Interactions between drought and elevated CO2 on growth and gas exchange of seedlings of three deciduous tree species

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1 New Phytol. (1995), 129, 63^71 Interactio between drought and elevated CO2 on growth and gas exchange of seedlings of three deciduous tree species BY T. J. TSCHAPLINSKI\ D. B. STEWART^, P. J. HANSON^ AND R. J. NORBY^ ^Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN , USA ^Department of Plant Biochemistry, Stanford University School of Medicine, Stanford, CA 94305, USA {Received 10 May 1994; accepted 31 August 1994) SUMMARY Interactio between elevated atmospheric COj and drought on growth and gas exchange of American sycamore {Platanus occidentalis L.), sweetgum {Liquidmnbar styraciflua L.) and sugar maple {Acer saccharum Marsh.) were investigated using 1-yr-old seedlings, planted in 8 1 pots and grown in four open-top chambers, containing either ambient air or ambient air enriched with 300 fimol mol"' COj. Two soil moisture regimes were included within each chamber: a 'well-watered' treatment with plants watered daily and a 'drought' treatment in which plants were subjected to a series of drought cycles. Duration and depth of the drought cycles were determined by soil matric potential. Mean soil water potential at rewatering for the water-stressed seedlings under ambient CO. for sugar maple, sweetgum and sycamore was -0-5, -0-7 and -1-8 MPa, respectively, compared with > -0-1 MPa for all well-watered plants. Elevated CO^ increased relative growth rate of well-watered sugar maple by 181 "o, resulting in a 4-3-fold increase in total plant dry weight after 81 d, compared with 1-4 and 1-6-fold increases for sweetgum and sycamore, respectively, after 69 d. Although elevated CO^ increased net CO^ assimilation rate of sugar maple by 115 %, there was a 10-fold increase in leaf area which contributed to the growth respoe. Although drought did not eliminate a growth respoe of sugar maple to elevated CO^, it greatly reduced the elevated COa-induced enhancement of relative growth rate. In contrast, relative growth rates of sweetgum and sycamore were not significantly increased by elevated CO^., under elevated CO.,, reduced leaf area of all three species to a greater extent than it reduced net CO^ assimilation rate. The respoe ranged from no effect in sugar maple to a 40% reduction in sycamore, with sweetgum exhibiting an intermediate respoe. Results indicate that drought may alter the growth respoe, gas exchange and water relatio of tree species growing in an elevated CO, atmosphere. Under high nutrient and water availability, sugar maple may benefit the most (of the three spec'ies studied) from a CO^enriched atmosphere, but productivity gai will be limited if frequent drought is prevalent. ey words: Acer saccharum (sugar maple), Liqiiidambar styracijlua (sweetgum), Plantanus occidentalis (American sycamore), elevated CO,, gas exchange, growth, drought. INTRODUCTION Davis (1989) both predict species migratio to T^. bigber latitudes. On tbe other hand, Bazzaz, ^ater availability controls tree growth, tree species Coleman & Morse (1990) state tbat what is often istribution and forest composition more than any neglected is the importance of the direct physio- ^^fterperennialfactor(hinckleye<a/., 1981). Global logical respoes to elevated CO,, concentratio, rculation models predict that increasing concentra- which could modify y the relatioh"ip p between trees. ^"' ^P^^"'^ ^^ 2 ^ other h g greenhouse h and climate. Given the importance of seedling seedg drou l7 7 '"'^'^^^'^'^ *^ frequency f and d severity i of f regeneration in determining forest succession, underfnr "f!d'" regio currently occupied by temperate standing seedling respoes is essential to assess how orestspastor&post, 1988; Neilson era/., 1989). It forest composition may be altered by CO,, and pred ' ^'""^ ^ '''''*' "^'^ respond to these drought. e icted climate changes. Pastor & Post (1988) and How elevated CO,, and drought will interact to

