Water relations in deciduous forest trees under elevated Co²

Water relations in deciduous forest trees under elevated CO ²

INAUGURALDISSERTATION

zur Erlangung der Würde eines Doktors der Philosophie vorgelegt der

PHILOSOPHISCH-NATURWISSENSCHAFTLICHEN FAKULTÄT der Universität Basel

von

SEBASTIAN LEUZINGER

aus Basel

Basel, 2006

Prof. Dr. Christian Körner Prof. Dr. Nina Buchmann

Basel, den 19. September 2006

Prof. Dr. Hans-Jakob Wirz

Dekan

Stomata orchestrate one of plant biology's greatest concerts

Stan Wullschleger

Acknowledgements

First and foremost thanks go to Christian Körner, for the excellent guidance he provided, for his enthusiasm towards my project, for tireless proof-reading of my manuscripts, for his faith in my ability to carry on.

Pascal Niklaus and Reto Stöckli from ETH Zürich provided most valuable advice, excellent collaboration and many hours of stimulating discussions. I wish to thank Roland Vogt for help and advice on thermography and micrometeorology and Barbara Köstner for interesting discussions and advice on sap ow. Thanks also to Nina Buchmann from ETH Zürich for co-examining my thesis.

I am greatly indebted to Erwin Amstutz for countless hours of (nightly) crane operations, technical help and deciphering thermal images. Also thanks to Roman Assho, Olivier Bignucolo, Patrick Cech, Georges Grun, Dieter Häring, Erika Hiltbrunner, Hans-Christoph Im Hof, Hamlyn Jones, Sonja Keel, Susanna Peláez-Riedl, Eva Spehn and Gerhard Zotz for general help and everybody else from the Botanical Institute for creating a very inspiring working environment.

I am further grateful to the supervisors of my MSc thesis in statistics which I pursued in parallel to the PhD programme, Marie-Agnès Moravie, Sylvain Sardy and Anthony Davison, their input greatly inuenced my PhD thesis.

Funding came from the Swiss National Science Foundation (NCCR climate, Project 3.3) and a research grant to C. Körner (3100-059769.99). The crane was sponsored by the Swiss Federal Oce for the Environment (FOEN).

I would like to thank you Alana for all the support you provided during these three years, for being patient when I came home late, for listening and encouraging me when things did not go so well. Thank you Matteo for giving me that smile before I leave home in the morning. Finally, I thank my parents, they rst awoke my interest in biology and supported me in all I have ever done since.

The title page shows a thermal image of the crane, taken from the gondola. Temperatures range from ca.

16 ^o C (black) to ca. 30 ^o C (red). The false-colour image contains the information of 320 x 240 = 76'800

temperature measurements, printed in numbers rounded to one degree Celsius on the back of the title page

(not for the farsighted among us).

Contents

1 General Introduction 9

2 Water savings in mature deciduous forest trees under elevated CO 2 - a multifac-

torial approach 15

3 Responses of deciduous forest trees to severe summer drought in Central Europe 27 4 Stomatal conductance in mature deciduous forest trees exposed to elevated CO 2 39 5 Tree species diversity aects canopy leaf temperatures in a mature temperate

forest 49

6 A sensitivity analysis of leaf temperature to changes in transpiration 59

7 A model predicting sap ow from environmental data 67

8 The analysis of relative data - A novel approach with the example of sap ow

data 77

9 Summary 85

Chapter 1

General Introduction

9

General Introdution

Global hange, limate hange, CO

2

and plants

Depending on whether one adopts a more anthro-

pogenioramoreeologialperspetive,povertyand

globalhangewillbethetwomostpressingproblems

humanity will haveto fae in the21

st

entury (The

Millenium Development Goals Report 2005, Mille-

nium Eosystem Assessment 2005). In many ways

however,theglobalsoialandeologialproblemsof

the 21

st

entury will interat, and any attempt to

ndasolutionmustinvolvetheonsiderationofboth.

The term `global hange'summarises all large-sale

alterations to theplanet'satmosphere, hydrosphere,

podosphere and biosphere brought along by human

ativity(Körner,2003). If biodiversityis takenasa

proxyfortheplanet'swellbeing,land-usehangehas

been estimated to havethe largestimpat, followed

byglobalwarming,N-deposition,biotiexhange(in-

trodutionofforeignspeiestoaneosystem)andthe

inrease of atmospheri CO

2

-onentration (Sala et al. 2000). Large-saleexploitationofmostterrestrial

eosystems,intensiveagriultureand theburningof

fossilfuelaretheresultsofanoverexponentialgrowth

of the humanpopulation duringthe last three en-

turies. Humanativitieshavealreadyresultedinsuh

drasti transformationsof theearth's globalproper-

ties(limate,vegetationover,biodiversity)thatthe

onsetofanewgeologialera(the`anthropoene')was

suggestedbyCrutzen (2002).

Climati hange as one faette of global hange

is mainly aused by the burning of fossil fuels and

the resultingenrihmentof CO

2

^{in the}atmosphere.

Changes in land-use (deforestation, agriulture, ur-

banisation) further ontribute to limate hange

throughthe reationof newarbonsinks, theburn-

ingofsoilarbon,othergreenhousegasemissionsand

ahange of thesurfaealbedo. Thehuman-indued

inreasein atmospheriCO

2

onentrationfromthe

`pre-industrial' 270 ppm (a. 1850) to the urrent

380 ppm (Figure 1) and its eet on global warm-

inghavebeenreognisedbythesientiommunity

(Rodhe 1990, Crowley 2000). The present level of

atmospheri CO

2

^{and therateofinreaseasexperi-}

thelast 640'000years(Petitetal.1999,Siegenthaler

et al. 2005). The projeted 500 to 900 ppm of at-

mospheri CO

2

^{are expeted to inrease the mean}

surfae temperature of the planet by a. 2 to 6

o

C

from 1990 to 2100, depending on the emission se-

nario(IPCCSynthesisReport, 2001).

Besidesitsroleasthemaingreenhouseagent,ar-

bondioxideonstitutesthemostimportantsubstrate

of photosyntheti arbon assimilation by plants. It

is therefore likely that any hange in atmospheri

CO

2

onentration also has plant physiologial ef- fets. This is beause of the saturation funtion of

the rate of photosynthesis over CO

2

onentration:

at the urrentatmospheri CO

2

^level, photosynthe- sis of C3 plants is not saturated and an be stim-

ulated with higher levels of CO

2

^{. Thus, plants are}

subjeted to bothCO

2

^{-indued limatihange and}

CO

2

^{-indued diret inuenes on their} metabolism.

