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(2003)

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Research

Blackwell Publishing Ltd.

Photosynthesis and stomatal conductance responses of poplars to free-air CO

2

enrichment (PopFACE) during the

first growth cycle and immediately following coppice

C. J. Bernacchi

1,2

, C. Calfapietra

3

, P. A. Davey

4

, V. E. Wittig

2

, G. E. Scarascia-Mugnozza

3

, C. A. Raines

4

and S. P. Long

2

1

Photosynthesis Research Unit, Agricultural Research Service, United States Department of Agriculture, Urbana, Illinois 61801, USA:

2

Departments of Plant

Biology and Crop Sciences, University of Illinois, Urbana, Illinois 61801, USA:

3

Universitá degli Studi della Tuscia, Department of Forest Environment and

Resources (DISAFRI), Via S. Camillo de Lellis, I-01100 Viterbo, Italy;

4

Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester,

CO4 3SQ, UK

Summary

• Using the Poplar Free Air CO

2

Enrichement (PopFACE) facility we investigated theeffects of elevated [CO

2

] on the diurnal and growth cycle responses of photosyn-thesis and conductance in three poplar species.•

In situ

diurnal measurements of photosynthesis were made on

Populus alba

,

P.nigra

and

P.

×

euramericana

and, in parallel,

in vivo

maximum capacity for carboxy-lation (

V

c

,

max

) and maximum rates of electron transport (

J

max

) were determined bygas exchange measurement.• Light saturated (

A

sat

) and daily integrated (

A

′

) photosynthesis increased at elevated[CO

2

] in all species. Elevated [CO

2

] decreased

V

c

,

max

and

J

max

for

P. nigra

and

J

max

for

P.

×

euramericana

but had no effect on stomatal conductance in any of the speciesthroughout the first growth cycle. During post-coppice re-growth, elevated [CO

2

]did not increase

A

sat

in

P. nigra and P.

×

euramericana

due to large decreases in

V

c

,

max

and

J

max

.• A 50% increase in [CO

2

] under these open-air field conditions resulted in a largeand sustained increase in

A

sat

. Although there were some differences between thespecies, these had little effect on photosynthetic rates at the growth [CO

2

]. Never-theless the results show that even fast growing trees grown without rooting volumerestriction in the open may still show some down-regulation of photosyntheticpotential at elevated [CO

2

].

Key words:

Free Air CO

2

Enrichment (FACE), atmospheric change, climate change,elevated CO

2

, Rubisco, photosynthetic electron transport,

Populus

spp.

©

New Phytologist

(2003)

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: 609–621

Author for correspondence:

C. J. Bernacchi

Tel: +1 (217) 333 3040

Email: bernacch@life.uiuc.edu

Received:

12 February 2003

Accepted:

9 May 2003

doi: 10.1046/j.1469-8137.2003.00850.x

Abbreviations

A

, net carbon assimilation (µmol m

−

2

s

−

1

);

A

′

, net daily integratedcarbon assimilation (mol m

−

2

);

A

sat

,

light saturated rates of netcarbon assimilation (µmol m

−

2

s

−

1

);

C

i

, leaf intercellular CO

2

concentration (µmol mol

−

1

); [CO

2

], atmospheric CO

2

concen-tration; FACE, free air CO

2

enrichment;

g

s

, stomatal conduc-tance (mmol m

−

2

s

−

1

);

J

max

, maximum

in vivo

rates of electron

transport through photosystem II (µmol m

−

2

s

−

1

); PopFACE,poplar free air CO

2

enrichment;

Q

, photosynthetically activephoton flux (µmol m

−

2

s

−

1

); Rubisco, Ribulose-1,5-bisphosphatecarboxylase/oxygenase; RuBP, Ribulose-1,5-bisphosphate;T

leaf

, leaf temperature (

°

C);

V

c

,

max

, maximum

in vivo

velocity ofcarboxylation (µmol m

−

2

s

−

1

); VPD, vapor pressure deficit (kPa);, maximum quantum efficiency of carbon assimilation

(mol CO

2

mol

−

1

Q

).ΦCO2

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Introduction

The Intergovernmental Panel on Climate Change (Prather

et al

., 2001) predicts that atmospheric CO

2

concentration([CO

2

]) will increase 50% by the middle of this century. Thescale of this response is unprecedented in recent geologicalhistory; therefore, it has become increasingly more importantto address the effect it will have on vegetation. Becausephotosynthesis is the primary process by which carbon entersthe biosphere and by which plants sense rising [CO

2

], it iscritical to understand how it will respond to an elevated[CO

2

] environment (Drake

et al

., 1997). Forest tree speciesare a critical component of the global carbon budget acc-ounting for over half of the total net carbon uptake intoterrestrial vegetation (Geider

et al

., 2001); therefore, it isimportant to address how photosynthesis for tree species willrespond in growth in elevated [CO

2

].Current studies on tree photosynthesis responses to ele-

vated CO

2

demonstrate a wide range of responses, however,on average a 30%

−55% increase is typically observed(Curtis, 1996; Curtis & Wang, 1998). While C3 photo-synthesis shows a well-defined immediate increase in res-ponse to elevated [CO2], long-term responses are far morevariable, affected primarily by feedback of carbohydrateand nitrogen availability on expression of genes coding forthe photosynthetic apparatus and in particular Rubisco(Rogers et al., 1998; Moore et al., 1999; Stitt & Krapp,1999). A loss in photosynthetic potential, defined here asmaximum attainable photosynthetic rate in saturatinglight, 25°C, and at ambient [CO2], is often a consequenceof long-term growth in elevated [CO2] (Gunderson &Wullschleger, 1994; Sage, 1994; Drake et al., 1997; Curtis &Wang, 1998; Stitt & Krapp, 1999; Rogers & Humphries,2000; Ainsworth et al., 2002). Evidence of photosyntheticdown-regulation is usually attributed to a loss in maximumrate of carboxylation (Vc,max), suggestive of changes inRubisco content and/or activation state (Sage, 1994; Rogers& Humphries, 2000; Ainsworth et al., 2002; Maroco et al.,2002). Less commonly, photosynthetic potential may alsodecrease as a result of lower rates of electron transport toRibulose-1,5-bisphosphate (RuBP) regeneration (Jmax; Rey& Jarvis, 1997; Centritto & Jarvis, 1999; Griffin et al., 2000;Murray et al., 2000; Centritto, 2002).