2 64 T. J. Tschapliki and others affect the growth and gas exchange of seedlings of with sand (4:1) to enhance drainage. The plants various tree species is an uncertainty. Elevated COj were kept in a greenhouse until they began to break generally increases itantaneous water-use effici- dormancy, and then traferred on 28 June to the ency (WUE) in potted tree seedlings (Jarvis, 1989), field site, the Global Change Field Research Site, but it is not known whether increased WUE will located at the Oak Ridge National Environmental impart increased drought resistance, as defined by Research Park, Oak Ridge, TN, USA (35 55' N, 84 Levitt (1980). Stomatal closure can be induced by 22' W). The potted seedlings of similar size (by leaf water deficits or by high internal COj concen- species) were then distributed among four open-top tratio (Hinckley et al., 1981; Morison, 1985). field chambers (3 m diam.) on 17 July. Two of the Whereas stomatal closure due to elevated COj chambers remained at ambient COj concentratio reduces trapirational water losses, the increased while the remaining two chambers were maintained COj concentration increases net COj assimilation at 300 fimo\ mol"' CO.^ above ambient, continuously rate, thereby compeating for the increase in for 24 h d ', commencing on 20 July. Chamber stomatal resistance to COj uptake (Morison, 1985; design, air fiow and the dispeing and monitoring of Jarvis, 1989). Many studies have shown an increase COj were as described elsewhere (Gunderson, in itantaneous WUK of plants growing under Norby & Wullschleger, 1993). The only modification increased concentratio of atmospheric CO.^, pri- was that these chambers were fitted with rain caps marily as a result of lowered stomatal conductance, elevated above the top of the chamber rim to exclude enhanced photosynthesis, or both factors in com- ambient rain. The chambers still operated as openbination; however, few studies have been conducted top chambers, with air circulating out of the top on tree species (Hollinger, 1987; Norby & O'Neill, between the chamber rim and the rain cap. To 1989). minimize the effects of being in different chambers, Sugar maple {Acer saccharum Marsh.) is con- each seedling was moved from its chamber to the sidered very seitive to drought with growth limited other chamber with the same COj concentration to moist environments (Davies & Kozlowski, 1977). midway through the study. The average CO^ mole {Liquidambar styracifiua L.), while grow- fraction over the duration of the study was 343 //mol ing best on sites most suitable for yellow-poplar mot' in the ambient chambers and 652//mol mor' {Liriodendroji tulipifera L.), is less exacting in soil in the elevated CO^ chambers, between and and moisture requirements. A field study of Ameri h, corresponding to the period after sunrise can sycamore {Platanus occidentalis L.), growing in a and before suet. For example, sunrise and suet plantation, indicated that sycamore resists drought on 20 August (1 month into the experiment) was by greatly reducing stomatal conductance and net and h, respectively. Carbon dioxide was CO2 assimilation rates (Tschapliki & Norby, continuously added over the whole 24-h period to 1991). For the three species coidered in this study, maintain the 300 /fmol mol ' CO.^ differential beprevious studies indicate the following series for tween ambient and elevated CO.^ chambers. For drought resistance: sweetgum > sycamore > sugar example, CO,^ mole fractio at midnight in July maple (Davies & Kozlowski, 1977; Roberts, Strain & through August were 479 //mol mol ' in the ambient Knoerr, 1980; Pallardy, 1981; Parker et al., 1982). chambers and 789 //mol mol ' in the elevated CO., Given that drought typically reduces growth, it was chambers. The maximum photosynthetically active hypothesized that a species' seitivity to drought radiation in the chamber was typically may limit its growth respoe to elevated CO^. //mol m^ s '. Air temperature peaked at C on Conversely, elevated CO, may ameliorate some of the hottest sunny days in August. the adverse effects of drought on trees. This study All of the pots were kept well watered for 10 d monitored net CO.