Ecophysiological responses to simulated canopy gaps of two tree species of contrasting shade tolerance in elevated CO2

Introduction

The occurrence of gaps in forest canopies allowspatches of increased solar irradiation of relativelylong duration, 10 min or greater, to penetrate to theforest floor. Increased carbon assimilation associ-ated with such sun patches may approach 60% of thedaily carbon gain of understorey plants (Chazdon1988). The ability of suppressed understorey treesto utilize sun patches for photosynthesis andimprove their daily carbon balance contributes tocanopy recruitment of those individuals. Thus, theformation of canopy gaps can be an important factorin successional replacement of tree species (Platt &Strong 1989).

However, species may be limited in their photosyn-thetic response to canopy gaps. Sudden illumination ofshade-adapted seedlings may be accompanied byincreased leaf temperature and a greater leaf-to-airvapour pressure gradient resulting in rapid develop-ment of leaf-water deficits and stomatal closure(Chazdon 1988; Pearcy 1990). Plant adaptations orenvironmental conditions that limit leaf-water loss andmaximize carbon gain during sun patches have thepotential to facilitate sun-patch utilization of under-storey trees. Some have hypothesized that moderate-to shade-tolerant species tend to exhibit the greatestamount of plasticity in their adaptations to understoreyconditions (Teskey & Shrestha 1985; Abrams andMostoller 1995; Kubiske & Pregitzer 1996), whichmay include sun-patch utilization (Woods & Turner1971). Any factor that may affect the ability of under-storey trees to utilize these light patches efficientlymay in turn influence forest dynamics.

FunctionalEcology 199711, 24–32

© 1997 BritishEcological Society

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Ecophysiological responses to simulated canopy gaps oftwo tree species of contrasting shade tolerance inelevated CO2

M. E KUBISKE* and K. S. PREGITZERMichigan Technological University, School of Forestry and Wood Products, 1400 Townsend Drive, Houghton,MI 49931-1295, USA

Summary

1. One-year-old seedlings of shade tolerant Acer rubrum and intolerant Betulapapyrifera were grown in ambient and twice ambient (elevated) CO2, and in full sunand 80% shade for 90 days. The shaded seedlings received 30-min sun patches twiceduring the course of the day. Gas exchange and tissue–water relations were measuredat midday in the sun plants and following 20 min of exposure to full sun in the shadeplants to determine the effect of elevated CO2 on constraints to sun-patch utilization inthese species. 2. Elevated CO2 had the largest stimulation of photosynthesis in B. papyrifera sunplants and A. rubrum shade plants. 3. Higher photosynthesis per unit leaf area in sun plants than in shade plants of B.papyrifera was largely owing to differences in leaf morphology. Acer rubrumexhibited sun/shade differences in photosynthesis per unit leaf mass consistent withbiochemical acclimation to shade. 4. Betula papyrifera exhibited CO2 responses that would facilitate tolerance to leafwater deficits in large sun patches, including osmotic adjustment and higher trans-piration and stomatal conductance at a given leaf-water potential, whereas A. rubrumexhibited large increases in photosynthetic nitrogen-use efficiency.5. Results suggest that species of contrasting successional ranks respond differently toelevated CO2, in ways that are consistent with the habitats in which they typicallyoccur.

Key-words: Leaf nitrogen, osmotic adjustment, photosynthesis, stomatal conductance, water relations.

Functional Ecology (1997) 11, 24–32

* Present address: Department of Forestry, Box 9681,Mississippi State University, Mississippi State, MS 39762-9681, USA.

25Response tocanopy gaps inelevated CO2

The effect of increased atmospheric CO2 has beenwidely studied in relation to growth and physiology intrees (Gates 1983; Poorter 1993) and numerous otherplants (Ceulemans & Mousseau 1994; Curtis 1996).For example, shifting the CO2:O2 ratio in favour ofcarboxylase relative to oxygenase activity of Rubiscooften significantly increases nitrogen-use efficiency(Woodrow 1994). Elevated atmospheric CO2 alsoincreases the gradient for CO2 diffusion throughstomata thus increasing water-use efficiency and, insome cases, decreasing stomatal conductance whichmay ameliorate or delay the deleterious effects ofleaf-water deficit (Tolley & Strain 1984, 1985). Inaddition, atmospheric CO2 concentration tends toinduce changes in leaf morphology (Woodward 1987;Radoglou & Jarvis 1990) which also plays an impor-tant role in plant water balance (Abrams, Kubiske &Mostoller 1994). Less well described is the possibleeffect that elevated atmospheric CO2 may have on thetissue–water relations of tree species (Tyree &Alexander 1993). Thus, the interaction of physiologi-cal and morphological responses of understoreyseedlings to elevated CO2 may play an important rolein future forest composition and dynamics.