Acclimation of stomatal conductance (gs) to growth in ele-vated [CO2] may also impact photosynthesis by increasing thediffusive barrier of CO2 into the leaf. Most previous work onstomatal conductance responses to elevated CO2 suggest adecrease in conductance, although these responses are gener-ally small or variable (Eamus & Jarvis, 1989; Gunderson &Wullschleger, 1994; Curtis & Wang, 1998; Lewis et al., 2002).Additionally, the responses of gs to growth in elevated CO2are shown to be highly variable depending on species, func-tional type, or plant age (Saxe et al., 1998; Mooney et al.,1999; Medlyn et al., 2001; Lewis et al., 2002).

Despite the wealth of previous studies, major limitationsexist on information for trees grown in elevated [CO2]. First,most studies have used enclosures which often show a largereffect on the vegetation than the treatment itself (Allen et al.,1992; McLeod & Long, 1999). Recent literature reviews alsosuggest that photosynthetic responses to elevated [CO2] maybe highly dependent on fumigation method (Curtis, 1996;Curtis & Wang, 1998; Ainsworth et al., 2002). Secondly,most studies concern juvenile individuals and are conductedbefore canopy closure; yet most terrestrial carbon assimilationis by mature trees in closed canopies (Lee & Jarvis, 1995;Norby et al., 1999). The use of Free Air CO2 Enrichment(FACE) technology provides the opportunity to determineplant responses to elevated [CO2] under field conditionswithout any direct alterations in the canopy microclimate(Hendrey et al., 1993; Ellsworth et al., 1997; McLeod &Long, 1999; Gunderson et al., 2002) and allows trees to growto canopy closure. Recent studies that address photosyntheticresponses of tree species to growth in FACE systems generallyshow a lack of photosynthetic acclimation responses (Tognettiet al., 1999; Herrick & Thomas, 2001; Gunderson et al.,2002); although pine trees exposed to elevated [CO2] usingFACE have shown some down-regulation (Ellsworth et al.,1997). Two such FACE facilities, the Duke Forest loblollypine and the Oak Ridge sweetgum FACE experiments, beganfumigation when the trees were already mature and the canopywas at or close to closure (Ellsworth et al., 1997; Gundersonet al., 2002). Stand development, growth and canopy closureat elevated CO2 might result in different responses thanthose observed from facilities where CO2 fumigation beganafter canopy closure.

The Poplar Free Air CO2 Enrichment (PopFACE) experi-ment (Miglietta et al., 2001) provides a unique opportunityto address how photosynthetic responses to elevated [CO2]are affected by growth from planting to canopy closure and toharvesting under open-air elevation of [CO2]. Poplars are sofast growing that they provide a rare opportunity to grow aplantation forest from planting to canopy closure of tall trees(> 9 m) in just 3 yr. Because PopFACE is a coppice system, itis also possible to determine the effect of elevated [CO2] onre-growth immediately after coppicing when the source-sinkbalances in these species have been significantly altered. Therewere two objectives of this study, to determine: diurnal andgrowth cycle responses of photosynthesis and conductancefor three poplar species grown in elevated [CO2]; and theresponse of leaf photosynthesis to elevated [CO2] duringpostcoppice re-growth.

Materials and Methods

The PopFACE facility is located near Viterbo in Central Italy(Tuscania; 42°22′-N, 11°48′-E, alt. 150 m). The 9 ha fieldwas originally used for wheat cultivation and consists of aheavy loam soil. The entire field was planted with a poplar

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hybrid, Populus × euramericana Dode (Guinier) (P. deltoidesBart. ex Marsh. × P. nigra L., I-214) at 2 m × 1 m spacing(5000 trees ha−1), with the exception of six 30 × 30 m squareplots. Within each plot, a 22-m diameter ring which includedabout 350 individual trees was established with three suchrings serving as controls (370 µmol mol−1) and three receiving[CO2] enrichment (550 µmol mol−1). Within each elevatedplot, pipes were installed in a 22-m diameter ring in order toelevate [CO2]. The pipes release CO2 in a precisely controlledmanner according to wind speed and direction to achieve auniform elevation of [CO2] within the three treatment plots.The construction and performance of this Free-Air CO2Enrichment (FACE) system is described in detail by Migliettaet al. (2001). Each ring was divided into six equal segments,pairs of opposing segments were planted to a single cloneof each of three poplar species P. alba L. (genotype 2AS11),P. nigra L. (genotype Jean Pourtet) and P. × euramericana(genotype I-214). Planting density within the experimentalplots was 1 m × 1 m spacing (104 trees ha−1). A drip irrigationsystem was installed to prevent drought-stress and was usedregularly to maintain soil moisture. Detailed informationon plant material and on plantation layout is presented inScarascia-Mugnozza et al. (2000) and Calfapietra et al. (2001).CO2 fumigation began immediately after planting in 1999and was maintained from bud burst (March) until leafsenescence (November) of each year. The trees were coppicedbetween the end of October 2001 and February 2002 and thefumigation was continued in March 2002 for the re-growth.

Gas exchange measurements

A vs Ci response Measurements of A vs Ci were first per-formed in late August, 1999 and were repeated during themonths of May, July and September, for the 2000 and 2001growing seasons and in June 2002, following the coppice. Theyoungest fully expanded mature leaves from two leaderbranches per species per plot were collected predawn andplaced in individual 0.5 l containers in the dark. The petiolesof the leaves remained under water from the time of cuttinguntil the measurements were completed. It should be notedthat cutting the leaves predawn and analyzing them in acontrolled environment measured the maximum potentialresponse of leaf photosynthesis. This procedure avoidedphotoinhibition, water stress or triose-phosphate utilizationlimitation that might develop over the diurnal course andensured that differences reflected long-term acclimationrather than short-term diurnal effects. Stomatal conductanceand rates of photosynthesis measured in these leaves equaledor exceeded rates measured in situ, suggesting that thisprocedure did not cause any loss in photosynthetic potential.