^ assimilation rate, stomatal con- following trafer to the chambers, until 27 July, ductance, itantaneous WUE, relative growth rate, when they were fertilized with 36 g per pot of a and final leaf area as indicators of drought respoe controlled-release fertilizer. Sierra with in seedlings of three eastern deciduous tree species minor nutrients (Sierra Chemical Co., Milpitas, CA, growing in ambient and elevated CO, USA). Soil moisture treatments were then initiated with two levels of soil moisture within each chamber - one-half of the tree seedlings were watered daily MATERIALS AND METHODS while the remainder of the seedlings experienced Plant culture periodic drought cycles, as described below. There Twenty four 1-yr-old, cm bare-rooted seed- were six replicate seedlings per species/treatment lings of American sycamore, sweetgum and sugar combination. Since ambient rainfall was excluded maple were obtained from Warren Co. Nursery, from the chambers, irrigation was applied daily to McMinnville, TN, USA. They were kept in cold maintain the soil water of the well-watered treatment storage until they were planted on 13 June 1990, in at volumetric water contents above 40%, corre8 1 pots containing a local forest soil (a highly sponding to a soil matric potential greater than 0-1 weathered Ultisol), which had been sieved and mixed MPa. Conversely, soil moisture contents were al-

3 Effects of drought and elevated CO., on trees 65 Table 1. Relative growth rate and final leaf area of sugar maple, American sycamore and sweetgum grown under ambient { + 0 /imol mot^) and elevated ( /mtol mol'^) CO Treatment + 0 /*mol mol-' CO, /*mol mol-' CO., Probability Species CO2 CO2 X H.p Relative growth Sugar maple Sycamore Leaf area (cm") Sugar maple Sycamore rate (d^') (0-004) (95) 5101 (507) 1059 (275) (80) 1407 (159) 508 (96) (0-005) (0-003) (0-008) 3143 (610) 6003 (499) 1785 (515) (0-005) () (0-015) 599 (232) 2343 (226) 815 (88) Relative growth rate was based on basal stem diameter squared times height, measured on 24 Aug. and 27 Sept. ed roughted plants () experienced periodic water stress cycles, and watered plants () were kept moist. Uata E mea (and one standard error of the mean in parentheses) offiveor six plants per treatment. Probabilities that weie are mea (and one greater than 0-10 were designated not significant (). lowed to fall to between 10 and 20% in the drought treatment. Time-domain reflectometry (TDR) was used for the measurements of volumetric soil water content using 20-cm wave guides in each pot. Three pots without plants were dried simultaneously with this experiment and weighed to test the validity of the published relatiohip between the TDR reading and soil volumetric water content (Topp & Davis, 1985). American sycamore depleted the soil water at a much greater rate than did sweetgum or sugar maple. Sugar maple and sweetgum seedlings completed a total of seven drying cycles, their soil reaching volumetric water contents from 15 to 18% at the maximum severity of the drought treatment. The sycamore seedlings, because of their greater leaf area and plant size, completed 11 cycles, reaching volumetric soil water contents of 11 to 14% at the maximum severity of the drought cycle. Wellwatered plants for all treatments began each day at full saturation, and their volumetric soil water contents were maintained above 40%. Soil water contents were converted to soil matric water potential by using a soil moisture-release curve for the soil medium. The mean soil water potential at rewatering for the droughted seedlings under ambient CO,, for sugar