood a lso i llustrate t hat t he t rees r eflected t his c limatic t ransition t oo . T he P urbeck g rowth r ings a re n arrow a nd v ariable i n w idth i n r esponse t o a r ather h arsh s emi-arid e nvironment i n w hich t he e rratic a vailability o f w ater a dversely a ffected g rowth . T he r ainfall w as e stimated t o b e a bout 4 00mm p er y ear b y F rancis ( 1983) b ased o n s edimentary a nalysis a nd b y c omparison w ith a v ery s imilar l iving c onifer f orest i n a l agoonal s etting o n R ottnest I sland, w est A ustralia . T he f ossil w ood i n t he W essex F ormation , h owever , h as n otably w ider a nd m ore u niform g rowth r ings s uggesting t hat l arger q uantities o f w ater w ere a vailable , p articularly i n t he s easons i mportant f or t ree g rowth . T he r ainfall e stimate o f 1 000mm b y S laden a nd B atten ( 1984) s eems a ppropriate f or g rowth o f t he W ealden t rees a lso . A lthough t he s edimentary e vidence d oes i mply a n i rregular d istribution o f r ainfa ll i n b oth P urbeck a nd W ealden t imes , t he o nly m eans o f a ssessing t he e xact n ature o f t he s easonality i s f rom t he g rowth p atterns i n t he f ossil t rees . T he c haracteristics o f i ndividual r ings h ighlight a n a nnual p attern c onsisting o f af avourable s eason o f g rowth f ollowed b y ap eriod i n w hich t he t ree b ecame a lmost d ormant ( Figure 2 ) . F or t he P urbeck t rees t his h as b een i nterpreted i n t erms o f a s easonal c limate w ith w arm , w et w inters f ollowed b y h ot , a rid s ummers , t hat i s , aM editerranean-type c limate ( Francis 1 984) . T he p eriod o f d rought w as q uite s evere i n t his c ase t o a llow g ypsiferous e vaporites t o f orm ( West 1 979) . E ven t he w inter s eason w as s omewhat h arsh w ith i ntermittent d ry s pells c ausing d rought r ings t o f orm i n t he t rees . T he g rowth r ing p atterns i n t he W ealden t rees , t hough l ess s ensitive , c ontinue t o s how t he s ame k ind o f a nnual p attern a nd s uggest t hat t he s ame k ind o f s easonal c limate c ontinued i nto t he e arly C retaceous , t hough p robably s omewhat a meliorated b y h igher r ainfall w hich a llowed t he t rees t o g row m ore s teadily . T his c limatic p attern p robably r eflects l ocal c onditions o ver s outhern E ngland . G lobally , t he M esozoic c limate h ad h igher a mbient t emperatures t han t oday w ith a n e xpanded t ropical b elt a nd t emperate z ones e xtending t o t he p oles ( Frakes, 1 979 ; H allam 1 984 ; 1 985) . T he c ontinents w ere t hen j oined t o f orm a l arge l and m ass ( Pangaea ) w hich w as s ubject t o am uch m ore c ontinental c limate w ith e xtremes o f w inter a nd s ummer t emperatures ( Frakes 1 979 ; B arron a nd W ashington 1 982) . P alaeoclimatic s imulation m odels o f t he C retaceous c limate ( Lloyd 1 982 ; B arron a nd W ashington 1 982 ; P arrish e t a l . 1 982) p redicted a lternating h igh a nd l ow p ressure c entres o ver t he c ontinents a nd o ceans , r esulting i n a s trongly s easonal c limate o ver t he c ontinental m argins . T he g rowth r ings i n t he f ossil w ood s upport t his I dea . B ritain w as t hen s ituated n ear t he w estern b orders o f t he T ethyan o cean , w hich f ormed b etween t he A sian c ontinent t o t he n orth a nd t he G ondwanan l and m ass t o t he s outh ( Hallam 1 984) . T he c hange f rom a s emi-arid t o am ore h umid c limate i n s outhern E ngland

3 1

w as u nlikely t o h ave b een d ue t o a l atitudinal s hift i nto a m ore n ortherly c limatic

z one

s ince

t he p alaeolatitudinal

p osition o f

t his

a rea

f or b oth

P urbeck a nd W ealden t imes r emains c onstant a t a bout 3 6 °N ( Smith e t a l . 1 981) . A n a lternative e xplanation i s l ocal u plift o f a djacent h ighlands ( Sladen a nd B atten 1 984 , A llen 1 981) , c oupled w ith a n i ncreasing m aritime i nfluence d ue t o t he w esterly e xtension o f t he T ethys s eaway a nd t he i nitial o pening o f t he N orth A tlantic ( Hallam 1 984).