The metaboli (photosyntheti) eets may sound

beneialforplantsat rstsight,theasadeofse-

ondaryproessesinduedbeyondasheershifttothe

rightinthephotosynthesis-CO

2

-onentrationurve, resultin amultitude, notneessarily beneial on-

sequenes. For instane stoihiometri onstraints

maylimitanygrowthresponse(Loladze2002,Körner

2003). Nutrientswillontrollong-termplantgrowth

stimulation under elevated CO

2

^{(Oren et al. 2001,}

Johnsonet al.2006). Seondly,biodiversityislikely

to beaeted throughdierentialresponsesofplant

funtional groups (Körner, 2000). For example,

lianasas shade-adapted plantstend to benet from

ashift in thelight-ompensation point ofphotosyn-

thesistolowerlightintensities. Avigorousgrowthof

lianashowevermaygreatlyinueneforestsuession

andeventuallyrevertinitialgrowtheetsofelevated

CO

2

^{(Körner,1998). Moreimportantthantheissue}

of aelerated plantgrowth is the question whether

this results in inreased arbon sequestration. Be-

auseevenifunder elevated CO

2

^{plantsgrewfaster,}

this maysimply speedup plantsuession, and not

neessarilytie upmorearbonin the plantbiomass

(Körner2004, Figure 4). This problem, although of

greatinterestforpoliymakers,isnotentirelysolved,

but major shifts in the amount of sequestered ar-

bon are unlikely for various reasons summarisedby

Figure 1: AtmospheriCO

2

-onentration overthe past640'000years,reonstruted from ie oresof the antartiie shield. The latest data are from CO

2

-measurementsat the Mauna-Loa observatory, Hawaii (Petitetal.1999,Siegenthaleretal.2005)

CO

2

^{, plants and water}

Theleafpores(stomata)ontrollingthegasexhange

between the leaf and the surrounding air, onsti-

tute the entrepieeof the plant's dilemmabetween

`hunger' (stomata losed, no CO

2

^{-uptakebut water}

isonserved)and`thirst'(stomataopen,CO

2

^-uptake

butwaterislost). Thistrade-ois,inarstapprox-

imation, shifted towards less water loss at onstant

(or even inreased)CO

2

^{-uptakeasaonsequeneof}

partialstomatallosureinresponsetoelevatedCO

2

^-

onentration. Thismehanismwasrstdoumented

in 1898 (Darwin, 1898 as ited in Rashke 1986).

Sine70%ofallwaterenteringtheatmosphereover

landpasses throughstomata,theirreationtorising

CO

2

^{isertainlyofkeyinteresttotheglobalwatery-}

le. It is omparativelystraight-forwardto quantify

redued stomatalondutane(a measureforstom-

atal opening) in response to dierent levelsof CO

2

under ontrolled (greenhouse) onditions (for a re-

view see Curtis &Wang 1998, Medlyn et al. 2001).

Topredittotalnetwatersavingsofafuture eosys-

tem however needs saling up from the leaf to the

whole anopy and nally the whole region (Jarvis

&MNaughton,1986). Further,it involvessoilpro-

esses(Morgan,2002)andtheonsiderationofaom-

bination of environmental drivers likely to oinide

in global hange senarios(warming, alteredpreip-

itation patterns, Shaw et al. 2002). Finally, water

savings will be ompromised or enhaned by atmo-

spherifeedbakeets,whihanonlybeunovered

bymodellingland-surfaeproesses(seebelow). An

plexityofnatural,open systems,arebiodiversity ef-

fets. If,inagreenhouseexperiment,speiesAshows

20%lessstomatalondutaneandAremainsunaf-

fetedbyelevatedCO

2

^{,aeldexperimentouldshow}

that speies B prots from A's savings and starts

transpiringevenmore.

Vegetation-atmosphere interations

The initial CO

2

^{-response of a plant in terms of re-}

duedstomatallosureentailsareduedhumidia-

tion of the atmosphere. From here, several imagin-

ableargumentationsforfeedbakeetsarepossible:

iftheatmosphereisdrier, thisouldause plantsto

transpireeven less beause of stomataldownregula-

tion. Alternatively,theplants'initialreationofless

transpirationouldbemitigatedbeauseleaftemper-

ature inreases and hene an inreaseof theleaf to

airvapourpressuregradient(Idso etal., 1993). Yet

anotherargumentationis that throughaless humid

atmosphere,thereislessloudformation,henemore

photosyntheti ative radiation and therefore more

transpiration (R. Stökli, unpublished data). Given

thereislesstranspirationin plantsunderhigherlev-

elsofCO

2

^{,theplanetaryboundarylayer(PBL)may}

simply grow higher during the diurnal yle, leav-

ingatmospherimoistureontententirelyunhanged.

Similar feedbak eets may our in the even less

aessible and studied soil system (Morgan, 2002).

Herein liesafundamental andintrinsidrawbak of

all studies exposingvegetation to elevated CO

2

^{, be}

11

sible: due to logistionstraints, the omparatively

small CO

2

^{-enrihed area will always be surrounded}

byvegetationgrowingunderambientonditions,pre-

venting all large-sale feedbak. Land-surfae mod-

els on the other hand, an aount for feedbak ef-

fets, but often suer from a ountless number of

parameters,whosequantiationritiallyinuenes

the model's outome. Most regional oupled soil-

vegetation-atmospheremodelsdonotyetaountfor

elevated atmospheri CO

2

^{(Vidale& Stokli, 2005).}

Jaobs&Debruin(1992)forthersttime modelled

possible feedbak mehanisms, but their vegetation

responsewasbasedonaratherunrealistiphotosyn-

thesismodel. Amorerealistiapproahwouldbeto

determine a series of transpiration response urves

under elevated CO

2

^{, in partiular the response of}

transpirationtoatmospherivapourpressuredeit.

Thus, a realisti plant response to a given atmo-

spherimoistureanbefed baktothemodel.

Water relations of forest trees

Whereas all all terrestrial and aquati biomes are

aeted by global hange and in partiular by li-

matehange,forestsdeserveprimeattentionbeause

they over about 30 % of the earth's land surfae

andontainabout80%ofallarbonboundinplant

biomass(Olson et al. 1983, RoyJ. 2001). Although

thearbonyleandespeiallyarbonsequestration

in forestsareeologiallyandpolitiallyverytopial

issues, I here onentrate on forest water relations.

Forests,partiularly tropialrainforests, haveama-

jor limate-regulatoryfuntion, where deforestation

haslargeeetsonthewholeeosystem(loudforma-

tion,preipitation, soilmoistureShuklaetal.1990).

Evenadelineinevapotranspirationratesandsubse-

quentsurfaewarmingouldpotentiallyauseaol-

lapse of the Amazon forests (Cox et al., 2000). In

order tobetrustedhowever,suh model resultsrely

on (mostly unavailable) eld data for parametrisa-

tionandmodeltesting. Moorroft(2006)pointsout

the potential errors that may arise in limate fore-

asts ifspeies-spei plantphysiologial responses

are not aountedfor. On the other hand, eld re-

searhinvolvingforest trees andtheir anopies usu-

ally requires heavy logistis, and in situ olletion

of leaf-level water status parameters was extremely

umbersome for alongtime. Sine theearly 1990s,

anopy ranesfailitate treerown aess andallow

large-sale studies of gas exhange at the leaf- and

whole treelevel(Bassetetal.,2003).

Thewaterstatus ofatree anbeassessedusing a

number methods that an generally be divided into

leaf-level and whole-anopy approahes. Leaf-level

measurements (gas exhange, porometry, water

potential, stable isotope analysis) have the advan-

they are often ompromised by large within-rown

variation. Every leaf experienes its very own

mirolimati onditions and history, depending on

the exat position and exposure within the anopy,

whih introduesa lot of variability. Whole-anopy

approahes (sap ow, thermal imagery, soil water

potential/moisture) in ontrast integrate signals

over the whole anopy, with the drawbak of only

yielding relativesignals. Partiularattentionhas to

be paid to hanges in leaf area index (LAI), whih

an onfound signals measured both at leaf and

anopylevel.