maple, sweetgum and sycamore was -0-5, -0-7 and -1-8 MPa, respectively, compared with > -0-1 MPa for all well-watered plants. Gas exchange measurements Stomatal conductance and net CO2 assimilation rate were determined with an LI 6200 Portable Photosynthetic System with a 1000 cm^ cuvette (Li- Cor, Inc., Lincoln, NE). An 11 cm" rectangular area of the third or fourth leaf down from the top of the stem was enclosed in the cuvette for measurement. All measurements were completed within the chambers under conditio of saturating sunlight (photosynthetic photon fluence rate > 1000/nnol m '" s^') and at the COj concentration in which the plants were growing. Each measurement lasted 30 s with relative humidity in the cuvette increasing less than 5% and leaf temperature increasing less than 1 C. Leaf water potential was determined with a Scholander-type pressure cylinder (PMS Itruments, Corvallis, OR), following the precautio of Ritchie & Hinckley (1975). Measurements were conducted concurrent with gas exchange measurements on the next leaf down the stem. On a given day, all seedlings of a species were measured between and h, which is typically a period of maximal and fairly cotant net CO2 assimilation rate. In general, one species was sampled within 2 h. Measurements were conducted at two different phases ofthe water stress cycles. The ' wet' phase was 1-3 d after rewatering the droughted seedlings, and the 'dry' phase was at the maximum severity of the drought cycles. After four drought cycles for sugar maple and sweetgum and eight cycles for sycamore, all seedlings were watered on 4 Sept. 'Wet' phase measurements were conducted on sycamore on 5 Sept., sugar maple on 6 Sept., and sweetgum on 7 Sept. Daily high temperatures were 33, 34 and 35 C, respectively. 'Dry' phase measurements were conducted on sycamore on 20 Sept.,

4 66 T. J. Tschapliki and others CO H2O COjx H2O (a) CO H2O CO2XH2O id) CO2 H2O T (b) H2O 150 T T n _ 2-5 H2O (f) Ambient CO2 Elevated CO2 Ambient CO2 Elevated CO2 Figure 1. Effects of drought and atmospheric CO,^ concentration on total plant dry weight {a-c) and root/shoot (R/S) ratio {d-f) of sugar maple, American sycamore, and sweetgum seedlings, respectively. Seedlings were grown in pots under either ambient ( + 0/fmol mor') or elevated ( + 300/tmol mol ') CO._,, and under periodic drought cycles (filled bars) or were kept moist (open bars). Data are mea (and one standard error of the mea) of five or six plants per treatment. Significance probabilities are indicated for the main effects of CO,^ and water treatments. The absence ofthe interaction probability indicates that it was not significant (P > 0-10). sweetgum on 23 Sept. and sugar maple on 24 Sept. Daily high temperatures were 30, 19 and 21 C, respectively. Growth measurements Mean aboveground relative growth rates (RGR) were determined using the square of basal stem diameter (D, 5 cm above the base of the stem) times height {D'^H) as a surrogate for dry weight of the stem (Norby & O'Neill, 1989). Height and diameter measurements were collected on 10 Aug, 24 Aug, 27 Sept. and upon harvesting. Measurements from 24 Aug. and 27 Sept. were used to calculate the RGR of plants. RGR was calculated as the natural log of final D~H minus the natural log of D^H at the beginning of the study, divided by the time interval. The final harvest occurred on 17 Oct. for sycamore, 18 Oct. for sweetgum and 30 Oct. for sugar maple. Leaf, stem and root dry weight and leaf area (LA) were measured. Data atialysis The experiment was a two by two factorial to determine the eftects of COj and drought on plant variables using six replicatio. The individual seedling was coidered the experimental unit, with seedlings rotated between chambers to minimize the