The purpose of this study was to test the hypothesisthat elevated atmospheric CO2 would improve thephotosynthetic performance of understorey treeseedlings exposed to sun patches owing to modifica-tion of N- and water-use efficiencies, leaf structureand tissue–water relations. A further prediction wasthat a shade-tolerant species would exhibit greaterplasticity to elevated CO2 than an intolerant speciesresulting in a greater CO2 response in shade pheno-types.

Materials and methods

EXPERIMENTAL DESIGN

One-year-old seedlings of shade intolerant Betulapapyrifera Marsh. and shade tolerant Acer rubrum L.were purchased from a commercial nursery (Michiganseed sources) in the spring of 1993. In May, twoseedlings of each species in leafless condition wereplanted in plastic lined, open-bottom wooden boxed(0·7 m3) that were buried into the C horizon of asandy, mixed, frigid Entic haplorthod (Rubicon sand)at the University of Michigan Biological Station innorthern lower MI, USA (45° 33 ' 30 "N, 84 ° 4 '28 "W). The boxes were filled with the C horizon ofRubicon sand overtopped by a 30-cm layer of mixedA horizon (Kalkaska sand, Typic haplorthod) and Chorizon (Rubicon sand, 1:2).

Open-top chambers approximately 1 m3 in volume,such as used by Curtis & Terri (1992) and Kubiske &Pregitzer (1996), were placed over the seedlings ineach soil box. Atmospheric CO2 was elevated toapproximately 700 p.p.m. in half of the chambers byinjecting pure CO2 into a blower system that provided

for two exchanges of air per min in the chambers.Crossed with CO2 in three replicate blocks, half of thechambers were also covered on the top and four sideswith neutral density shade cloth. On the south face ofeach shaded chamber and extending across the top,two parallel slots were cut about 10-cm wide and30-cm apart to allow the passage of approximately30-min sun patches across the seedlings 1·5-h apartduring midday, simulating the presence of smallcanopy gaps. This shade regime reduced daily inte-grated irradiance to about 27% of that in theunshaded chambers on cloud-free days (see Kubiske& Pregitzer 1996). Thus, the experiment consistedof 12 open-top chambers comprising four light ×CO2 treatments in each of three blocks, with sixindividuals of each species in each treatment.

DATA COLLECTION

Air temperature and incident photosynthetic photonflux density (PPFD) were recorded every 15 min ineach light treatment with a shaded thermocouple and aLI-190SA quantum sensor connected to a LI-1000data logger (LI-COR, Inc., Lincoln, NE, USA).Chamber relative humidity under each light environ-ment was monitored with a hygrothermograph thatwas calibrated every 6–7 days.

In a companion study, photosynthetic lightresponse was examined in detail, and daily C balanceestimated, among the study species (Kubiske &Pregitzer 1996). In the present study, midday eco-physiological measurements were made on each ofthe study seedlings on relatively cloud-free days (7,11 and 18 August 1993). On the morning of each mea-surement day, predawn (0500 h solar time) waterpotential (ψ) measurements were made on a B.papyrifera and A. rubrum seedling in each box with apressure chamber (Model 1000, PMS Instrument Co.,Corvallis, OR, USA). Following predawn ψ measure-ments, the open slots in the shade cloth were closed toprevent entry of direct solar radiation. Beginning at1100 h solar time on those days, the cloth wasremoved from the shaded chambers in one block at atime to simulate full irradiation under a canopy gap.This was necessary to time the illumination of thetrees accurately. Following 20 min of acclimation tofull sun, net CO2 and H2O vapour exchange, incidentPPFD and temperature were measured on a fullyexpanded, mid-crown leaf of each seedling using aLCA-2 portable photosynthesis system (ADC Co.,Ltd, Herts., UK). Two LCA-2 systems were used, onedesignated for each CO2 level on any given measure-ment day, to reduce instrument equilibration timeamong the four chambers of each block. Each LCA-2was calibrated at the CO2 concentration for which itwas dedicated on that day. The leaf cuvettes were alsocalibrated for water vapour pressure with a dew pointgenerator (Li-Cor, model LI-610, Lincoln, NE, USA)and the PAR sensors equalized against a quantum

© 1997 BritishEcological Society,Functional Ecology,11, 24–32

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sensor (Li-Cor, model 190SA). Gas-exchange measure-ments were conducted by reaching in through the top ofthe chambers with the leaf cuvette. Air supplied to theleaf cuvette was drawn from within each chamber andpassed through a desiccant (the standard configurationfor the LCA-2 system) for 5 min prior to measuringeach chamber to allow the instrument to equilibrate. Itis not likely that exposure of the leaves to dry air for thebrief period of time required for each gas-exchangemeasurement (< 2 min), resulted in a stomatal response.Immediately following each gas exchange measure-ment, the leaf was harvested and its midday ψ deter-mined with the pressure chamber. All three blocks weremeasured in random order on each day.