Gas exchange measurements were made using a portablegas exchange system (Li-Cor 6400; Li-Cor, Inc., Lincoln, NE,USA). The gas exchange system was zeroed daily using anhy-drous calcium carbonate (Drierite, W.A. Hammond Drierite

Company, Ltd, Xenia, OH, USA) to remove water and usingsoda lime (sofnolime granules, Morgan Medical, Ltd, Kent,UK) to remove CO2 from the air entering the cuvette. Leaftemperatures were set at 25°C for all measurements, thoughactual temperature ranged from 25 to 30°C. Leaves were illum-inated using a red-blue light source attached to the gas-exchangesystem and photosynthetically active photon flux density (Q)was maintained between 1000 and 1500 µmol m−2 s−1, depend-ing on measurement date, for the duration of the A vs Ci res-ponse curve. Levels of Q varied based on saturating light levelsderived from photosynthetic light response curves measuredbefore experimental sampling. By using light levels slightlyabove saturating, we were able to reduce the chances of induc-ing photoinhibition during the measurements. Leaf vaporpressure deficits were maintained between 0.5 to approx. 1.6 kPa.Measurements of A were made starting at 400 µmol mol−1 CO2surrounding the leaf, decreased stepwise to 50 µmol mol−1,returned to 400 µmol mol−1, and increased stepwise to 1600 µmolmol−1 CO2. Each complete curve consisted of at least eightseparate measurements. Values for A and Ci were calculatedusing the equations of von Caemmerer & Farquhar (1981)and were used to solve for Vc,max and Jmax using the equations ofFarquhar et al. (1980). When necessary, measurements werecorrected to 25°C using the temperature responses of Bernacchiet al. (2001) and Bernacchi et al. (2003) for the Rubisco andRuBP-limited portions of the A vs Ci curves, respectively.

Asat Sampling and measurements of A vs Q curves were per-formed in September 2000 and in May, July and Septemberduring the 2001 growing season and in June 2002, followingthe coppice. Leaves were sampled as described for A vs Cimeasurements above. Leaves were placed in the cuvette andilluminated until steady-state light-saturated rates of photo-synthesis and stomatal conductance were achieved; typicallythis required 5 min. The [CO2] surrounding the leaf was setto the growth [CO2] for each treatment. Plots of A vs Q werethen measured starting at saturating light (2000 µmol m−2 s−1)and decreasing stepwise to complete darkness. Curves consistedof a minimum of 13 measurements at different Q and wereusually completed within three hours of leaf collection.Completed A vs Q curves were fitted to a nonrectangularempirical function to estimate light saturated photosynthesis(Asat). The apparent maximum quantum efficiency of CO2assimilation ( ) was calculated as the slope of A measuredat a range of Q below 100 µmol m−2 s−1 where data points didnot deviate from linearity (Long et al., 1993). For eachmeasured curve, values of stomatal conductance (gs) weretaken from the photosynthesis measurement made at a Q of1500 µmol m−2 s−1, which was always saturating.

Diurnal photosynthesis

The diurnal response of leaf photosynthesis was measuredthree times during the 2000 growing season, mid-May, mid-July

ΦCO2

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and mid-September. Beginning predawn and finishing afterdusk, photosynthesis of three leaves per species was measuredin one ambient and one elevated plot; measurements wererotated among the three blocks at approx. 1.5 h intervals.Measurements were made using a portable gas exchangesystem (Li-Cor 6400, Li-Cor Inc.) with a clear chamber headto allow for natural sunlight to illuminate the leaf. Measure-ments were made at the respective growth [CO2] of 370 µmolmol−1 or 550 µmol mol−1. Measurements were confined tothe youngest fully expanded leaf of the leader branch as for themeasurements of A vs Ci above. These leaves constituted thebulk of the plant canopy surface. The leaves were measured ina horizontal position to minimize within leaf variation and theeffect of leaf angle on incident photon flux (Garcia et al.,1998). To ensure steady-state photosynthesis measurementswhile avoiding significant increases in leaf temperature, astabilization period of at least 40 s was used before measure-ments (Long et al., 1996). Dew was observed on the leavesmost mornings; visible moisture was removed with tissuesbefore measurement. Mid-day values of stomatal conduc-tance were analyzed to determine whether elevated [CO2]resulted in any differences in situ. Daily integrated rates ofphotosynthesis, A′, were estimated from the diurnal mea-surements of photosynthesis by fitting a fourth orderpolynomial to the data collected. The area under the fourthorder curve was estimated in 15-min increments and summedfor the whole day.

Modeled values

To determine whether changes in Vc,max and Jmax could predictresponses of assimilation over the diurnal course, values ofVc,max and Jmax as estimated from the A vs Ci curves were usedto predict diurnal rates of photosynthesis during July, 2000.This date was chosen because this was when diurnalmeasurements and A vs Ci measurements coincided mostclosely. For each measured diurnal time period, modeledvalues of A were estimated using the leaf model of pho-tosynthesis (Farquhar et al., 1980) parameterized using theequations for Rubisco-limited A from Bernacchi et al. (2001)and for RuBP regeneration-limited A from Bernacchi (2003).The measured and modeled values of A were then comparedby coplots of their diurnal variation.

Statistical analysis

Results of Asat, gs, Vc,max and Jmax determinations for controland elevated [CO2] grown leaves were analyzed using acomplete block repeated measures using species,time and [CO2] treatment as main effects (Mixed Procedure,The SAS System 8.1; SAS Institute, Cary, NC, USA). Pre-determined contrasts between control and elevated [CO2]were performed on each species separately, regardless whetherspecies by treatment interactions were shown to be statistically

different between the two treatments. Measurements made,after the coppice, in June 2002 were analyzed separately fromthe precoppice measurements using a complete block ANOVAwith species and treatment as main effects.

A comparison of regressions technique was used to test dif-ferences in diurnal rates of photosynthesis among the ambientand elevated grown [CO2] treatments (Mead & Curnow,1983). Each day yielded only one value of A′ for each of thethree species in the control and elevated [CO2] treatments.Therefore, the interactions between date of measurement andspecies were not possible to test statistically. The comparisonof regressions technique required that the diurnal plots werefit to a polynomial that accurately reflects the diurnal changesin photosynthesis. A 4th order polynomial was shown to fitthis criteria and was fitted first to both treatments togetherand then to each treatment separately. The 4th order acc-ounted for significantly more variation than lower order poly-nomials, while 5th and higher order polynomials did notaccount for significantly more variation than the 4th order.The goodness-of-fit for each separate curve was comparedwith the goodness-of-fit for one line representing all data. Thetype 3 sum of squares (i.e. error variances around each model)and associated degrees of freedom for lumped vs separate fitswere used to compute F-ratios associated with tests of homo-geneity of the fitted polynomials (Mead & Curnow, 1983;Potvin et al., 1990). Significance values were set a priori at P< 0.1; this value is justified by the low level of replication, thatis, three plots per treatment.