5 Effects of drought and elevated CO., on trees 67 Table 2. Gas exchange and water relatio of sugar maple, American sycamore and szveetgum grown under ambient { + 0 nmol mot') and elevated { imiol mol') CO.,, during the 'wet' phase of the watering cycle Treatment + 0 /imol mol"' CO,^ //mol mol ' CO,2 Probability Species CO2 H.O CO2 X H2O Net CO,^ assimilation rate (//mo Sugar maple 3-59 (0-53) Sycamore (0-58) (0-87) Stomatal conductance (mol m"" Sugar maple (0-016) Sycamore (0-018) (0-068) 2-38 (0-83) 6-73 (0-95) (1-85) s"') (0-012) (0-018) (0-045) Itantaneous water-use efficiency (mmol Sugar maple Sycamore 0-88 (0-16) 1-21 (0-05) 0-96 (0-09) 0-92 (0-40) 1-79 (0-13) 1-09 (0-21) Middav leaf water potential (MPa) Sugar maple (0-26) (0-22) Sycamore (0-08) (0-02) (0-10) (0-15) 5-37 (1-11) (1-52) (2-45) (0-009) (0-042) (0-044) mol') 1-79 (0-31) 2-51 (0-08) 2-16 (0-14) (0-17) (0-11) (0-11) 5-37 (0-42) (2-00) (1-44) (0-007) (0-029) (0-033) 2-48 (0-39) 3-43 (0-29) 2-65 (0-34) (0-14) (0-11) (0-06) ed plants () experienced periodic drought cycles, and watered plants () were kept moist. Data were ivere collected on 5-7 Sept. and are the mea (and one standard error of the mean in parentheses) of six plants per treatment. Probabilities that were greater than 0-10 were designated not signihcant (). eflpects of growing in different chambers. Data were analyzed using a two-way ANOVA to test for significant (P < 0-10) treatment differences in gas exchange, RGR, LA and root/shoot ratio. The total number of degrees of freedom was 22 or 23. Data of net CO2 assimilation rates and stomatal conductance for sycamore during the 'dry' phase of the drought cycle were log traformed to eure homogeneous variances. RESULTS Growth variables Carhon dioxide enrichment significantly increased RGR of sugar maple, by 50% for droughted plants and 181 % for well-watered plants (Table 1), resulting in a 4-3-fold increase in plant dry weight when well-watered (Fig. 1). The growth respoe resulted from a large increase in leaf area production (Table 1), with net CO.^ assimilation rate also increased but to a lesser extent (Table 2). Carbon dioxide enrichment increased leaf area of sugar maple 2-fold in droughted seedlings and 10-fold in well-watered seedlings. The stimulation of shoot growth under elevated CO., was also reflected in the reduction of root/shoot ratio of sugar maple; effects on other species were not significant (Fig. 1). Plant dry weight of sycamore and sweetgum was increased by elevated CO2, but to a lesser extent than that observed for sugar maple, with increases of 58 o <i'id 41 /o, respectively, when well-watered. However, elevated CO2 increased the dry weight of sycamore and sweetgum 78% and 77%, respectively when droughted, which was greater than that observed for sugar maple (47%) (Fig. 1). Leaf area of sycamore was also increased by elevated CO.,, but the increase in leaf area of sweetgum was not statistically significant. As expected, drought reduced the RGR of American sycamore, sugar maple, and sweetgum seedlings (Table 1). RGR of sycamore was reduced 45% in ambient CO,, and 60 % in elevated CO,, by drought. RGR of sugar maple was not reduced by drought in

6 68 T. y. Tschapliki and others Table 3. Gas exchange and water relatio of sugar maple, American sycamore and sweetgum grown under ambient { + 0 /imol mol~^) and elevated { /imol mot'') CO.^, during the 'dry' phase of the watering cycle Species Treatment + 0 //mol mo1-1 CO2 Net COg assimilation rate (//mol Sugar maple 4-09 (0-68) Sycamore (1-28) (1-35) m-''' s-') 4-16 (0-73) 0-12 (0-16) 8-22 (1-29) Stomatal conductance (mol m-'^ 3-1) Sugar maple Sycamore (0-011) (0-075) (0-027) (0-010) (0-030) Itantaneous water-use efficiency (mmol nnol-') Sugar maple (0-07) (0-09) Sycamore (0-02) (0-31) (0-14) (0-30) Midday leaf water potential (MPa) Sugar maple Sycamore (0-18) (0-06) (0-06) (0-19) (0-17) (0-09) /imol mol-i CO, 6-22 (1-12) (1-54) (2-57) (0-006) (0-038) (0-067) 2-93 (0-28) 2-91 (0-35) 3-27 (0-44) (0-06) (0-09) (0-09) 5-07 (0-86) 0-87 (0-20) 9-72 (1-59) (0-006) (0-009) 2-58 (0-25) 1-82 (0-55) 5-90 (0-44) (0-21) (0-04) (0-03) Probability CO CO., X Ufi : ed plants () experienced periodic drought cycles, and watered plants () were kept moist. Data were collected on 20, 23, 24 Sept. and are the mea (and one standard error of the mean in parentheses) of six plants per treatment. Probabilities that were greater than 0-10 were designated not significant () i - ambient COj, but was reduced by 53 "/ by drought in elevated CO2. displayed a coistent reduction of RGR under drought by one-third under both CO.