On the next cloud-free day following the 7 and 11August sampling days (8 and 12 August), leaves werecollected for pressure–volume (p–v) analysis. Theshade cloth was removed from all shaded chambers atabout 1100 h solar time. After 20 min, a fullyexpanded, mid-crown leaf was harvested from aseedling of each species, sealed in a dark containerwith moist paper and transported to the laboratory.Owing to logistical constraints, only shaded plantswere utilized in this analysis. Pressure–volume analy-sis was performed within 1 h of harvest by repeatedlymeasuring ψ and fresh mass as the leaves slowly driedon the laboratory bench. Leaves were not rehydratedprior to p–v analysis to prevent artificial shifts in thep–v relationship (Kubiske & Abrams 1991).

Leaf area (LI-3000 area meter) and oven dry masswithout the petiole (49 h at 80 ° C) were determinedfor calculation of specific leaf area (area/dry mass) ona leaf from each species from each chamber. Leafstomatal frequency and guard cell length were mea-sured with cellulose-acetate impressions of the abax-ial surface of a leaf from all study seedlings using alight microscope. Leaf N was determined on leavescollected from each seedling following the third sam-ple date using a Carlo Erba 1500 CN analyser.

DATA ANALYSIS

Net photosynthetic rate (A), stomatal conductance toH2O vapour (gw) and transpiration rate (E) were cal-culated according to von Caemmerer & Farquhar(1981). Instantaneous water-use efficiency (WUE)was calculated as AE –1 and nitrogen-use efficiency(NUE) was calculated as µmol CO2 assimilated permol N per s.

From moisture release curves of p–v data (X = ψ vsY = fresh mass), leaf saturated mass was determinedby extrapolating to ψ = –0.1 MPa using linear regres-sion on data above the wilting point (Kubiske &Abrams 1991). Leaf relative water content (RWC) ateach ψ measurement was calculated from the extrapo-lated saturated mass and was used to construct p–vcurves (X = RWC vs Y = ψ–1). From the p–v curves,osmotic potential at full turgor (ψπ

100) was deter-mined as the inverse Y-intercept of the straight line

region. Relative water content at zero turgor (RWC0)and osmotic potential at zero turgor (ψπ

0) were deter-mined as the X–1 and Y co-ordinates, respectively, ofthe first point in the straight line region (i.e. the turgorloss point). Tissue elastic modules (ε) was calculatedas the change in turgor pressure divided by the changein RWC for the full range from full to zero turgor,which provides a more integrated representation ofcell-wall elasticity than does maximum ε (Melkonian,Wolfe & Steponkus 1982).

Data were analysed with a split-plot analysis ofvariance in a randomized complete block (with cham-bers treated as whole units and species within chamberstreated as sub-units) and with least-squares liner regres-sion using proc GLM in SAS (SAS Institute 1985).Means were compared using Fisher’s least-significantdifference procedure at the 0·05 level of significance forpair-wise contrasts among species within a light/CO2

treatment and among treatments within each species.

Results

CHAMBER MICROCLIMATE

Atmospheric CO2 concentration in the ambient andelevated chambers averaged 374·8 ± 2·9 and 714·4 ±2·4 p.p.m., respectively, throughout the experiment.On cloud-free days, daily (500 h to 1840 h solar time)integrated PPFD in the shaded chambers was approxi-mately 27% that of the unshaded chambers, and thetwo sun patches accounted for 78% of the daily PPFDin the shaded chambers (data not shown, see Fig. 1 ofKubiske & Pregitzer 1996). On measurement days,removal of the shade cloth resulted in significantlyincreased air temperature and decreased relativehumidity, although values did not reach those of theunshaded chambers (Fig. 1). Maximum ambient airtemperature on the three measurement days (7, 11 and18 August) were 27, 35 and 37 ° C, respectively, andthe sunlit chambers averaged 2·6 ° C warmer thanambient (cf. Vogel & Curtis 1995).