The number of water relation studies, onduted

onmatureforesttreesunderelevatedCO

2

^isstilllim-

iteddue to logisti onstraints. Currently, there are

approximately 20 free air CO

2

^{-enrihment (FACE)}

experiments, but only three outof these are testing

trees taller than ve meters (Ellsworth et al. 1995,

Wullshleger&Norby2001,Shäferetal.2002,Ceh

et al. 2003). The Swiss Canopy Crane (SCC) site

hoststhe only FACE devie testing the inuene of

elevatedCO

2

^ontallbroad-leavedforesttrees,anim- portantvegetationtypeinlargepartsofthetemper-

atezone.

In summary, reported responses of forest trees to

elevated CO

2

^{in terms of redued water onsump-}

tion vary widely from no response (Ellsworth 1999,

for Pinus taeda) to25 % in Liquidambar styraiua

(Wullshleger & Norby 2001, Shäfer et al. 2002).

Most studies report sap ow or stomatal ondu-

tane data. While this is a needed rst step, it is

importanttonotethatneithersapowdensity(on-

founded with stem repletion) nor stomatal ondu-

tane(inserieswithother ondutanes,i.e. anopy

ondutane) are linearly related to tree transpira-

tion. Summarising results from dierent studies is

also deliate, beause the exat weather onditions

greatlyinuene netCO

2

^{eets. Long-termstudies}

helptoaverageresponsesarossmanydierentenvi-

ronmentalonditions. Ofpartiularinterestarestud-

iesthatombineenvironmentaldriverslikelytooin-

idein thefuture, aboveallwarming,elevated CO

2

andpreipitationfrequeny/abundane. However,in

eld experimentswith talltrees, suh ombinations

ofenvironmentaldriversannotbeplanned(beause

weatheronditionsarenotforeseeable). Thespeies-

spei response of tree water onsumption on the

interation ofthese driverswill be thekeytofuture

preditionsofforestwateruse.

Inthestudies itedabove,onesinglemethod(sap

oworstomatalondutane) isapplied toestimate

redued water onsumption of forest trees (mostly

oneortwospeies)under elevated CO

2

^{. Comparing}

severalspeiesandespeiallytheparalleluseof var-

moisture as a relatively simple measure of net wa-

ter use (integratingeets of stomatalondutane,

LAI,anopyresistane)israrelyreportedinsuient

temporalandspatialresolutiontodisernCO

2

^-eets

(Ellsworth1999,Gundersonetal.2002,Shäferetal.

2002). Latest advanes in high-resolution infrared

thermographynotonlyallowanauratewayof as-

sessing plant transpiration Leinonen et al. (2006),

they also permit to desribe temperature frequeny

distributionsofplantleavesinanunpreedentedres-

olution. The methodologyhas notyet been applied

to detet leaf warming under elevated CO

2

^{due to}

reduedlatentheatowinforest trees.

Thisdotoralthesisattemptstoassesswaterrela-

tions of tall deiduous forest trees subjeted to ele-

vated CO

2

^{under normal weather onditionsand in}

ombination with drought. A series of partlynovel

tehniques(e.g. thermography)wereusedtothisend.

After the general introdution, hapter 2 estimates

totalwatersavingsusing sapow,soilmoistureand

thermographydata (Global Change Biology, in revi-

sion). Chapter 3reports responses offorest trees to

the entennial drought over Central Europe in the

summer of 2003, partly ombined with the eet of

CO

2

^{(published 2005 in Tree} Physiology). Chapter 4(o-authored)ompilesstomatalondutanedata

overtherst4yearsofCO

2

^{-enrihmentattheSCC}

site (Trees, in revision). Chapter 5 analyses spatial

and temporal temperature distributions of deidu-

ous forest trees, irrespetive of the CO

2

^treatment

(submitted to Agriultural and Forest Meteorology).

Chapters6to8areshortunpublishedanalyses: hap-

ter6isasensitivityanalysistoassesstheplausibility

of deteting reduedtranspiration by leaf warming,

with thegivenvariation atthe site. Chapter 7sug-

gests a preditive model for sap ow with environ-

mental variables aspreditors. Chapter 8isa short

noteontheproblematiofanalysingrelativedatain

general, withsapowdata asan example. Chapter

9,thegeneralsummary,onludesthethesis.

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Chapter 2

Water savings in mature deciduous forest trees under elevated CO 2 - a multifactorial approach

15

Water savings in mature deiduous forest trees under elevated

CO

2

^{- a} multifatorial approah

GlobalChangeBiology,inrevision

Sebastian Leuzinger and ChristianKörner

AbstratStomatalondutaneofplantsexposed

to elevated CO

2

^{is often redued. Whether this}

leads to water savings in tall forest-trees under fu-

tureCO

2

onentrationsislargelyunknownbutould havesigniantimpliations forlimate andhydrol-

ogy. Forest-trees of a. 35 m height and 100 years

of age exposed to elevated CO

2

onentration (540 ppmduringdaylighthours)usingfreeairCO

2

^enrih-

ment(FACE)andtheSwissCanopyCrane(SCC)in

NW-Switzerlandindiatereduedwateronsumption

by year4 and 5, but signalsare highly variable be-

tweenspeies (Querus petraea, Fagus sylvatia and

Carpinus betulus). Aross speies and a widerange

ofweatheronditions,sapowwasreduedby14%

in trees subjeted to elevated CO

2

^{, but this signal}

is likely to diminish as atmospheri feedbak omes

into play at landsapesale. Beause sapwood an-

not reliably be assessed in these experimental trees

and beausethepositionof individualrownsinthe

upperanopyvaries,westandardisedthesapowof

eah individual tree with its own maximum during

theperiodin onsideration. Thisanalysisarrivedat

watersavings of a. 16 to 20 %. TheCO

2

^{-eet is}

greatest at low vapour pressure deit (vpd). The

eet waslargelyprodued by Carpinus and Fagus,

with Querus ontributing little. Inline with these

ndings,soilmoistureat10mdepthdereasedata

slowerrateunderhigh-CO

2

^{treesthanunder ontrol}

trees duringrainless periods, with areversal of this

trend during prolonged drought when CO

2

^-treated

treestakeadvantagefrominitialwatersavings. High

resolution thermal images taken at dierent heights

abovetheforest-anopydidonlydetetreduedwater

lossthroughalteredenergybalaneatlosedistane

(0.44 K leaf warming of CO

2

^{-treated Fagus trees).}

Short disontinuations of CO

2

^{supply had no mea-}

surable anopy temperature eets, most likely be-

ausethestomataleetsweresmallomparedtothe

aerodynamionstraintsinthesedense,broad-leaved

anopies.