Results

Pre-coppice 1999–2001

Growing three poplar species under field conditions in anatmosphere with [CO2] predicted for 2050 resulted inincreased light-saturated rates of leaf photosynthesis (Asat;Fig. 1, Table 1). Statistical analysis of the data shows Asat to behighly variable with time and a significant time by treatmentinteraction was observed (Table 1). The statistically signi-ficant time by treatment interaction is likely caused by therelatively lower stimulation in Asat with elevated [CO2] duringthe initial measurements in the 2001 growing season. Pre-determined treatment contrasts were performed on eachspecies separately and all show statistically significant increasesin Asat with growth in elevated CO2 (P < 0.001 for P. alba andP. nigra, P < 0.005 for P. × euramericana). Results for themaximum quantum efficiency of carbon assimilation ( )are similar to the results observed for Asat, with the only excep-tion being that there was no treatment by measurement dateinteraction observed (Fig. 2, Table 1). Growth in elevated[CO2] only altered leaf level stomatal conductance for thespecies by measurement date interactions; there was noobserved effect of [CO2] whether extracted from thephotosynthetic light response curves measured on detached

ΦCO2

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leaves or from mid-day measurement of attached leaves(Fig. 3, Table 1).

Summarizing the results from A vs Ci curves for the dura-tion of the experiment, a statistically significant decreasein Vc,max was observed for plants grown in elevated CO2(Table 1, Fig. 4) though individual contrasts show that onlyP. nigra demonstrated a decrease in Vc,max (P < 0.01) withgrowth in elevated [CO2]. Measured values of Vc,max were

shown to change for all three species over measurement date(Fig. 4, Table 1). A statistically significant decrease in Jmaxwith growth in elevated [CO2] was also observed for the dura-tion of the experiment (Fig. 5; Table 1), however, individualcomparisons show that only P. alba (P < 0.03) and P. nigra(P < 0.04) resulted in a down-regulation of Jmax. Over theduration of the experiment, Jmax also varied with measurementdate for all species.

Table 1 Repeated measures analysis of variance of Vc,max, Jmax, , Asat, and gs for two species and one hybrid of poplar grown in control (370 µmol mol−1) and elevated (550 µmol mol−1) CO2 concentrations in FACE with species, date and treatment as main effects. There were three replicate plots per treatment for the poplar species with a minimum of two subsamples per plot

Effects Asat gs Vc,max Jmax

Species < 0.001 < 0.05 < 0.01 ns nsTreatment < 0.001 < 0.0001 ns < 0.001 < 0.01Species × treatment ns ns ns ns nsDate < 0.001 < 0.001 < 0.001 < 0.001 < 0.001Species × Date 0.0217 0.0045 0.0603 0.0012 < 0.001Treatment × Date < 0.005 ns ns ns nsSpecies × Treatment × Date ns ns ns ns ns

ΦCO2

ΦCO2

Fig. 1 Light saturated rates of photosynthesis (Asat) at 25°C for three species of poplar grown in ambient or elevated CO2 at the end of the 2000 and throughout the 2001 growing season. Each point represents the mean of three replicate plots with a minimum of two subsamples per plot. Bars on the right-hand side show the averages across all dates.

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To assess the in situ responses of growth in elevated [CO2],diurnal patterns of photosynthesis were measured at threepoints during the 2000 growing season. For each of thesethree dates, patterns of Q, vapor pressure deficit (VPD), and

leaf temperature (Tleaf) were measured and are presented inFig. 6. Growth and measurement in elevated [CO2] resultedin higher rates of photosynthesis for all three species over mostof the diurnal course on each day (Fig. 6, Table 2). Diurnal

Fig. 2 Maximum quantum efficiency of CO2 assimilation ( ) at 25°C for three species of poplar grown in ambient or elevated CO2 at the end of the 2000 and throughout the 2001 growing season. Each point represents the mean of three replicate plots with a minimum of two subsamples per plot. Bars on the right-hand side show the averages across all dates. CO2 concentrations and symbols are as in Fig. 1.

ΦCO2

Table 2 The daily integral of net carbon assimilation (A′) for two species and one hybrid of poplar for three measurement days. Values were determined by estimating the area under a fourth order polynomial fitted to diurnal measurements of photosynthesis. P-values are for comparison of regressions technique on diurnal photosynthesis measurements (Mead & Curnow, 1983, see Materials and Methods for details)

Control mol m−2

Elevated CO2 mol m−2 % Increase P-values

May, 2000 P. alba 0.55 0.82 50.4 < 0.01P. nigra 0.64 0.90 39.8 < 0.01P. × euramericana 0.63 1.00 57.9 < 0.001

July, 2000 P. alba 0.47 0.84 77.2 < 0.001P. nigra 0.69 1.02 48.9 < 0.001P. × euramericana 0.85 1.16 35.3 < 0.001

September, 2000 P. alba 0.70 1.15 64.1 < 0.05P. nigra 0.66 1.24 86.5 < 0.001P. × euramericana 0.76 1.09 43.7 < 0.001

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Fig. 3 Stomatal conductance (gs) for three species of poplar grown in ambient (open symbols) or elevated (filled symbols) CO2 at the end of the 2000 and throughout the 2001 growing season. Each point represents the mean of three replicate plots with a minimum of two subsamples per plot. Circular symbols represent gs values extracted from photosynthetic light response curves at 25°C and square symbols represent mid-day in situ gs at ambient temperature. Bars on the right-hand side show the averages across all dates. CO2 concentrations and symbols are as in Fig. 1.

Fig. 4 Maximum velocity of carboxylation (Vc,max) at 25°C for three species of poplar grown in ambient (open symbols) or elevated (filled symbols) CO2 through one complete coppice rotation (three growing seasons). Each point represents the mean of three replicate plots with a minimum of two subsamples per plot (± 1 standard error). Bars on the right-hand side show the averages across all dates. CO2 concentrations are ∼370 µmol mol−1 for the control plots and ∼550 µmol mol−1 for the elevated plots.

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photosynthesis measured in situ was closely predicted from Vc,maxand Jmax measured on detached leaves from the same popula-tion. The only exception was in the late afternoon for P. alba whenthe modeled value over-estimates the in situ value (Fig. 6).

Integrated over the day, growth at elevated [CO2] resultedin an increase in daily carbon uptake for upper canopy leavesranging from 35% to 86%, depending on species and measure-ment date (Table 2). While there were not enough degrees offreedom to statistically compare the effect of elevated [CO2]among the three species, there were some noticeable trends. P.× euramericana apparently showed the highest daily integralsof net CO2 uptake (A ′ ) for both treatments, but the relativestimulation by elevated [CO2] was only 46%. P. alba showedthe lowest values of A′, though the relative stimulation was thelargest (Table 2).