PHOTOSYNTHESIS AND LEAF NITROGEN

Mean light-saturated A per unit leaf area (Aarea) wassignificantly higher in B. papyrifera than in A. rubrumin nearly all treatments (Table 1). In B. papyrifera,light-saturated Aarea was higher in sun plants thanshade plants (P < 0·05), but in A. rubrum the reversewas true in both CO2 treatments. Elevated CO2

resulted in higher light-saturated Aarea in both speciesand in both light regimes except for A. rubrum sunplants. With A expressed on a leaf mass basis (Amass),B. papyrifera seedlings exhibited similar light satu-rated A between sun and shade plants in both CO2 lev-els, whereas A. rubrum had higher Amass in shade thanin sun. The largest CO2-induced increases in Amass

occurred in B. papyrifera sun plants (162%) and A.rubrum shade plants (127%).


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26M. E. Kubiske &K. S. Pregitzer

Shade leaves of A. rubrum had significantly higherN concentration than sun leaves (Table 1). There wereno significant differences among treatments in leaf Nconcentration in B. papyrifera. In both species, N con-centration tended to be lower in elevated than ambientCO2, Elevated CO2 resulted in significantly higherlight-saturated NUE in both light environments,compared with that in ambient CO2, and shaded A.rubrum in elevated CO2 had the highest NUE of thestudy. Across both light treatments, trees in elevatedCO2 exhibited a steeper relationship betweenmaximum measured A vs N than those in ambient CO2

on both an area and mass basis (Fig. 2).

PLANT–WATER RELATIONS

There were no significant species or treatment effectsin predawn ψ, but there were interesting interactions

among species and treatments in midday ψ (Table 2).Sun-grown B. papyrifera had significantly lowermidday ψ than did shade-grown seedlings following20-min exposure to full sun and this difference wasgreater in elevated CO2. Acer rubrum did not exhibitthese differences.

Betula papyrifera had higher gw than A. rubrum innearly all treatments and B. papyrifera had higher gw

in sun than in shade. Also, B. papyrifera sun plantshad significantly higher gw in elevated than in ambientCO2. Higher gw in sun than in shade in B. papyriferawas reflected in the differences in midday ψ betweenlight regimes, but higher gw in elevated CO2 was notreflected as lower ψ in elevated CO2. In B. papyrifera,but not A. rubrum, higher rates of E were supported ata given midday ψ in elevated CO2 (Fig. 3).

Both species tended to have higher WUE in sunplants than in shade plants during exposure to sun,except for A. rubrum in elevated CO2. Shade plants ofA. rubrum had a significant CO2-induced increase inWUE. Elevated CO2 did not result in a WUE increasein sun plants, which was likely related to higher gw inelevated CO2. There were no differences in WUEbetween light regimes in B. papyrifera in either CO2

treatment.

LEAF MORPHOLOGY AND TISSUE–WATER RELATIONS

Both species exhibited significant responses in leafstructural characteristics to shade but elevated CO2

had little effect (Table 3). In particular, SLA in shadeplants was more than twice that of the sun plants,which was accompanied by greater area per leaf (P <0·05) in shade than in sun. In addition, sun-grownplants of A. rubrum had significantly more stomataper unit leaf area in elevated CO2 than in ambientCO2. In terms of species comparisons, A. rubrum hadsmaller guard cells but greater stomatal frequencythan B. papyrifera in all treatments and A. rubrum hadsmaller shade leaves (area) than B. papyrifera.

Shade-grown B. papyrifera seedlings had an obvi-ous CO2-related shift in tissue–water relations, with agreatly extended range of positive turgor and signifi-cantly lower ψπ

100 and RWC0 (Fig. 4, Table 4). Therewere significantly higher osmotic potentials and ε inB. papyrifera than in A. rubrum seedlings in both CO2

treatments. The mean difference between ψπ0 and

midday ψ (in effect, midday turgor) for B. papyriferawas 0·18 MPa in ambient CO2 and 0·39 MPa in ele-vated CO2. In contrast, those values for A. rubrumwere 0·22 MPa in both CO2 treatments.

Discussion

Species in this study exhibited interesting interactionswith light and CO2 treatment combinations following20 min of exposure to full sun. Differences in Aarea

between sun and shade plants in elevated CO2 weresimilar to sun and shade plant differences in ambient



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Fig. 1. Incident photosynthetic photon flux density (PPFD),air temperature (T) and atmospheric vapour pressure deficit(VPD) in a sun-lit (open symbols) and shaded (closedsymbols) open-top chamber typical of a cloud-free day onwhich gas exchange and tissue–water relations weresampled. Data are point samples recorded every 15 min. Thefigure reflects that, at 1150 h solar time, the shade cloth wasremoved from the shaded chamber for about 1 h.