Introdution

PartialstomatallosureunderelevatedCO

2

^anlead

tosubstantialwatersavings(Morison&Giord1984,

Medlynetal.2001)andthereforehasthepotentialto

signiantlyinuene ourlimate(Field etal. 1995,

Morgan et al. 2004). However, a lot of the infor-

mation available to date omes from small plants,

in the ase of trees from seedlings, young saplings

or branhes and muh less is known for entire tall

forest trees exposed to elevated CO

2

^{. On average,}

stomatal ondutane (g

s

^{) of trees is redued by 9}

%, (non-signiant, leafbased data from 10 studies

in adult broad-leaved and oniferous trees, Medlyn

etal.2001). Outoftheurrentlya. 15freeairCO

2

enrihment(FACE)experiments,onlyvearetesting

trees, out of these, three use trees higher than 5m

(Ellsworthet al. 1995, Ellsworth 1999, Wullshleger

and Norby 2001, Shäfer et al. 2002, Ceh et al.

2003). Thesestudiesreportwatersavingsunderele-

vatedCO

2

^{rangingfromnoresponseatall(Ellsworth}

1999,forPinustaeda)overanaverageof13to25%in

Liquidambarstyraiua (Wullshleger&Norby2001,

Shäfer et al. 2002), to a maximum of (marginally

signiant)22% at lowvpd forsixdierentdeidu-

ousforest-treespeiesduringtheinitialexperimental

monthat theSwissCanopyCrane (SCC)site(Ceh

etal.,2003).

Depending on the aerodynami oupling, net wa-

ter savings of forests under elevated CO

2

^{- the a-}

tual quantity of interest - are lower than expeted

from stomatal response alone. Stomata appear to

respond in suh a way that the total transferresis-

tane(inludingtheaerodynamiomponent)meets

the plant's needs (Meinzer et al., 2001). Therefore,

it is not possible to aurately infer atual tran-

spirationfrom stomatalondutanedata often ol-

letedinCO

2

experiments. Asanexample,wellou- pledonifers,lessoupleddeiduoustreesandpoorly

oupledgrasslandsupposedlyarriveatsimilar water

savings at strongly diverging stomatal ondutane

(Körneret al.,2006): there isnomeasurablehange

in g

s

^{in tallonifers (Ellsworth1999, Körner 2003),}

50 % redution of g

s

^{in dense grassland(Niklaus &}

Körner2004,Morganetal.2004)withadultdeidu-

oustreesholdinganintermediateposition(e.g. Keel

et al. 2006, in review). Even ifatual transpiration

responsestoelevatedCO

2

^{areknown,itstillremains}

unleartowhatextentatmospherifeedbak(lesshu-

midied air and slight inreasesin foliage tempera-

ture) will mitigate plot sale CO

2

^{-eets on water}

onsumption (Field et al. 1995, Jaobs & de Bruin

1997, Amthor1999, Morganet al. 2004). Assessing

whole tree transpiration and in partiular its vpd-

response to CO

2

^{enrihment in tall trees is a muh}

needed rst step. Even if feedbak proesses miti-

gate or even enhane the initially found water sav-

ings eets, speies spei vpd-response urvesare

thebasisforallmodellingattemptsaimed atunov-

eringthenalnetwatersavingsourringinaforest

eosystem.

There are three ways to study tree-forest water

vapour loss: (1) stem ow measurements, (2) soil

moistureresponsesand(3)measurementsinorabove

theanopyusingleafgasexhangetehniquesormi-

rometeorologialmethods. Weadoptedallthreeap-

proahes,but emphasiswasonsapowwiththead-

vantagethat the signalaverages arossleaf, branh

orrown-levelvariationandpermitsontinuousread-

ingswithoutinterferingwiththeleaves'environment.

Sapowanbeassessedinvarious ways(Köstner

et al., 1998), for thepresentlong-term study in tall

trees,wefoundtheleastinvasivemethodbyGranier

(1985)mostappropriate.

Soilmoistureoersanindiretmeasureofredued

wateronsumption, is easyto measurebut is rarely

reported in the CO

2

literature. Only three studies from the other FACE experiments with trees show

data for soil moisture orsoil water potential dier-

enes. Twondanon-signianttrendforhighersoil

moistureorsoilwaterpotentialunder elevated CO

2

(Ellsworth1999, Gundersonet al. 2002). Shäfer et

al. (2002) nd no dierenes in the alulated soil

waterstoragebetweenCO

2

-treatments. Intheother ases,dierenesbeamegreaterwithdryingsoil. In-

terpreting soil moisturesignals in terms of absolute

wateruptakewould require data for the full rooted

prole,whihisimpossibletoobtainin ourase,be-

auseof therokyundergroundand unknownmaxi-

mumrootingdepth. However,topsoilsignalsprovide

aqualitativeestimateofwaterusebytrees.

Forassessingatualanopywaterloss,theonven-

tionaleddyuxovarianemethodannotreliablybe

applied at the small sales needed, but the leaf en-

ergybalaneanbeassessedusingtheanopyrane.

We employed an energy balane approah using a

latest tehnology, high-resolution thermal amera,

whih yields anopy surfae temperature (a proxy

forlatentheatuxthroughalteredleaftemperature;

Jones 1999). Allfators being onstant, aredution

anopytemperatureashasbeendemonstratedinthe

laboratory and for agriulturalrops (Fuhs, 1990).

Thistehniquehasneverbeenappliedtotalltreefo-

liage. Although the leaf energy balane of leaves is

diulttoquantifyinaroughnaturalforestanopy,

arelativerelativeomparisonisstillpossible(Jones,

1999).

This study builds upon earlier results from the

samesitebyCehetal.(2003)who,intherstthree

monthsof CO

2

-enrihment, onlyfound aslightten- denyofreduedwateronsumptionoftreatedtrees,

basedonsapowdataonly. Here,weapplytwoad-

ditional,ompletelyindependentapproahes,namely

soil moistureand thermal imaging, to assess poten-

tialwatersavingsofdeiduousforesttreesunder fu-

tureCO

2

onentration. Mostimportantly,however, we provide speies spei vpd-response urves of

transpiration at ambient and elevated CO

2

^{. These}

responses permit modelling of the water balane of

forestsunderfuture limationditions.

Materials and Methods

Sitedesription andstudiedspeies

Theexperimentwasondutedinadiversemixedfor-

est standloated 15km southof Basel,Switzerland

(47

o

28' N, 7

o

30' E; elevation: 550 m a.s.l.). The

forest is approximately 100 years old with anopy

tree heights between 30 and 38 m. The stand has

a stem density of 415 trees ha

− 1

(diameter

≥

¹⁰

m), a total basal area of 46 m

2

ha

− 1

and a leaf

area index of a. 5 in the experimental area. It is

dominatedbyFagussylvatiaL.andQueruspetraea

(Matt.) Liebl.,withCarpinus betulus L.,Tiliaplaty-

phyllos Sop.,Aer ampestre L. andPrunus avium

L.presentasompanionspeies. Inaddition,thesite

has a strong natural presene of onifers outside of

theCO

2

^{-enrihed zone(Abies alba Mill., Piea abies}

L., Pinus sylvestris L.). For the present investiga-

tion,weseletedthemostabundantQueruspetraea,

Fagussylvatia and Carpinus betulus,heneforthre-

ferredtobytheirgenusonly. Weusedsixtreeseah

(threetreated,threeontrols)forallanalyses,i.e. 18

treesintotal.