Post-coppice 2002

Re-growth after coppicing shows strong evidence of photo-synthetic down-regulation for P. nigra and P. × euramericanawhere photosynthesis for leaves grown and measured inelevated [CO2] are similar to leaves grown and measured incurrent ambient [CO2] (Fig. 7). Since elevated [CO2] onlyresulted in a statistically significant decrease in gs for P. nigra(F1,10 = 18.85, P < 0.002; Fig. 7) the responses of A likelyresulted from down-regulation of Vc,max (F1,26 = 9.23, P < 0.006)and Jmax (F1,26 = 5.13, P < 0.04; Fig. 8). Pre-determined

contrasts revealed that P. nigra had the strongest down-regulation response of both Vc,max and Jmax, while only Vc,maxwas observed to down-regulate for P. × euramericana and nodown-regulation was apparent for P. alba (Fig. 8).

Discussion

The results show that even fast growing trees grown withoutrooting volume restriction in the open may still show a down-regulation of photosynthetic potential at elevated [CO2].Despite down-regulation for two of the species, all speciesshowed higher rates of carbon uptake for the upper canopyleaves when grown under elevated [CO2]. Against expectationthere was no significant effect of growth at elevated [CO2] onstomatal conductance. In sharp contrast to all measurementsbefore coppicing, an increase in Asat with growth in elevated[CO2] was not apparent for P. × euramericana and P. nigraduring re-growth after the first coppice. This is explained by thelarge decrease in photosynthetic potential in these two genotypes.The following will address the responses of photosynthesis,conductance, and acclimation for the first growth cycle(1999–2001) and then for the postcoppice re-growth (2002).

Pre-coppice 1999–2001

Elevated [CO2] stimulated photosynthesis for all three speciesof poplar for the entire first growing cycle. Reviews of tree

Fig. 5 Maximum in vivo rates of electron transport through photosystem II (Jmax) at 25°C for three species of poplar grown in ambient or elevated CO2 through one complete coppice rotation (three growing seasons). Each point represents the mean of three replicate plots with a minimum of two subsamples per plot. Bars on the right-hand side show the averages across all dates. CO2 concentrations and symbols are as in Fig. 1.

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responses to elevated [CO2] show a mean increase in Asatranging from 44 to 63%, though the response is highlyvariable (Gunderson & Wullschleger, 1994; Norby et al.,1999). These reviews, however, are heavily weighted to studieson isolated seedlings and saplings grown in artificialconditions with CO2 at or above 700 µmol mol−1 comparedwith 550 µmol mol−1 used in this FACE study. The meanresponse for the three poplars presented in this study is anapproximate 38% increase in Asat measured at 25°C (Fig. 1).This increase in Asat for a 49% increase in [CO2] is likely closeto the maximum theoretically possible without up-regulationof photosynthetic capacity (Long, 1991). Additionally,elevated [CO2] resulted in significantly higher values of for all three species (Fig. 2); this is commonly predicted tooccur for plants growing under [CO2] enrichment (Long &Drake, 1991; Osborne et al., 1998). This increase in isattributed to a decrease in photorespiration as the availabilityof CO2 at the Rubisco active site increases (Drake et al., 1997).

A range of gs responses to growth in elevated [CO2] havebeen reported for woody plants (Ceulemans & Mousseau,

1994; Drake et al., 1997; Saxe et al., 1998; Medlyn et al.,2001; Lewis et al., 2002). In a meta-analysis by Curtis &Wang (1998), a 10% decrease in gs was reported for woodyplants grown in elevated [CO2], however, this response wasnot statistically significant. Previous studies addressing theresponses of elevated [CO2] on gs specifically for poplar rangefrom a significant decrease to no effect (Radoglou & Jarvis,1990; Loats & Rebbeck, 1999; Tognetti et al., 1999). However,as one of only two studies (FACTS-II; Dickson et al., 2001)to grow a forest canopy from sapling to canopy closure underopen-air elevation of [CO2], lack of response in gs (Fig. 3) maycall for some re-evaluation of expectations of forest transpira-tion in a future elevated [CO2] world (Bucher et al., 2000).

There was significant photosynthetic down-regulation forP. nigra, Vc,max and Jmax, and for P. alba, Jmax (Figs 4 and 5).These decreases in photosynthetic potential were not suffi-cient to offset the stimulation of A by elevated [CO2]. ThusAsat for leaves grown and measured at elevated [CO2] alwaysexceeded those of leaves grown and measured at ambient[CO2] (Fig. 1). Previous studies on woody species exposed to

Fig. 6 The diurnal courses of leaf temperature (Tleaf), vapor pressure deficit (VPD) and photosynthetically active photon flux (Q) (top panels). The diurnal progression of photosynthesis for the same dates during the 2000 growing season (lower three panels). For the July measurements, rates of diurnal photosynthesis predicted using the leaf model of photosynthesis are represented by the solid lines overlaying the symbols for both CO2 treatments. Each symbol represents the mean (± 1 SE) of three subsamples. CO2 concentrations and symbols are as in Fig. 1.

ΦCO2

ΦCO2

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elevated [CO2] using growth-chambers or glasshouses havedemonstrated more significant down-regulation responses(Pettersson & McDonald, 1992; Gunderson & Wullschleger,1994; Sage, 1994; Loats & Rebbeck, 1999; Norby et al., 1999).One of the few other studies that address long-term tree responsesto FACE did not show any evidence of photosynthetic down-regulation after 3 yr of growth in elevated [CO2] (Liquidambarstyraciflua; Herrick & Thomas, 2001). These trees, however,were grown first at ambient [CO2] and then subjected to astep increase in [CO2] whereas at PopFACE, the trees weregrown from planting to canopy closure in elevated [CO2].

Despite the 38% increase in Asat associated with growth inelevated [CO2], daily integrated rates of in situ photosynthesiswere increased by 40 to almost 90% (Table 2, Fig. 6). This isexplained by the fact that daytime leaf temperatures were typ-ically over 30°C resulting in a larger stimulation of leaf pho-tosynthesis by elevated [CO2] than would be evident at 25°C(Long, 1991). All species showed a large increase in daily inte-grated photosynthesis despite the photosynthetic down-regulation responses observed for P. nigra and P. alba. Becausethere were no observable changes in LAI (Gielen et al., 2001),it is likely that the increase in A′ resulted in higher rates of car-bon assimilation at the canopy level for all three species. Thestimulation in photosynthesis with growth under elevated[CO2] is consistent with the large increases in above- andbelow-ground biomass for these three species at PopFACE(Calfapietra et al., 2003).