CO2 for both species: –43% and –49% in ambient andelevated CO2 for B. papyrifera and +63% in both CO2

levels for A. rubrum. Thin shade leaves in shade-intol-erant B. papyrifera resulted in lower Aarea becauseboth sun and shade leaves had similar Amass. In A.rubrum, shade leaves had higher Aarea despite beingthinner because of a large increase in Amass. Theseresults suggest that B. papyrifera responded to shadeprimarily by altering specific leaf area, whereas A.rubrum responded in large part via biochemical accli-

mation of the photosynthetic apparatus. While A.rubrum typically has intrinsically low A under the bestconditions (4–6 µmol m–2 s–1: Abrams & Kubiske1990; Kloeppel, Abrams & Kubiske 1993), relativelyhigh temperatures at the time of this study may haveresulted in even lower values (Fig. 1).

An increase in photosynthetic NUE in elevatedCO2, reported in almost every C3 species studied, isrelated to an increase in carboxylase vs oxygenaseactivity of rubisco (Tolbert & Zelitch 1983). It isinteresting, however, that in this study sun and shade-grown A. rubrum had nearly threefold increases inNUE, while B. papyrifera had twofold increases witha doubling of atmospheric CO2. There are several pos-sible explanations for differences in NUE response toelevated CO2 between the study species. It is likelythat a combination of factors contributed to the resultsseen here. One possibility is that the kinetic propertiesof Rubisco produced by B. papyrifera are differentfrom that of A. rubrum (i.e. CO2-induced change inthe carboxylase/oxygenase ratio is different for thetwo species; cf. Seemann & Berry 1982). Anotherpossibility is that of a functional difference in theability of the two species to partition N among rate-limiting reactions of photosynthesis. It has beensuggested that shade-tolerant species have greaterintrinsic photosynthetic plasticity to irradiance thanintolerant species (Evans 1989). The greatestresponses of photosynthesis to elevated CO2 in thisstudy occurred in B. papyrifera sun plants and A.rubrum shade plants, and shade plants of A. rubrumexhibited the greatest increase in NUE with elevatedCO2. In a related study, photosynthetic light responseanalysis indicated that the photosynthetic apparatus ofA. rubrum may be more plastic to irradiance and toatmospheric CO2 than that of less tolerant B.papyrifera (Kubiske & Pregitzer 1996). Moreover, itwas hypothesized in that study that photosyntheticplasticity to irradiance exhibited by many shade-toler-ant species may be related to the ability of somespecies to acclimate to elevated CO2, because of bio-chemical similarities in photosynthetic acclimation toirradiance and to elevated CO2 (Sage 1990, 1994). Forexample, it has been shown in biochemical modelsthat leaf N could, theoretically, be partitioned betweenlight and dark reactions in elevated CO2 to optimizeNUE (Sage 1990; Woodrow 1994). With increasedCO2 availability, carboxylation may be limited byRuBP regeneration which could be compensated forby increased N investment into ATP production (lightreactions) or Calvin-cycle enzymes. The largeincreases in NUE by A. rubrum are consistent with thehypothesis that shade-tolerant species are better ableto acclimate photosynthetically to CO2 enrichmentcompared with intolerant species, owing to plasticityin partitioning of rate-limiting photosyntheticreactions (cf. Evans 1989). The hypothesis of Kubiske& Pregitzer (1996) is further supported by studies inwhich shade-tolerant species exhibited greater


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Fig. 2. Maximum photosynthesis rate (out of three measurement dates) vs leaf N con-centration of Acer rubrum and Betula papyrifera seedlings grown in ambient (circles)and twice ambient (squares) CO2. Data are for both sun-grown (open symbols) andshade-grown (closed symbols) plants. Measurements were conducted on shade-grown plants following 20 min of exposure to full sun. The left panel is expressed ona leaf area basis in which regression equations are, for ambient CO2,y = 3·947x + 0·772 (r2 = 0·86) and for elevated CO2, y = 9·185x + 1·338 (r2= 0·76). Theright panel is expressed on a leaf mass basis in which regression equations are, forambient CO2 y = 45·956x + 37·849 (r2 = 0·43) and for elevated CO2, y = 114·701x +22·192 (r2 = 0·91).