The soil is asilty-loamy rendzinathat developed

onalareousbedrok. Deepsoilwaterwasassessed

independently of the CO

2

^{-treatment to roughly es-}

timate the vertial soil moisture distribution using

four90mPRB-Ftimedomainreetometryprobes

(EnvironmentalSensor,Vitoria, BC,Canada). Soil

waterwasnearlyuniformlydistributedbetween0and

90mthroughoutthe2004and2005growingseason.

However,withinreasing rokdensityat depth, ne

substrate beomes very sare and the atual mois-

turepertotalbulkvolumebeomesverysmall. The

17

haraterised bymild winters andmoderatelywarm

summers. During the two study years (2004 and

2005), the growing season of deiduous trees lasted

fromtheendofApriltotheendofOtober(a. 180

days,Asshoet al.2006,Körneretal.2005andun-

published data). Mean January and July tempera-

turesare2.1and19.1

o

C.Totalannualpreipitation

atthestudysitewas950mmin2004and910mmin

2005,whihisinloseorrespondenetotheexpeted

long-term averageat that site(990 mm). Normally,

two-thirdsfallduringthegrowingseason. Thegrow-

ing season 2004 an be onsidered muh drier than

the one in 2005, as only 41 % of the preipitation

ourred duringMay to September(64 % in 2005),

while 36 % ourred in January and Otober om-

bined.

At the start of the two 22-day test periods (see

below)studiedin2004and2005,initialsoilmoisture

wassimilar,buttotalumulativerainfallin the2004

period amounted to 16 mm only, while 60 mm of

rainfellduringthe2005testperiod.

Free air CO

2

^{enrihmentsystem(FACE)}

A 45-m freestanding tower rane equipped with a

30-m jib and a working gondola provided aess to

64 dominant trees in an area of a. 3000 m

2

. A

group of 14 adult broad-leaved trees (3 Fagus, 4

Querus,4Carpinus,1Tilia,1Aer and1Prunus),

overingaanopy areaof a. 550 m

2

were seleted

for CO

2

^{enrihment. For this study, only nine}

CO

2

^{-treated trees (three eah from Querus, Fagus,}

and Carpinus) were used. Nine non-treated trees

(three per speies) loated in the remaining rane

areaatsuientdistane fromtheCO

2

^releasezone

were used as ontrols. A series of pre-treatment

measurements assured that there was no hidden

bakground eet between the two groups (Ceh

et al. 2003,Asshoet al. 2006). CO

2

^-enrihmentof

the forest anopy was ahieved by a free-air, pure

CO

2

^{releasesystemthatonsistedofawebof 4mm}

plasti tubes (a. 0.5 km per tree) with 0.5 mm

laserpunhedholes(spaedat30-mintervals)emit-

ting pure CO

2

^{into the tree anopy (FACE). For a}

moredetaileddesription,seePepin&Körner(2002).

Climati data

Wind speed, photon ux density, rainfall, air tem-

peratureandrelativehumidityweremeasuredabove

the tree anopy using a weather station loated at

the top of the rane (anemometer AN1, quantum

sensor QS and a tipping buket rain gauge RG1,

all from Delta-T, Cambridge, UK). Measurements

wereperformedevery30sanddatawerereordedas

10-minmeansusingadatalogger(DL3000,Delta-T,

Cambridge,UK).Vapourpressuredeit(heneforth

referred to as vpd) was alulated from wet and

averages)loatedat anopyheight.

Soil moisturedata

Weassessedpotentialtreatmenteetsonsoilmois-

turebythreedierentprobetypes: rstly,weuseda

handheld TDR devie (Trime-FM, Imko, Ettlingen,

Germany, here referred to as the `handheld TDR

probe') for manual sampling of soil moisturein the

top 10 m on May 17 and August 12 2004. The

devieallowedusto takereadings froma. 80spots

within two hours. For better temporal resolution,

we also used probes working with the apaitane

priniple (ECH2O, Deagon, Pullman, Washington;

heneforth referred to as `eho probes'). Thirty of

theseprobes(halfof whihin thetreatedarea)were

plaed at 10 m depth and read out daily during

dry periods. Thirdly, eight (5 ambient, 3 elevated)

time domain reetometry probes (ML2x probes,

Delta-T, Cambridge, UK; heneforth referredto as

`TDR probes') logged hourly in order to provide

hightimeresolution.

Sapow measurements

Sapowsensorsonsistedoftwo20-mmlongand2-

mm diameter probes(eah equipped with aopper-

onstantan thermoouple and oated with heating

wire)insertedradiallyintothesapwoodafterphloem

removal at breast height with a vertial spaing of

15 musing 2 mmaluminium tubes (UP, Kolkwitz,

Germany). Two pairs of sensors per tree (installed

onoppositesidesofthestem)wereusedandtheav-

erage of both readings per tree was taken for fur-

ther analysis. Sensors were proteted against rain

andexternalthermalinueneswithaluminiumov-

ers lledwith polyester wool. Readings were taken

at 30 s intervals and reorded as10-min means us-

ingmulti-hanneldataloggers(DL2e,Delta-T,Cam-

bridge,UK).Sensorswereremovedduringthewinter

and re-installed in new positions in spring 2005, as

reommendedbyKöstneretal.(1998). Theupperof

thetwoprobesreeivedaonstantpowerof200mW

while the lowerreferene probe remained unheated.

Duringonditionsofminimalsapowsuhasatnight

orduringprolongedraineventsthetemperaturedif-

ferene,

∆

T,betweenthetwoprobesreahedamaxi- mum(a. 9-15K,dependingonthermalondutivity

ofthesurroundingwood,exatsensorposition et.).

Sap ow during the day thus auses a derease in

∆

T. Sap ow measurements were performedduring

most of the growing season in 2004 and 2005, but

due to periodi sensor and logger failure, we had to

selet periods with reliable and ontinuous data for

the analyses(twotimes 22 days, June 16to July 6,

2004 and July 20 to August 10, 2005). Due to a

time-stamp problem with one data logger in 2004,

weanonlyshowtheumulativedata(Figure1)for

densityaordingto Granier (1985),exept that we

divided bythe absolute maximum(extreme outliers

removed)ofthe whole 22-dayperiodinstead ofpre-

nightlymaxima. This had marginal(< 5%) eets

on sap ow density beause our readings were sta-

ble andouromparisons restritedto 22days. This

proedure alsoavoideddaily step hanges of signals

(monotoni signalsrequiredfor ouralternativedata

analysis,seebelow). Anotheradvantageisthepossi-

bilityof analysingnight-timesapow. Fortheright

panel in Figure 1, we dividedthe sapowseries by

themaximum(meanof10highestvaluespertree)of

the aording speies (again per 22 day period), in

order topreventthe pooledresultsfrom beingover-

riddenby aspeies eet. Giventhe unknownstem

harateristis,itwasimpossibletoarriveatanabso-

lutesapowdensity(J

s

^,m

3

H

2

^Om

− 2

persapwood

areas

− 1

). Anysuhalulationsusingexistingequa-

tions would be highly pretentious. Wetherefore do

not speify units and refer to our data as`relative'

sapow. ForFigure 1,wealulatedumulativesap

ow(sumoverall10minutetimestepsofeah22-day

period. Wehavenopretreatmentsapowdata(from

yearsbefore 2001),but anumber ofother treevari-

ables(Ceh et al., 2003)suggest that there were no

apriori dierenesbetween thetwogroupsof trees.