S UMMARY D uring t he l ate J urassic a nd e arly C retaceous t he v egetation i n s outhern E ngland c onsisted o f c onifer f orests w hich a re n ow p reserved a s t he P urbeck i n s itu f ossil f orests a nd t he W ealden d rifted p lant b eds . T he g rowth r ings i n t he f ossil w ood s how t hat t he t rees g rew i n a s easonal , t emperate e nvironment . T he t ree-rings i n t he P urbeck w ood a re n arrow a nd v ariable i n w idth f rom y ear t o y ear , r eflecting g rowth i n am arkedly s easonal , s emi-arid c limate w here t he e rratic a vailability o f w ater s trongly i nfluenced t ree g rowth . T he c limate s eems t o h ave b een o f M editerranean-type w ith w arm , w et w inters s uitable f or t he g rowth o f t he t rees f ollowed b y h ot , d ry s ummers w hen t he e vaporites f ormed . T he r ainfall i s e stimated t o h ave b een a bout 4 00 - 5 00 m m p er y ear, b ased o n b oth f loral a nd s edimentary e vidence. G rowth r ings r ocks t imes .

i n

t he

w ood

a nd

a nalysis

o f

t he

a ssociated

s edimentary

i ndicate t hat t he s ame p attern o f s easonality c ontinued i nto W ealden H owever

t he g reater w idths

o f

t he

r ings

a nd

t he

s lightly

l ower

( though s till ' sensitive ') M ean S ensitivity v alues s uggests t hat t he r ainfall w as s ignificantly h igher t han d uring t he P urbeck . T he p roportion o f f luvially-influenced a nd k aolinite-rich s ediments i s e vidence i n s upport o f t his . T he a nnual r ainfall w as e stimated t o b e o ver 1 000 mm . T his s easonality c orresponds w ith p alaeoclimatic s imulation m odels w hich p redict t he p resence o f s easonal c limates o ver c ontinental m argins i n t he M esozoic . T he i ncrease i n r ainfall f rom P urbeck t o W ealden t imes h as b een r elated t o t he i ncreased e levation o f t he s urrounding h ighlands a nd t he i ncreasing m aritime i nfluence r elated t o t he o pening o f t he T ethyan s eaway a nd t he N orth A tlantic o cean .

A cknowledgements Ia m g rateful f or r esearch f acilities p rovided a t S outhampton U niversity a nd B edford

C ollege

( University o f

L ondon) d uring t he

r esearch g rants , a nd t he B ritish A ntarctic S urvey . G .T . C reber f or v aluable a dvice a nd d iscussion .

3 2

I a m

t enure o f i ndebted

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3 6

T he C ontribution o f G rowth R ing S tudies t o t he R econstruction o f P ast C limates G .T . C reber a nd W .G . C haloner B otany D epartment R oyal H olloway a nd B edford N ew C ollege E gham H ill, E gham S urrey T W20 O EX

A BSTRACT T he f ossil r emains o f t he e arliest k nown t rees f rom t he U pper D evonian a nd C arboniferous P eriods a re e ither w ithout g rowth r ings o r o nly h ave t hem v ery weakly d eveloped . S ince t hese t ree g rew l argely w ithin t he l ow p alaeolatitudes o f t hose P eriods , t heir r ing c haracteristics s eem q uite c onsistent w ith g rowth i n a n a lmost s easonless c limate . I n m ore r ecent g eological p eriods , p articularly t he M esozoic , f ossil w ood w ith v ery w ide r ings i s f ound i n v ery h igh p alaeolatitudes w here a t t he p resent d ay t ree g rowth i s e ither v ery l imited o r a bsent a ltogether . I t s eems l ikely t hat t he m ain c limatic d ifference b etween t he p resent d ay a nd t he M esozoic h igh l atitudes i s t he l ow a mbient t emperature w hich d epresses m etabolic p rocesses s uch a s t ransport a nd g rowth r ather t han p hotosynthesis . F or t he l atter , t he s upply o f l ight e nergy a t h igh l atitudes w ould a ppear t o b e a dequate t o e xplain t he h igh p roductivity i n t he M esozoic p rovided t here h ad b een h igher a mbient t emperatures .