Table 1. Mean (±SE) light-saturated photosynthesis and leaf N of two tree specieswith contrasting shade tolerance grown in ambient and twice ambient CO2, and infull sun and 73% shade. Means are of three replicates over three measurement dates,measurements of shaded seedlings were conducted following 20 min of exposure tofull sun. Parameters are: net photosynthesis per unit leaf area (Aarea, µmol m–2 s–1);net photosynthesis per unit leaf mass (Amass, nmol g–1s–1); leaf nitrogen concentration(N, mg g–1); nitrogen-use efficiency (µmol mol–1s–1). Values of a parameter within arow or column with the same letter are not significantly different (P > 0·05)

CO2 Light Betula papyrifera Acer rubrum

Aarea ambient sun 6·51 ± 0·74 a 1·79 ± 0·21 bambient shade 3·69 ± 0·28 b 2·91 ± 0·22 belevated sun 12·92 ± 0·93 c 2·98 ± 0·48 belevated shade 6·64 ± 0·34 a 4·87 ± 0·42 c

Amass ambient sun 82·9 ± 11·7 a 56·4 ± 17·4 aambient shade 132·7 ± 18·1 ab 105·6 ± 16·2 belevated sun 216·9 ± 29·1 c 61·3 ± 12·4 aelevated shade 216·7 ± 20·8 c 239·3 ± 53·3 c

N ambient sun 24·1 ± 2·2 a 10·9 ± 0·7 bambient shade 30·1 ± 1·0 a 24·9 ± 2·5 aelevated sun 23·2 ± 2·2 a 8·0 ± 0·1 belevated shade 24·6 ± 0·5 a 18·9 ± 0·6 a

NUE ambient sun 53·5 ± 8·4 a 42·6 ± 13·1aambient shade 54·4 ± 6·8 a 66·2 ± 7·3 aelevated sun 128·0 ± 12·4 b 128·5 ± 28·5 belevated shade 109·4 ± 29·0 b 165·0 ± 64·8 b

relative increases in growth than intolerant species inelevated CO2, and greater growth enhancement byelevated CO2 in shade than in sun (Bazzaz, Coleman& Morse 1990; Bazzaz & Miao 1993).

All treatment-related increases in A were accompa-nied by significant increases in gw. In contrast, theclassic response of plant species to elevated CO2 isstimulation of A with a decrease or no change in tran-spiration or conductance (Bunce 1992; Curtis & Teeri1992; Gunderson, Norby & Wullschleger 1993). In atleast one case, decreased gw contributed to heat stressby decreasing transpirational cooling (Surano et al.1986). A change in gw may result from a change in vari-able stomatal aperture or a more fixed change in thenumber and/or size of stomata. In this study, elevatedCO2 did not affect stomatal frequency significantlyexcept for an increase that occurred in A. rubrum sunplants. The increase in gw associated with elevated CO2

more likely resulted from an increase in variable stom-atal aperture, possibly in response to a change in whole-tree morphology or function. Mechanistically, thewater absorption/conduction/transpiration systemappears to be very well integrated in plants (Joyce &Steiner 1995). For example, high transpirationaldemand on the south-facing side of a mature Fraxinusamericana tree resulted in higher leaf-specific conduc-tivity in those branches compared with branches on thenorth side of the crown (Joyce & Steiner 1995). Open-grown Picea abies trees had 1·4- to 3·1-times greaterxylem conductivity than shade-grown trees (Sellin1993). In Sugar cane, removal of leaf area resulted in

increased gw of the remaining leaves to maintain a con-stant relationship between crown E and stem flow and,conversely, root pruning resulted in reduced gw

(Meinzer & Grantz 1990). While several tree specieshave been shown to increase C allocation to roots inelevated CO2 (Rochefort & Bazzaz 1992; Rogers et al.1992; Pregitzer et al. 1995), which may translate toincreased water absorbing capacity of root systems andhence higher gw, it is virtually unknown whether this isaccompanied by changes in stem hydraulic conduc-tance in elevated CO2. Either case (high root/shoot ratioor increased hydraulic conductance) could account forthe increased rates of E and gw vs ψ of B. papyrifera inelevated CO2 in this study. However, we are aware ofno studies that address this issue. The effect of elevatedCO2 on interactions of root absorption, water transportand physiological and morphological aspects of tran-spiration clearly remains a large gap in our understand-

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Table 2. Mean (±SE) plant water relations parameters for seedlings grown in ambi-ent and twice ambient (elevated) CO2 and full sun and 73% shade. Values are fromthree sample dates in August. Parameters are leaf water potential (ψ, MPa) measuredat predawn (0500 h solar time) and at midday (c. 1200 h solar time), stomatal conduc-tance to water vapour (gw, µmol m–2 s–1) and instantaneous water-use efficiency(WUE, mmol mol–1). Measurements of shade plants were conducted following20 min of exposure to full sun. Values of a parameter within a row or column with thesame letter are not significantly different (P > 0·05)


Predawn ψ ambient sun –0·30 ± 0·04 a –0·32 ± 0·04 aambient shade –0·26 ± 0·02 a –0·24 ± 0·03 aelevated sun –0·31 ± 0·04 a –0·32 ± 0·03 aelevated shade –0·28 ± 0·03 a –0·31 ± 0·03 a