Mostimportantly,thepre-treatmentrateofgrowthof

trees(asassessedbytreerings)didnotdier(Assho

et al.,2006).

The variation in sap ow between trees was high

despite using themean of twosensors pertree. We

attribute this to the natural growth onditions of

these treesandtheirvariationin stemdiameter and

anopy position. We divided eah tree's sap ow

readings by itsownmaximum, i.e. westandardised

the 2005 22-day time series to a range from 0 to 1

(herereferredtoas`standardised'sapowdata). We

alulatedthe ratioof themeanuxunder elevated

CO

2

^{(E)to that under ambient air (A) (allspeies}

pooled). TheresultantE/Aratiourveof22diurnal

ourseswasthenompatedbyrandomlyresampling

(withreplaement)99timeseverytimeofday(from

0:00 to 23:50, 10 min intervals) out of the 22-day

series,inordertoyieldoneaveragedailyE/Aurve.

As will be disussed later, standardisation by noon

maxima inevitablyremovesanysignalfrom midday,

but retains dierenes at the two diurnal tails at

ontrastingvpd(morning, evening),forwhih water

savingsunderelevatedCO

2

^{anbeestimated.}

Thermal imaging

Leaf temperatures were measured using a state-

of-the-art thermal amera (VarioCam, Infrate,

Dresden, Germany) with a resolution of 240 x 320

pixels, providing 76'800 temperature readings with

a 0.1 K resolution. We measured at three dierent

sales: from the rane gondola (a. 5 m above the

anopy), form the ounter jib of the rane (a. 12

m abovetheanopy)andfrom aheliopter(a. 200

m above the anopy). At the smallest sale, we

monitored adjaent, similarly exposed, treated and

non-treated treeson twosunnydays in 2004 taking

IR-pitures at 0.2 Hz from the rane gondola of

Querus and Fagus during a period of 50 minutes.

Piturematries were aumulated(averaged),then

the mean of the ontrol and the treated area was

alulated. Due to the given distane between

treated and non-treated Carpinus trees, there are

nodata for thisspeies (within-pitureanalysiswas

impossible). Forthe on/oexperiments (disontin-

uationof CO

2

^{supply duringonehour), theamera}

wasattahedtoapolereahingoutfromtheounter

jib of the rane in order to entre the amera over

the CO

2

^{-treated area. We took IR-pitures at a}

frequeny of1 Hzon twoloud-freedayswith little

wind. OnJune24,wesannedfrom7:30to10hours

(true loal time) and swithed o the CO

2

^supply

from 8 to 9 hours. On August 17, we monitored

temperature of a dierent part of the anopy from

7:2011:50 hours, interruptingthe CO

2

^supplyfrom

7:50to8:50andagain from9:50to 10:50hours. All

thermal images were leaned from what waslearly

identied as bakground and non-leaf temperatures

(e.g. branhes,forestsoil). In2004,thermalpitures

of the SCC FACE site (inluding a large ontrol

area) were taken from a heliopter at a. 200 m

above ground around noon on a loud-free day.

Hene we have overed a spatial range from 100

m to a few m and temporal ranges from simple

imagestoafrequenyof1Hzduringnearlyonehour.

Dataproessingandstatistialanalysis

A non-parametriWiloxon ranksum testwasused

where repliation was low and/or error distribution

wasnotnormal. Readings ofthe`TDRprobes'were

ompared by ranks, in order to avoida violation of

the assumption of normally distributed data. For

analyses of thermal images, Irbis professional (In-

frate, Dresden, Germany)was used. Thefree soft-

warepakage`R',version2.1.0(RDevelopmentCore

Team,2004)wasusedforalldataproessing, statis-

tialanalysesandgraphis.

Results

Sapow

Meanrelativesapow(sumoverall10-minreadings)

was a. 14 % lower in CO

2

^{-treated trees than in}

ontroltreesduringtwoperiodsof22daysinsummer

2004and2005(allspeiesandbothyearspooled,two

sampleWiloxontest,p-value=0.028,Figure1right

panel). In line withearlier ndingsobtainedduring

therstthree monthsofCO

2

^-enrihment

19

2004

0 200 400 600 800 1000 1200

+ 3.5 %

−22.6 %

−3.9 %

Qp Fs Cb

Sap flow (relative units)

2005

0 200 400 600 800 1000 1200

+ 2.3 %

−29.8 %

−34.5 %

Qp Fs Cb

Species pooled per year

Sap flux (relative units, standardised to spec. max) 0.0 0.2 0.4 0.6 0.8 1.0

−7.7 % −20.3 % −14.3 %

04 05 04+05

Figure1: Totalumulativesapow(relativeunits)ofQuer-

us(Qp),Fagus(Fs)andCarpinus(Cb)fromJune16toJuly

6 2004 (22 days, leftpanel) and fromJuly 20to August10

2005(entrepanel)withperenthange fromambient(white

olumns) to elevated (lled olumns) trees indiated. Error

barsrepresentonestandarderror. Rightpanel: speiespooled

forsameperiods,butwithdatastandardisedtohighestvalue

per speies and year toexlude absolutedierenes between

speies. Onlypooledspeies weretestedusing atwo sample

Wiloxontest,p-value=0.26for2004,0.16for2005and0.028

forbothyearspooled,n=9treespertreatment.

Daytime

1 − E/A (Reduction in transpiration, %)

06:00 12:00 18:00 00:00

0 5 10 15 20 25

Figure 2: The ratio of 1 minusthe CO

2

^{-treated to ambi-}

ent(1-E/A)seriesofstandardisedsapowdata(bootstrapped

dataofthe200522-dayperiod(July20toAugust10)andall

speiespooled, eahtreestandardisedto itsown meanmax-

imumwithin the period, n=9 trees per treatmen). Time is

plottedagainstoneminusthisratiowhihorrespondsto%re-

duedtranspirationoftreatedtreesomparedtoontroltrees.

Note thetwohumpsinthe morningandlate afternoon indi-

ate reduedtranspirationinCO

2

^{-treated trees,whereasthe}

middaydepressionoriginatesfromthefatthatalltreeshave

been standardisedto their ownmaximuminsap ow. This

proedureremovesanytreeortaxonspeiityofsapow,at

theprieofloosingmiddaysignals.