^ regimes. The reduced RGR was reflected in final biomass, where drought reduced the plant dry weight of sycamore by 63 % under ambient COj and 53"/,, under elevated COj. -induced reductio in plant dry weight were less (39% and 31%, respectively) in sweetgum (P = 0-076) and only evident in sugar maple under elevated COg. reduced final leaf area of all species, and caused increased root/shoot ratio of sweetgum under both ambient and elevated COj (only evident in sugar maple under elevated COj and not evident in sycamore). Gas exchange variables Under elevated COj, there was an increase in net CO2 assimilation rate, ranging in well-watered plants, from a 34% increase in sycamore to a % increase observed in sweetgum and sugar maple (Table 2). In contrast, net COg assimilation rate in droughted sugar maple, during the 'dry' phase, was only 22 % higher under elevated CO.^. Coupled with the increase in CO2 assimilation under elevated CO, was an increase in the itantaneous WUE of droughted and well-watered seedlings of all three species during both phases of the drought cycle. The increase in WUE of well-watered seedlings, was the result of both the increase of net CO2 assimilation rate and the reduction of trapiration rate (data not shown) of seedlings of all species (Tables 2, 3). Stomatal conductance declined under elevated CO2 in sweetgum and well-watered sycamore. Well-watered sugar maple exhibited conductances that were generally < 20% that of the other species, but the decline in stomatal conductance in sugar maple was not significant {P = 0-102), Midday leaf water potential was increased by elevated CO.^ in sycamore and sugar maple seedlings that were wellwatered, whereas sweetgum seedlings maintained high leaf water potentials in all treatments.

7 Effects of drought and elevated CO., on trees 69 (Norby & O'Neill, 1991), or it remai unchanged, as reported for loblolly pine (Tolley & Strain, 19846; Tschapliki, Norby & Wullschleger, 1993). Several factors suggest that changes in root/shoot ratio, observed in short-term studies on seedlings in small pots, may have limited relevance to mature trees growing in a forest. These include the diversity of respoes reported in the literature, the use of static measures of root/shoot ratio, the potentially confounding effect of the change in the allometric relatiohip between roots and shoots with age, and the effects (if any) of nutrient and water availability (Norby, 1994). The growth respoe of all three species to elevated CO., was a result of an increase in both leaf area production (but the "o increase in leaf area of sweetgum was not significant {P = 0'12)) and net CO2 assimilation rate. The preferential allocation of carbon to leaf area production in sugar maple has to be coidered an important factor contributing to its maximal growth respoe, given the relatively low DISCUSSION net CO.3 assimilation rates. Elevated CO. increased Effects of elevated CO.^ on utressed tree seedlifigs net CO2 assimilation rate of well-watered plants in Many studies have indicated that RGR and whole- all three species, resulting in large increases in plant dry weight of tree seedlings increase in itantaneous WUE. Enhanced net CO., assimilation respoe to elevated CO, (Tolley & Strain, 1985; rate under elevated CO. has been reported for a Jarvis, 1989; Norby & O'Neill, 1989). Only a single variety of tree species, including white oak (Norby & study has reported a relative growth respoe as O'Neill, 1989; Gunderson et al. 1993), red oak great as the 4-3-fold increase in plant dry weight (Williams et al., 1986), sweetgum (Tolley & Strain, 1984a, b), sycamore (Williams et al., 1986), and observed for sugar maple after 81 d in the present yellow-poplar (Williams et al., 1986; Norby & study. Conroy, Milham & Barlow (1992) reported O'Neill, 1991 ; Gunderson et al., 1993), resulting in that under high N and P additio, elevated CO.