Midday ψ ambient sun –1·31 ± 0·06 a –1·29 ± 0·18 aambient shade –1·15 ± 0·04 b –1·41 ± 0·18 aelevated sun –1·46 ± 0·08 a –1·29 ± 0·19 aelevated shade –1·06 ± 0·04 b –1·47 ± 0·16 a

gw ambient sun 123·4 ± 19·9 a 28·9 ± 3·5 cambient shade 91·3 ± 7·6 a 69·3 ± 5·5 cdelevated sun 251·2 ± 31·3 b 55·6 ± 5·0 cdelevated shade 127·6 ± 7·7 a 84·6 ± 8·6 d

WUE ambient sun 1·53 ± 0·17 a 1·32 ± 0·20 acambient shade 0·98 ± 0·09 b 0·91 ± 0·05 belevated sun 1·77 ± 0·13 a 1·14 ± 0·21 bcelevated shade 1·28 ± 0·08 bc 1·38 ± 0·13 c

Fig. 3. Leaf transpiration rate (E) vs midday leaf water poten-tial (ψ) for two tree species grown in ambient (circles) andtwice ambient (squares) atmospheric CO2. Data are fromboth sun-grown (open symbols) and shade-grown plants(closed symbols). Measurements were conducted on a shade-grown plants following 20 min of exposure to full sun. For A.rubrum, the linear relationship was y = –0·768x + 2·052(r2= 0·21). For B. papyrifera, the elevated CO2 relationshipwas y = –6·342x + 1·322 (r2= 0·64) and the ambient CO2 rela-tionship was y = –2·951x + 0·366 (r2= 0·34).


ing of plant water use (Morison 1993; Tyree &Alexander 1993).

In this study, shade-grown plants of both speciesexhibited significant increases in WUE as is typicallyseen (Samuelson & Seiler 1992; Gunderson et al.1993; Townend 1993) and these values were similarto those reported elsewhere (Abrams et al. 1994).However, increased gw likely prevented an increase inWUE for sun plants. Elevated CO2 has been known toameliorate the effect of drought in some species,partly owing to increased WUE and root:shoot ratios(Tolley & Strain 1984, 1985; Townend 1993) andpartly owing to changes in tissue–water relations(Conroy et al. 1988; Miao, Wayne & Bazzaz 1992).These responses could serve to decrease the rate ofwater use, increase the rate of water absorption and

extend the range of positive turgor, respectively,which would also have implications for improvedwater economy in understorey trees during extendedsun patches. In this study, pressure-volume analysisindicated that shade plants of B. papyrifera had signif-icantly lower ψπ

100, and slightly lower ψπ0, in ele-

vated than in ambient CO2. In contrast, A. rubrumexhibited no significant responses in tissue–waterrelations parameters to elevated CO2, consistent withthe species’ general lack of tissue–water relationsadjustments during drought (Abrams & Kubiske1990). Similarly, elevated CO2 had a greater amelio-rating effect on Betula populifolia than it did for A.rubrum as leaf water deficits developed duringdrought (Miao et al. 1992). In another study, B. pop-ulifolia had lower ψπ

100 in elevated than in ambientCO2, which resulted in higher turgor as leaf ψdecreased during drought (Morse et al. 1993). Ourdata clearly indicate an extension of the range of posi-tive turgor and a decrease in RWC0 in B. papyriferashade plants in elevated CO2, which would help tomaintain stomatal opening as leaves lose water (Tyree& Jarvis 1982). Although differences in gw betweensun and shade plants of B. papyrifera were similar inboth CO2 treatments, the development of lower ψπ

100

in B. papyrifera shade leaves may help to maintainhigher gw with CO2 enrichment under conditions ofgreater water stress than seen here. In large sunpatches, this could augment the daily carbon balanceof understorey trees by limiting leaf water deficits.

Shade-adapted understorey seedlings may sufferrapid turgor loss and stomatal closure when fully illu-minated for extended periods of time (Chazdon 1988),which limits their daily carbon balance and affectsunderstorey survival and recruitment (Platt & Strong1989). Our data suggest that elevated CO2 increasedresource-use efficiency in understorey phenotypes,

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Table 3. Mean (±SE) Specific leaf area (SLA, cm 2g–1), stomatal frequency(# mm–2), guard cell length (µm) and area per leaf (cm2) of two tree species of con-trasting shade tolerance grown in ambient and twice ambient CO2, and in full sun and73% shade. Values of a parameter within a row or column followed by the same let-ter are not significantly different (P > 0·05)