(Cehet al., 2003),this response waslargelydue to

Carpinus and Fagus, but statistial power was too

low for testing eets at the speies level. Querus

did not show any redution in transpiration under

elevated CO

2

^{in both years. The CO}

2

^{-eet aross}

trees wasless pronounedduring thedrier 2004pe-

riod. ApartfromtheCO

2

^{eet,treesize,anopyar-}

hitetureandsapwoodthiknessinuenethediret

sapowreadings. Standardiseddatainsteadpermit

a qualitative omparison of the sap ow responses,

irrespetiveoftheatualreadingsoftheprobes. The

mean bootstrapped 1-elevated/ambient (1-E/A) ra-

tio urvefor anaverage day outof the 2005 22-day

period(Figure2)shows16and20%watersavingsin

themorningandintheafternoonrespetively. Thisis

in loseagreementwiththewhole-daywatersavings

of thenon-standardiseddata forthis year presented

in Figure 1. The urve is obviously fored to near

zeroaround midday and thus ontainsno signalfor

this period (peak ow wasset to 1.0 irrespetive of

treatmentand tree).

The fat that the 1-E/A ow ratio urve stays

above zero at night indiates that some water sav-

ings may our even during the night, i.e. there is

lessowtoreplenishdaytimedeitsintreesexposed

to elevatedCO

2

^{duringtheday. Flowmaximaaver-}

agedin tree-spei relativesignalswerereahed a.

30minearlierinCO

2

^{-treatedtrees(Wiloxontest,p}

= 0.028), a time shift whih may explain the small

humparoundnoonin the1-E/A ratioseries(Figure

2).

To determine the dependeny of transpiration on

evaporativedemandin aCO

2

^-enrihed atmosphere, weplottedrelativesapowdataagainstvpd(Figure

3). At any given vpd, sap ow of CO

2

^treated

trees(all speies pooled) waslowerthan sapow of

ontrol trees, but there wasaa. 20 % diminishing

of the water saving eet as vpd rose from lose

to 0 to 17 hPa (translating into a 20 % inrease

in the E/A perentile ratio over this vpd range,

Figure3,bottompanel). Dierenesbetweenspeies

were pronouned: Querus showed no hange in

the vpd response urve under enrihed CO

2

^{, both}

ambientandelevatedtreesreahedtheturningpoint

(where saturation of sap ow starts) at around 4.5

hPa (Figure 3, top panel). However, in Fagus and

Carpinus, the sap ow saturation point wasshifted

markedlytotheright(highervpd)under high CO

2

^.

Forallspeiespooled,theratiooftheelevatedtothe

ambient slope (linear regression with the rst four

95

th

perentiles, see Figure 3) is 0.68, representing

the dierene in vpd-response that is attributable

0 0.2 0.4 0.6 0.8 1

Qp 4.4

0 0.2 0.4 0.6 0.8 1

Fs 4.8

5.7

0 0.2 0.4 0.6 0.8 1

Cb

7.3 4.5

0 0.2 0.4 0.6 0.8 1

All 5.7

4.6

0 0.6 0.8 1

95 ^th percentile, all species

r ² = 0.84 p < 0.001 0

0.6 0.8 1

0 5 10 15

Vapour pressure deficit (hPa)

E/A percentiles Sap flow (relative units)

Figure3: Responseofsapowtovapourpressuredeitbe-

tweenJuly20andAugust102005forQuerus,Fagus,Carpi-

nusandallspeiespooled,n=9treespertreatment. Opengrey

irlesareontroltrees(A),lledirlesCO

2

^{-treatedtrees(E).}

Thinlinesshow95

th

perentilesforvpd (lasswidth1hPa)

of ontroltrees,boldlinesrepresent the equivalentforCO

2

^-

treatedtrees. Themarkedturningpointsatwhihsaturation

starts,areinferredfromthe intersetionof twolinearregres-

sions(rstfourandlastsevenperentilelasses). Thebottom

panelshowstheE/Aratioofthe95

th

perentilesofallspeies.

Notethedereaseoftheeetasvpdinreases(translatinginto

ariseoftheE/Aperentileratio).

aerodynamis).

Soil moisture

With two out of the three methods applied to

monitorsoilmoistureundertreatedandnon-treated

trees, the mean was higher under trees exposed to

elevatedCO

2

^{thanunderontroltrees,exeptduring}

and immediately after signiantrainfall (Figure 4,

bottompanel) orafterlong periods ofdroughtsuh

as in the year 2003 (Leuzinger et al. 2005). The

spatially highly repliated measurements with the

handheld TDR probe yielded aa. 2% higher soil

moistureinthetop10mofthetreatedarea(mean

valuesofMay25andAugust122004,

x ¯

^=28.6%

± 0 . 62

s.e.,n=56intheontrolareaand

x ¯

^=30.7%

±

0.64s.e.,

n = 61

inthetreatedarea,t-test,

p < 0 . 05

).

Beauseoflessspatialrepliation,thesignalwasnot

detetable between the2x 15 ehoprobes (beause

of the high spatial variation) neither in absolute

terms nor when eah sensor was standardised to

its own maximum, i.e. its eld apaity after rain

(Wiloxontest,p-value=0.70). Finally,thetreated

versus ontrol readings of the TDR probes were

entirely apart by their ranks both in their mean

values and, more importantly, in their slopes (rate

of drying) during both 22-day periods (n=5 in the

ontrol areaandn=3in thetreatedarea,Figure4).

The relatively slowerdrying of soils duringrainless

periods in the CO

2

^{-treated area provides stronger}

evidene for redued transpiration under elevated

CO

2

^{than the mere means of absolute dierene in}

soilmoistureatonepointintime,whiharestrongly

inuened by the loal topography and varying

soil bulk density due to vole ativity. During the

nearlyompletely rainless period in 2004 (Figure 4,

upper panel), weould observethe deviation of the

twodrying rates to stop (a. day of year 175) and

evenaslightreverseeetlater(a. dayofyear182).

Canopysurfae temperature

Atonly oneoutofthe three salesat whih anopy

temperature was monitored, we were able to dis-

ern foliage temperature dierenes due to altered

leaf transpiration. Atthe smallestsale, omparing

treated and non-treated parts of the anopy within

the same image, the temperature distributions of

CO

2

^{treated and} non-treated Fagus foliage diered learly (

x control = 25 . 89 ^o

C,

x treated = 26 . 33 ^o

C;

T

air

^=24.7

o

C), suggesting less latent heat loss of

theCO

2

^{-treatedanopy.} Correspondingwithresults from sapowdata,this dierenedidnotappearin

Querus (

x control = 24 . 04 ^o

C,

x treated = 23 . 97 ^o

C;

T

air

^=23.8

o

C, Figure 5; no data for Carpinus be-

ause CO

2

^{-treated trees and ontrol trees were too}

21

10 20 30 40

0 10 20 30

166 168 170 172 174 176 178 180 182 184 186 188 E

A

P S

1 2 3 4

10 20 30 40

0 10 20 30

201 203 205 207 209 211 213 215 217 219 221 223

Precipitation, P (mm)

Day of year

Soil moisture, S (% vol)

Figure4:Soilmoisture(TDR-probes,S

A

^,S

E

^at10mdepth,

n=3elevatedareaandn=5ontrolarea)andrainfall(P)dur-

ing the two 22-dayperiods in2004 (June 16to July6) and

2005 (July20 until August10). Thik lines represent data

from the CO

2

^{enrihedarea. Note the reverse eet inthe}

nearlyompletelyrainless2004period. Thefourdryperiods

inthe bottompanelmarked1-4areusedtoalulate theav-

erageratioofslopesbetweenelevatedtoambientsoilmoisture

derease(=0.62,seetext).

far apart to be aptured in one image). At the in-

termediate sale (a. 12 m above the anopy), we

performedon/oexperiments(swithingotheCO

2

supplyforonehour),takingsequelimages(1Hz)from

theounterjib oftheraneontwoloudfreemorn-

ings. Counterexpetationsneitherinreasedsapow

signiantly,nordiditauseanopysurfaetemper-

ature to drop within the resolution of the available

equipment(Figure6). Lastly, singlethermalimages

were taken from a heliopter at a. 200 m above

ground around noon. They inluded the omplete

CO

2

^{-treatedareaplusalargeontrolareawith sim-}

ilar speies omposition. The mean temperature of

thetreatedarea(a. 2000measurementpointsaver-

aged)was21.91

o

Canddidnotdierfromthemean

oftheaveragesof12equallysizedareasloatedinad-

equate distane aroundthe CO

2

^area(21.88

∓

^0.15

o

Cs.e.,t-testp=0.5;T

air

^=24.2

o

C,Figure7).