I NTRODUCTION F ossil w ood i s f ound i n g reat a bundance w ith a w orld-wide d istribution . T his u biquity o f f ossil wood i s d ue t o an umber o f f actors:

1 . W ood i s

f ormed i n c onsiderable b ulk ; 2 . I t i s a r elatively r esistant m aterial w hich may s urvive u ndecayed o n l and o r i n water f or s ome t ime b efore f ossilization ; 3 . I t a ppears t o h ave a p ositive a ttraction f or i onized s ilica a nd i s most c ommonly f ossilized b y c alcification ( in c alcium c arbonate ) o r b y p yritization ( in i ron s ulphide) o r i ncorporated ( coalified , i n v arying d egree ) i n p eat o r l ignite . E nvironmental d eductions m ay b e m ade f rom t he t ype o f s iliceous p ermineralization o f t he w ood ( Francis, 1 983 , 1 984 , ( this v olume)) . T he v arious p rocesses o f f ossilization m ay p reserve t he c ellular d etails o f t he w ood s o w ell t hat g rowth-ring s tudies m ay b e c arried o ut u pon i t e xactly a s i f i t w ere m odern w ood ( Creber , 1 977 ; J efferson , 1 982 ; F rancis , 1 983) . N evertheless, t he a pproach t o t he s tudy o f g rowth r ings i n w oods u p t o m any m illions o f y ears o ld d iffers i n an umber o f i mportant r espects f rom t hat o f h istorical material a t m ost a f ew t housand y ears o ld . Most r ecords o f f ossil w ood a re o f f ragments which may o nly s how a l imited n umber o f g rowth r ings . t he

s ite o f g rowth ( Jefferson ,

E ven w hen ' fossil 1 982 ;

F rancis,

f orests ' a re f ound

i n

1 983) t here i s l ittle t o b e

g ained b y a ttempting t o c ross-date t he s tumps . T he p rincipal i nterest i n s uch m aterial i s t o d etect a g eneral i ndication o f t he c limatic e nvironment

3 7

u nder w hich t he t rees g rew , r ather t han t o i dentify f luctuations e ven o f t he o rder o f c enturies o r m illennia . T his i s b ecause t he i dentity o f a ny w ood o ver a f ew m illion y ears o ld i s a t b est o nly t o b e e xpressed a t t he l evel o f a l iving o r f ossil g enus, s o t hat n o k nowledge o f i ts p recise r esponse t o t he c limatic e nvironment i s a vailable . F urthermore ,

s ingle s mall p ieces o f f ossil w ood m ake u p t he b ulk o f o ur

r ecord a nd t heir p osition w ithin t he t ree may b e o nly i mperfectly e stablished . T he s ite o f g rowth a nd i ts t opographic r elations may b e e qually u nknown . D espite t hese l imitations ,

a ncient f ossil w oods r epresent a n i mportant

s ource o f p alaeoclimatic i nformation w hich a s y et h as b een l ittle e xploited . E ven t hough t his m ay a ppear c rude o r b lurred t o t he d endrochronologist o r d endroclimatologist o f t he h istorical p eriod , i t d oes c onstitute a s ource o f i nformation o n t he l evel o f b iological p roductivity a nd e specially o n t he c haracter o f t he s easonality ( or l ack o f i t) i n t he p alaeoclimatic r egime .

O CCURRENCES O F F OSSIL W OOD 1 : T he D evonian F ossil w ood f irst a ppears i n t he g eological r ecord a bout 3 70my B P .