The strong agreement between the modeled and measuredrates of A throughout the diurnal time course shows thatVc,max and Jmax closely predict A under field conditions. P. alba,however, shows a period in the late afternoon when the mod-eled and measured values do not agree. Specifically, the mod-eled values suggest that rates should be higher than measuredin the field for the elevated [CO2] treatment. Higher rates ofphotosynthesis associated with the elevated [CO2] wouldlikely result in the accumulation of triose phosphates in leavescompared with the control treatments (Rogers et al., 1998;Isopp et al., 2000) and could lead to TPU-limitation ofphotosynthesis (Sharkey et al., 1986).

Post-coppice 2002

Despite only modest down-regulation responses of photo-synthesis during the first rotation cycle, the spring followingthe harvest resulted in large down-regulation responses fortwo species such that elevated [CO2] did not stimulate ratesof carbon assimilation compared with the control (Fig. 7). Ofthe three species, P. nigra shows the strongest evidence ofdown-regulation for both Vc,max and Jmax while P. × euramericanashowed a down-regulation response for only Vc,max (Fig. 8).Coupled with our precoppice measurements (Figs 4 and 5)these results suggest that P. nigra may be more sink limitedthan the other two species, i.e. it has less capacity to utilize the

Fig. 7 Estimates of the light saturated rates of photosynthesis (Asat) and stomatal conductance (gs) at 25°C for the three species of poplar grown under ambient or elevated concentrations of atmospheric CO2. Data was collected in June 2002 following coppicing in October, 2001. Each bar represents the mean of three replicates and two subsamples per replicate. CO2 concentrations, symbols and error bars are same as Fig. 1. Unmarked treatment comparisons show no statistically significant differences; marked are significant at the P < 0.05 level (**).

Fig. 8 The maximum rate of carboxylation (Vc,max) and maximum rate of electron transport (Jmax) at 25°C for the three species of poplar grown under ambient (open bars) or elevated (closed bars) concentrations of atmospheric CO2. Data was collected in June 2002 following coppicing in October, 2001. Each bar represents the mean of three replicates and two subsamples per replicate. CO2 concentrations, symbols and error bars are same as Fig. 1. Unmarked treatment comparisons show no statistically significant differences; marked are significant at the P < 0.1 level (*) or the P < 0.05 level (**).

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additional photosynthate generated under elevated CO2. Thisalso corresponds with the observation that P. nigra is the onlyspecies to show a significant decrease in gs (Fig. 7). It is likelythat the decrease in gs is the result, rather than a cause, of theloss of photosynthetic potential, as has been demonstratedpreviously for soybean (Fiscus et al., 1997) and for otherpoplars (Noormets et al., 2001).

As a result of the down-regulation responses, the enhance-ment of net photosynthesis by decreased photorespirationunder elevated [CO2] is offset by decreased photosyntheticpotential during the initial stages of re-growth. Coppice re-growth depends on remobilization of reserves from the vascu-lar parenchyma of the root system. Calfapietra et al. (2003)have shown that root biomass of these trees was increased by20–30% under elevated [CO2]. If the coppice developed inelevated [CO2] contains a higher reserve of carbohydrate,then it is possible that more soluble carbohydrates are trans-located to the regrowing shoot and that this causes a down-regulation of photosynthetic potential for P. nigra and P. ×euramericana. Increased soluble carbohydrate concentrationsin the leaf have been linked to suppression of some of thegenes coding for the photosynthetic apparatus, in particularrbcS which codes for the small subunit of Rubisco (Sheen,1990; Drake et al., 1997; Moore et al., 1998, 1999). This mayexplain the photosynthetic down-regulation observed forP. nigra and P. × euramericana grown in the elevated [CO2]treatment during re-growth compared with the first rotationcycle when the roots and shoots were developing simultane-ously. P. alba, however, did not demonstrate a down-regulationin photosynthetic potential, though root biomass was alsohigher in elevated [CO2] for this species (Calfapietra et al.,2003).

Acknowledgements

This research was conducted within the framework of theEU-POPFACE (ENV4-CT97-0657) program. Research waspartially funded by the Integrative Photosynthesis Researchgrant to CJB (NSF DBI96-02249) and by the SpecialUndergraduate Research on the Environment (SURE) fromthe University of Illinois Environmental Council to VEW.The authors acknowledge Patrick Morgan, Shawna Naidu,David Moore, Emily Heaton, and Andrew Leakey for com-ments on this manuscript, Franco Miglietta for design andmaintenance of the PopFACE Facility, and Beverly Bryant forassisting with measurements. We further acknowledge GuyBernacchi and Best Travel and Tours, Inc. of Chicago, IL USAfor supplying airfare.

ReferencesAinsworth EA, Davey PA, Bernacchi CJ, Dermody OC, Heaton EA,

Moore DJ, Morgan PB, Naidu SL, Ra HY, Zhu X, Curtis PS, Long SP. 2002. A meta-analysis of elevated [CO2] effects on soybean (Glycine max) physiology, growth and yield. Global Change Biology 8: 695–709.

Allen LH Jr, Drake BG, Rogers HH, Shinn JH. 1992. Field techniques for exposure of plants and ecosystems to elevated CO2 and other trace gases. Critical Reviews in Plant Sciences 11: 85–119.

Bernacchi CJ, Pimentel C, Long SP. 2003. In vivo temperature response functions for parameters required to model RuBP-limited photosynthesis. Plant, Cell & Environment in press:.

Bernacchi CJ, Singsaas EL, Pimental C, Portis AR, Long SP. 2001. Improved temperature response functions for models of Rubisco-limited photosynthesis. Plant, Cell & Environment 24: 253–259.

Bucher WIK, Sonnleitner MA, Egli P, Gunthardt GMS, Tarjan D, Schulin R, Bucher JB. 2000. Effects of elevated CO2, increased nitrogen deposition and soil on evapotranspiration and water use efficiency of spruce-beech model ecosystems. Phyton 40: 49–60.

von Caemmerer S, Farquhar GD. 1981. Some relationships between the biochemistry of photosynthesis and the gas-exchange of leaves. Planta 153: 376–387.

Calfapietra C, Gielen B, Galema ANJ, Lukac M, De Angelis P, Moscatelli MC, Ceulemans R, Scarascia-Mugnozza G. 2003. Free-air CO2 enrichment (FACE) enhances biomass production in a short-rotation poplar plantation (POPFACE). Tree Physiology (in press.)

Calfapietra C, Gielen B, Sabatti M, De Angelis P, Scarascia Mugnozza G, Ceulemans R. 2001. Growth performance of Populus exposed to ‘Free Air Carbon dioxide Enrichment’ during the first growing season in the POPFACE experiment. Annals of Forest Science 58: 819–828.

Centritto M. 2002. The effects of elevated (CO2) and water availability on growth and physiology of peach (Prunus persica) plants. Plant Biosystems 136: 177–188.