Disussion

Two ompletely independent approahes (sap ow

and soil moisture) revealed water savings under el-

evated CO

2

^{in these adult deiduous forest trees.}

The third approah, high-resolution thermal imag-

0 5 10

15 mean = 25.89 mean = 26.33

Fagus

0 10 20 30 40

Temperature class (°C)

Relative frequency (%)

mean = 24.04 mean = 23.97

Quercus

20 21 22 23 24 25 26 27 28 29 30

ambient CO 2

elevated CO 2

Figure 5: Temperature distributions in the CO

2

^-treated

anopy (lass width = 0.5 K, blak olumns) and ontrol

anopy (white olumns) of Fagus (top panel) and Querus

(lower panel). Pitures were taken at 0.2 Hz on loudfree,

nearlyalmdaysduringnoon(windspeed<1ms

−1

.Temper- aturematrieswereoverlainandaveragedover50minutes.

thesmallestsale(pairsofpartsofelevated/ambient

tree rowns). Though not signiant, the trends in

dierenes among speies in relative sap ow were

pronounedandonrmedinitialdatasoonafterthe

onsetofCO

2

^{-enrihment(Cehetal.,2003)andmon-}

itoringwithporometryintheanopyoverfouryears

(Keeletal.,inreview)withCarpinusalwaysshowing

thestrongestresponseto CO

2

^(Figure1).

Sapowmeasurementshaveoftenbeeninterpreted

in absolutetermsperunit sapwoodareaorto alu-

late the total amount of watertranspired by a tree

(Granier, 1985). This may work in individual trees

with known sap wood properties or in homogenous,

monospei plantations with equally sized individ-

uals. Toobtainsuhqualityofdata, wewouldhave

todamageourtreesorusemoreintrusivetehniques,

whihisnotanoptionin thislong-termexperiment.

Wehadenoughtreesandsensorshowevertoobtaina

reliablerelativesapowsignalthatpointsatasignif-

iantredutionintreewateronsumptionofa. 14%

arossallyearsand speies(Figure1). Thevariane

waslarge however. This isdue to the natureof our

experimentthat representsnaturalonditionswhere

individualsdierbeyondtheirtaxonomiidentityin

size,exposure,sapwoodthiknessandanopyarhi-

teture. Whenstandardisedtotheirownmaximum,

transpiration data aross speies beome more ho-

mogenous at the expense of losingthe absolute sig-

nal at maximum sap ow (i.e. during late noon).

However,astheowrates dereaseto either side of

themiddaypeak, the standardisationbiasdereases

to loseto zeroearlyin themorningand latein the

0 0.2 0.4 0.6 0.8

1 ON Quercus OFF ON

−1 0 1

0 0.2 0.4 0.6 0.8

1 Fagus

−1 0 1

0 0.2 0.4 0.6 0.8

1 Carpinus

−1 0 1

08:00 09:00 10:00

Time of day

Deviation from mean ambient T trend (K)

Relative sap flow

24 June 2005

0 0.2 0.4 0.6 0.8

1 Quercus ON OFF

−1 0 1

0 0.2 0.4 0.6 0.8

1 Carpinus

−1 0 1

07:50 08:50 09:50 10:50

Time of day

Deviation from mean ambient T trend (K)

Relative sap flow

17 August 2005

Figure6: Relativeowanddeviationfromthemeananopy

temperaturetrend duringon/oexperimentsonJune24(up-

per panel) and August17(lower panel). Non-hathed areas

showtimeswhenCO

2

^supplywasinterrupted. Threespeies were monitored on June 24(Querus n=3, Fagus n=1 and

Carpinus n=2) and on August 17 (Querus n=3, Carpinus

n=2). During`o'periods,sapowwouldbeexpetedtoin-

reaseandtemperaturetodereaseinCO

2

^{-enrihedtrees,but}

nosuhpatternwasobservedinanyoftheshownexamples.

Figure 7: Airborneinfraredphotographofthe CO

2

^-treated

area(blakline)and12equallysizedadjaentontrolareason

aleardayinJuly2004.Themeansurfaetemperatureofthe

CO

2

^-plotis21.91

o

and doesnot dierformthe meanofthe

12ontrolareas(21.88

∓

^0.15

^o

^{Cs.e.) atanair}temperature of24.2

o

Catthetopoftherane.

tween the two mean, bootstrapped diurnal ourses

(symmetri for morning and evening, Figure 2). If

the high and low CO

2

^{-urves of the two groups of}

treeshad the sameshape,but only dieredsystem-

atiallyinabsolutevaluesbeauseofstemproperties

(sap wood thikness), the standardisation to noon

maxima should removeelevated vs. ambient dier-

ential signals at all times, whih is learly not the

ase. Theombinationoftheanalysisofboth(noisy)

raw data and standardisedrelative data from three

speiesprovidesstrongevideneforoverallwatersav-

ingsofCO

2

^{-treatedtreesofaround16%. Cehetal.}

assumeda. 10%redutionforthesametreesduring

the rst three months of CO

2

-enrihment. We now haveonsolidatedsignalsforwatersavingsafterase-

riesofyearsofgrowthunderelevatedCO

2

^{. Themag-}

nitudeoftheresponseiswithinthe13to25%range

ofsapowredutionfoundfor Liquidambarstyrai-

ua(L.)(WullshlegerandNorby2001,Shäferetal.

2002). Remarkably, the CO

2

^{-eet on sap ow was}

lowerintheoveralldrieryear2004(Figure1and4).

Thismayoriginatefromsubstantialimprovementsin

treewaterstatusinelevatedCO

2

^{duringrainlesspe-}

riods, mitigating or even reversingthe initial eet

over a transition period, before soils beome dessi-

atedunderelevatedCO

2

^{-treesaswell(Figure4,top}

panel). Prolonged redution of water onsumption

under CO

2

^{-enrihment leading to suh areverse ef-}

fetwasfoundearlierbyShäferet al. (2002).

WatersavingeetsinaCO

2

^{-enrihedatmosphere}

need to be ompared for dened evaporative on-

ditions. We found that responses were most pro-

nouned when atmospheri humidity washigh (low

vpd),whihonrmstheearlierobservationsbyCeh

23