SLA ambient sun 145·8 ± 14·1 a 188·1 ± 5·0 aambient shade 353·2 ± 25·1 b 437·7 ± 66·2 belevated sun 170·1 ± 10·4 a 150·7 ± 7·7 aelevated shade 353·2 ± 27·7 b 419·1 ± 45·2 b

Stomatal ambient sun 164·8 ± 8·0 a 324·7 ± 13·5 bfrequency ambient shade 121·3 ± 4·0 a 333·9 ± 15·0 b

elevated sun 149·4 ± 5·0 a 409·3 ± 32·5 celevated shade 106·8 ± 7·3 a 324·7 ± 13·5 b

GCL ambient sun 43·7 ± 0·7 a 16·3 ± 0·4 bambient shade 39·8 ± 1·2 a 16·0 ± 0·3 belevated sun 47·7 ± 0·9 a 17·1 ± 0·4 belevated shade 41·2 ± 0·8 a 17·0 ± 0·4 b

Area ambient sun 39·4 ± 2·6 a 25·0 ± 1·5 aambient shade 61·2 ± 0·6 b 31·0 ± 3·1 celevated sun 41·9 ± 2·6 a 28·1 ± 1·5 aelevated shade 60·1 ± 6·2 b 34·5 ± 2·9 c

Fig. 4. Höfler diagrams for leaves of two tree species that were grown in 73% shade inambient and twice ambient (elev) CO2, and measured following 20 min of exposure tofull sun. Lines were fit by eye. Corresponding pressure–volume parameters are givenin Table 1.ψ = water potential, ψπ = osmotic potential, ψp = turgor pressure.

Table 4. Tissue-water relations (mean ± SE) parameters fortwo tree species grown in 73% shade and in ambient andtwice ambient (elevated) CO2. Values are from two mea-surement dates in August. Parameters are: osmotic potentialat full and zero turgor (ψπ

100, ψπ0, respectively, MPa), rela-

tive water content at zero turgor (RWC0, %) and elasticmodulus (ε, MPa). Means of a parameter followed by thesame letter within a row or column are not significantly dif-ferent (P > 0·05)

CO2 Betula papyrifera Acer rubrum

ψπ100 ambient –0·90 ± 0·08 a –1·32 ± 0·07 b

elevated –1·21 ± 0·07 b –1·28 ± 0·09 b

ψπ0 ambient –1·33 ± 0·08 a –1·45 ± 0·04 a

elevated –1·63 ± 0·08 b –1·69 ± 0·13 b

RWC0 ambient 72·9 ± 2·4 a 79·8 ± 8·7 aelevated 56·6 ± 1·1 b 75·1 ± 8·2 a

ε ambient 3·64 ± 0·40 a 12·45 ± 5·60 belevated 2·39 ± 0·11 a 8·22 ± 2·74 b



partially supporting our original hypothesis, but theeffect differed between the two species. Shade tolerantA. rubrum is a common component of forest understo-ries in the eastern United States and is known to bevery plastic in physiological response to environment(Abrams & Kubiske 1990). In this study, it exhibitedlarge changes in Amass and NUE to elevated CO2, par-ticularly in shaded conditions. In contrast, B.papyrifera, a shade-intolerant, early successional treespecies, exhibited ecophysiological responses to ele-vated CO2 that would augment its characteristics forgrowth in exposed conditions (cf. Bazzaz 1979):higher A in sun plants, lower ψπ

100 and RWC0, and agrater capacity to replenish transpirational losses ofwater. It is not clear from our data how the interac-tions of elevated CO2 and plant acclimation to iradi-ance would affect the dynamics of photosynthesis intransient light (e.g. stomatal reactivity and Rubiscoinduction), if at all (Woods & Turner 1971; Knapp &Smith 1990; Pearcy 1990; Küppers & Schneider1993, Sassenrath-Cole, Pearcy & Steinmaus 1994).Nonetheless, the results of this study reflect the adap-tive mechanisms of these two species for the respec-tive environments in which they typically occur andsuggest that, as atmospheric CO2 increases, speciesmay become more efficient competitors in theirrespective habitats.

Acknowledgements

This research was supported by USDA competitivegrant 90-37290-5668, NIGEC, and DOE grant DE-FG02-93ER61666. Technical expertise in construct-ing and maintaining the CO2 exposure system wasprovided by C. S. Vogel and the LCA-2 analyserswere supplied by D. I. Dickman and J. A. Teeri. Helpwith data collection was provided by C. Williamsand A. Kubiske. James A. Teeri and the UMBS staffcontinue to provide critical leadership and logisticalsupport.

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Received 7 February 1996; revised 11 April 1996; accepted24 April 1996


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Ecophysiological responses to simulated canopy gaps of two tree species of contrasting shade tolerance in elevated CO2