Centritto M, Jarvis PG. 1999. Long-term effects of elevated carbon dioxide concentration and provenance on four clones of Sitka spruce (Picea sitchensis). II. Photosynthetic capacity and nitrogen use efficiency. Tree Physiology 19: 807–814.

Ceulemans R, Mousseau M. 1994. Effects of elevated atmospheric CO2 on woody plants. New Phytologist 127: 425–446.

Curtis PS. 1996. A meta-analysis of leaf gas exchange and nitrogen in trees grown under elevated carbon dioxide. Plant, Cell & Environment 19: 127–137.

Curtis PS, Wang XZ. 1998. A meta-analysis of elevated CO2 effects on woody plant mass, form, and physiology. Oecologia 113: 299–313.

Dickson RE, Lewin KF, Isebrands JG, Coleman MD, Heilman WE, Riemenschneider DE, Sober J, Host GE, Zak DR, Hendrey GR, Pregitzer KS, Karnosky DF. 2001. Forest Atmosphere Carbon Transfer and Storage – II (FACTS-II) – the Aspen Free-Air CO2 and O3 Enrichment (FACE) Project: an Overview. General Technical Report NC-214, North Central Experiment Station. St. Paul, MN, USA: USDA Forest Service.

Drake BG, Gonzalez-Meler MA, Long SP. 1997. More efficient plants: a consequence of rising atmospheric CO2? Annual Review of Plant Physiology and Plant Molecular Biology 48: 609–639.

Eamus D, Jarvis PG. 1989. The direct effects of increase in the global atmospheric CO2 concentration on natural and commercial temperate trees and forests. Advances in Ecological Research 19: 1–55.

Ellsworth DS, LaRoche J, Hendrey GR. 1997. Photosynthesis and Leaf Nitrogen in a Maturing Pine Forest Under Free-Air CO2 Enrichment (FACE). Report BNL #61741. Department of Applied Science, Brookhaven National Laboratory, Upton, NY, USA: Brook haven National Laboratory.

Farquhar GD, von Caemmerer S, Berry JA. 1980. A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species. Planta 149: 78–90.

Fiscus EL, Reid CD, Miller JE, Heagle AS. 1997. Elevated CO2 reduces O3 flux and O3-induced yield losses in soybeans: possible implications for elevated CO2 studies. Journal of Experimental Botany 48: 313.

Garcia RL, Long SP, Wall GW, Osborne CP, Kimball BA, Nie GY, Pinter PJ Jr, Lamorte L, Wechsung F. 1998. Photosynthesis and conductance of spring-wheat leaves: Field response to continuous

www.newphytologist.com © New Phytologist (2003) 159: 609–621

Research620

free-air atmospheric CO2 enrichment. Plant, Cell & Environment 21: 659–669.

Geider RJ, Delucia EH, Falkowski PG, Finzi AC, Grime JP, Grace J, Kana TM, La Roche J, Long SP, Osborne BA, Platt T, Prentice IC, Raven JA, Schlesinger WH, Smetacek V, Stuart V, Sathyendranath S, Thomas RB, Vogelmann TC, Williams P, Woodward FI. 2001. Primary productivity of planet earth: biological determinants and physical constraints in terrestrial and aquatic habitats. Global Change Biology 7: 849–882.

Gielen B, Calfapietra C, Sabatti M, Ceulemans R. 2001. Leaf area dynamics in a closed poplar plantation under free-air carbon dioxide enrichment. Tree Physiology 21: 1245–1255.

Griffin KL, Tissue DT, Turnbull MH, Whitehead D. 2000. The onset of photosynthetic acclimation to elevated CO2 partial pressure in field-grown Pinus radiata D. Don. after 4 years. Plant, Cell & Environment 23: 1089–1098.

Gunderson CA, Sholtis JD, Wullschleger SD, Tissue DT, Hanson PJ, Norby RJ. 2002. Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation during 3 years of CO2 enrichment. Plant, Cell & Environment 25: 379–393.

Gunderson CA, Wullschleger SD. 1994. Photosynthetic acclimation in trees to rising atmospheric CO2: a broader perspective. Photosynthesis Research 39: 369–388.

Hendrey GR, Lewin KF, Nagy J. 1993. Free air carbon dioxide enrichment: development, progress, results. Vegetatio 104/105: 17–31.

Herrick JD, Thomas RB. 2001. No photosynthetic down-regulation in sweetgum trees (Liquidambar styraciflua L.) after three years of CO2 enrichment at the Duke Forest FACE experiment. Plant, Cell & Environment 24: 53–64.

Isopp H, Frehner M, Almeida JPF, Blum H, Daepp M, Hartwig UA, Luscher A, Suter D, Nosberger J. 2000. Nitrogen plays a major role in leaves when source-sink relations change: C and N metabolism in Lolium perenne grown under free air CO2 enrichment. Australian Journal of Plant Physiology 27: 851–858.

Lee HSJ, Jarvis PG. 1995. Trees differ from crops and from each other in their responses to increases in CO2 concentration. Journal of Biogeography 22: 323–330.

Lewis JD, Lucash M, Olszyk DM, Tingey DT. 2002. Stomatal responses of Douglas-fir seedlings to elevated carbon dioxide and temperature during the third and fourth years of exposure. Plant, Cell & Environment 25: 1411–1421.

Loats KV, Rebbeck J. 1999. Interactive effects of ozone and elevated carbon dioxide on the growth and physiology of black cherry, green ash, and yellow-poplar seedlings. Environmental Pollution 106: 237–248.

Long SP. 1991. Modification of the response of photosynthetic productivity to rising temperature by atmospheric CO2 concentrations: Has its importance been underestimated? Plant, Cell & Environment 14: 729–739.

Long SP, Drake BG. 1991. Effect of the long-term elevation of carbon dioxide concentration in the field on the quantum yield of photosynthesis of the C3 sedge, Scirpus olneyi. Plant Physiology 96: 221–226.

Long SP, Farage PK, Garcia RL. 1996. Measurement of leaf and canopy photosynthetic CO2 exchange in the field. Journal of Experimental Botany 47: 1629–1642.

Long SP, Postl WF, Bolhar-Nordenkampf HR. 1993. Quantum yields for uptake of carbon dioxide in C3 vascular plants of contrasting habitats and taxonomic groupings. Planta 189: 226–234.

Maroco JP, Breia E, Faria T, Pereira JS, Chaves MM. 2002. Effects of long-term exposure to elevated CO2 and N fertilization on the development of photosynthetic capacity and biomass accumulation in Quercus suber L. Plant, Cell & Environment 25: 105–113.