T he

e arliest t rees , ( the w ood i s a ssigned t o t he g enus C allixylon , t he f oliage t o A rchaeopteris) ( Figure 1 ), c ould b e v ery s ubstantial, w ith s tump d iameters i n e xcess o f l m . I t i s i nteresting t o n ote t hat t hey h ad e volved o nly s ome 3 0my a fter t he e arliest v ascular p lants ( with p rimary x ylem) r ecorded i n t he f ossil r ecord ( Chaloner a nd S heerin , 1 979) . D etails o f t he p rincipal l ocalities w here t hese f ossil t rees o ccur a re g iven i n T able 1 a nd F igure 2 . I n s ome s pecimens t he w ood h as n o g rowth r ings w hile i n o thers t he r ing b oundaries a re v ery f aint a nd i n i rregular s equences . M any r ing b oundaries a ppear n ot

t o e ncircle t he e ntire t runk ( Lemoigne e t a l"

1 983) .

2 : T he C arboniferous D uring t he l ater p art o f t his p eriod f rom a bout 3 30my B P t o 2 50my B P a l arge a rea o f

l owland s wamp

f orest

e xtended

f rom p resent-day K ansas

e astwards a cross t he M id-West o f t he U nited S tates t o t he e ast c oast a nd f rom W estern E urope t o t he D onetsk B asin i n t he U SSR ( Figure 3 ) ( the s oc alled E uramerian p rovince) . . T he p eat-accumulating v egetation i ncluded many a rborescent s pecies, s uch a s L ycopods ( club-mosses), a rticulates ( horse-tails) a nd s eedferns w hich p roduced e xtensive s econdary w ood a nd a ttained

t he

s tature o f

t rees .

T hus

a v ery

l arge

q uantity

o f

w ood

w as

p roduced . S tudies o f t he f ossilised r emnants a gain s how t hat much i s t otally d evoid o f g rowth r ings . H owever ,

i n w ood s pecimens d rawn f rom t he e ntire C arboniferous p eriod

s ome g rowth r ings a re r ecorded ( Figure 3 , T able I I) w hich o ccur r ather h aphazardly i n b oth s pace a nd t ime . I n g eneral , t he c limatic c onditions a pparently p ermitted m uch c ontinuous w ood g rowth , b ut t he o ccurrence o f, s ay , f luctuations i n t he a vailability o f w ater c aused i nterruptions i n t he f ormation o f w ood . S ome f ossil w ood r ecorded f rom t he C arboniferous s hows s trongly d eveloped g rowth r ings .

3 8

F igure 1 . Ag eneralised r econstruction o f am ember o f t he g enus A rchaeopteris , t he e arliest k nown g enus o f t rees, f rom t he U pper D evonian ( ca . 3 70my B P) . N ote t he c onsiderable d imensions o f t he t runk . ( After B eck , 1 962) .

3 9

T able I .

U pper D evonian f ossil w oods p lotted o n F igure 2

G ENUS

L OCALITY

M AP S ITE

P ALAEOL ATITUDE

C allixylon

I ndiana U SA

1

1 9°S

E lkins &Wieland ,1914 A rnold ,1 931

C allixylon

N ew Y ork S tate U SA

2

1 7°S

H ylander ,1922 A rnold ,1 930 B eck ,

S phenoxylon

A UTHORS

1 953

2

1 7°S

M atten & B anks,1967

C allixylon

E llesmere I sland

3

1 6°N

A ndrews e t a 1,1965

I ndet. g ymnosperm w ood

S pitzbergen W . G ermany

4 5

2 0°N 7 °S

H oeg ,1 942 K räusel & W eyland ,1937

C allixylon

T iman U SSR

6

1 8°N

L emoigne e t a 1,1983

C allixylon

D onetsk B asin U SSR

7

3 °N

Z alessky ,1909 ,19 11

C allixylon

K azakhstan U SSR

8

3 °N

L epekhina ,1963

C allixylon

T annu T uva U SSR

9

3 9°N

L emoigne e t a 1,1983

P alaeos piroxylon

K araganda U SSR

2 6°N

I ürina & L amoigne,1972

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