McLeod AR, Long SP. 1999. Free-air carbon dioxide enrichment (FACE) in global change research: a review. Advances in Ecological Research 28: 1–54.

Mead R, Curnow RN. 1983. Statistical methods in agriculture and experimental biology. New York, NY, USA: Chapman & Hall.

Medlyn BE, Barton CVM, Broadmeadow MSJ, Ceulemans R, De Angelis P, Forstreuter M, Freeman M, Jackson SB, Kellomaki S, Laitat E, Rey A, Roberntz P, Sigurdsson BD, Strassemeyer J, Wang K, Curtis PS, Jarvis PG. 2001. Stomatal conductance of forest species after long-term exposure to elevated CO2 concentration: a synthesis. New Phytologist 149: 247–264.

Miglietta F, Peressotti A, Vaccari FP, Zaldei A, deAngelis P, Scarascia-Mugnozza G. 2001. Free-air CO2 enrichment (FACE) of a poplar plantation: The POPFACE fumigation system. New Phytologist 150: 465–476.

Mooney HA, Canadell J, Chapin FS, Ehleringer J, Körner Ch, McMurtrie BE, Parton WJ, Pitelka L, Schulze E-D. 1999. Ecosystem physiology responses to global change. In: Walker BH, Steffen WL, Canadell J, Ingram JSI, eds. The terrestrial biosphere and global change. Implications for natural and managed ecosystems. Cambridge, UK: Cambridge University Press, 141–189.

Moore BD, Cheng SH, Rice J, Seemann JR. 1998. Sucrose cycling, Rubisco expression, and prediction of photosynthetic acclimation to elevated atmospheric CO2. Plant, Cell & Environment 21: 905–915.

Moore BD, Cheng SH, Sims D, Seemann JR. 1999. The biochemical and molecular basis for photosynthetic acclimation to elevated atmospheric CO2. Plant, Cell & Environment 22: 567–582.

Murray MB, Smith RI, Friend A, Jarvis PG. 2000. Effect of elevated (CO2) and varying nutrient application rates on physiology and biomass accumulation of Sitka spruce (Picea sitchensis). Tree Physiology 20: 421–434.

Noormets A, Sôber A, Pell EJ, Dickson RE, Podila GK, Sôber J, Isebrands JG, Karnosky DF. 2001. Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O3. Plant, Cell & Environment 24: 327–336.

Norby RJ, Wullschleger SD, Gunderson CA, Johnson DW, Ceulemans R. 1999. Tree responses to rising CO2 in field experiments: Implications for the future forest. Plant, Cell & Environment 22: 683–714.

Osborne CP, Laroche J, Garcia RL, Kimball BA, Wall GW, Pinter PJ Jr, Lamorte RL, Hendrey GR, Long SP. 1998. Does leaf position within a canopy affect acclimation of photosynthesis to elevated CO2? Analysis of a wheat crop under free-air CO2 enrichment. Plant Physiology 117: 1039–1045.

Pettersson R, McDonald AJS. 1992. Effects of elevated carbon dioxide concentration on photosynthesis and growth of small birch plants (Betula pendula Roth.) at optimal nutrition. Plant, Cell & Environment 115: 911–919.

Potvin C, Lechowicz MJ, Tardif S. 1990. The statistical analysis of ecophysiological response curves obtained from experiments involving repeated measures. Ecology 71: 1389–1400.

Prather M, Ehhalt D, Dentener F, Derwent R, Dlugokencky E, Holland E, Isaksen I, Katima J, Kirchhoff V, Matson P, Midgley P, Wang M. 2001. Atmospheric chemistry and greenhouse gases. In: Houghton JT, Ding Y, Griggs DJ, Noguer M, van der Linder PJ, Dai X, Maskell K, Johnson CA, eds. Climate change 2001: the scientific basis. Contribution of working group to the third assessment report of the intergovernmental panel on climate change. Cambridge, UK: Cambridge University Press, 881.

Radoglou KM, Jarvis PG. 1990. Effects of carbon dioxide enrichment on four poplar clones. II. Leaf surface properties. Annals of Botany 65: 627–632.

Rey A, Jarvis PG. 1997. An overview of long-term effects of elevated atmospheric CO2 concentration on the growth and physiology of birch (Betula pendula Roth.). Botanical Journal of Scotland 49: 325–340.

Rogers A, Fischer BU, Bryant J, Frehner M, Blum H, Raines CA, Long SP. 1998. Acclimation of photosynthesis to elevated CO2 under low-nitrogen nutrition is affected by the capacity for assimilate utilization. Perennial ryegrass under free air CO2 enrichment. Plant Physiology 188: 683–689.

© New Phytologist (2003) 159: 609–621 www.newphytologist.com

Research 621

Rogers A, Humphries SW. 2000. A mechanistic evaluation of photosynthetic acclimation at elevated CO2. Global Change Biology 6: 1005–1011.

Sage R. 1994. Acclimation of photosynthesis to increasing atmospheric CO2: The gas exchange perspective. Photosynthesis Research 39: 351–368.

Saxe H, Ellsworth DS, Heath J. 1998. Tansley review, 98. Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139: 395–436.

Scarascia-Mugnozza GE, DeAngelis P, Sabatti M, Calfapietra C, Ceulemans R, Peressotti A, Miglietta F. 2000. A FACE experiment on short rotation, intensive poplar plantation: objectives and experimental set up of POPFACE. In: Sutton MA, Moreno JM, van der Putten W, Struwe S, eds. Terrestrial ecosystem research in Europe: successes, challenges and policy. Final conference of the terrestrial ecosystem research initiative – concerted

action (TERICA). Ecosystem research report, European Commission, Luxembourg. Luxembourg: European Commission, 136–140.

Sharkey TD, Stitt M, Heineke D, Gerhard TR, Raschke K, Heldt HW. 1986. Limitation of photosynthesis by carbon metabolism. II. Oxygen-insensitive carbon dioxide uptake results from limitation of triose phosphate utilization. Plant Physiology 81: 1123–1129.

Sheen J. 1990. Metabolic repression of transcription in higher plants. Plant Cell 2: 1027–1038.

Stitt M, Krapp A. 1999. The interaction between elevated carbon dioxide and nitrogen nutrition: The physiological and molecular background. Plant, Cell & Environment 22: 583–621.

Tognetti R, Longobucco A, Raschi A, Miglietta F, Fumagalli I. 1999. Responses of two Populus clones to elevated atmospheric CO2 concentration in the field. Annals of Forest Science 56: 493–500.

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