^(660 increased itantaneous WUE in several tree species //mol mol"^) increased the total dry weight of (Hollinger, 1987; Conroy et al., 1986; Norby & Eucalyptus grandis seedlings by x 39 that at ambient O'Neill, 1989; Gunderson er o/., 1993). Jarvis (1989) COg (340 //mol mor^) after 6 wk. Other studies have concluded that most tree species will have an increase indicated lesser degrees of respoe for sugar maple in itantaneous WUE when grown in elevated CO.j to elevated CO.^, with total dry weight increasing by because of increased net CO.^ assimilation rate, if not X 1 44 after 60 d at 642 //mol mol"' COj (Lindroth, from the decrease in stomatal conductance. Although Kinney & Platz, 1993), x 1-55 after 85 d at 800 /nnol sugar maple was the only species that did not exhibit m o r ' CO., (Noble et al., 1992), and x 2-52 after 100 (statistically significant) lower stomatal conductance d at 700//mol mol"' COj (Bazzaz et al., 1990). The under elevated CO.^, conductance was already low, respoe of sycamore ( x 1-58) and sweetgum ( x 1-41) relative to the other species studied. Furthermore, in our study was coistent with that of three studies sycamore growing under drought showed no adon sweetgum which report relative increases of ditional stomatal closure associated with elevated X in total dry weight, with plants grown at CO.,. Therefore, reductio in stomatal conductance, //mol mol-' CO^ for d duration induced by elevated CO., may be more likely (Tolley & Strain, 1984a, b; Sionit et al., 1985). A observed when conductance is initially high. The lower respoe of x 109 for sycamore after 90 d effect of elevated CO2 on stomatal conductance was growth at 700 /<mol mol ' CO, has also been variable in a long-term field study of yellow-poplar reported (Williams et al., 1986). {Liriodendron tulipifera L.) and white oak {Quereus Carbon allocation to roots vs. shoots was not alba L.), where supplemental irrigation was not affected by elevated COj in sycamore and sweetgum, applied (Gunderson et al., 1993). Similarly, Conroy but the COj-induced increase in dry weight of sugar et al., (1988) found that conductance of Pinus radiata maple was preferentially allocated to shoots under was not affected by elevated CO., and Bunce (1992) well-watered conditio. This finding contrasts with reported that conductance of Malus domestica, that which is typically observed, where the relative Quercus primis and Quercus robur were not reduced carbon allocation to roots increases under high CO.^, by elevated CO.^. An exception was that Acer as reported for white oak {Quercus alba L.) (Norby, saccharinum seedlings had reduced conductances O'Neill & Luxmoore, 1986) and yellow-poplar The reduction of RGR in sycamore by drought was paralleled during the 'dry' phase by reductio in net COg assimilation rate, stomatal conductance and itantaneous WUE (Table 3). Net CO. assimilation rate and stomatal conductance were lower in these seedlings even during the 'wet' phase, than in seedlings that were well-watered (Table 2). resulted in lower midday leaf water potentials only in American sycamore (Table 3). In sweetgum, drought reduced net CO., assimilation rate (41% in ambient CO^ and 59"o in elevated.^; Table 3) and stomatal conductance in both J levels and during both phases of the water stress cycle (Tables 2, 3), although itantaneous WUE of sweetgum actually increased in droughted sweetgum during the 'dry' phase (Table 3). did not aftect any of the gas exchange variables of sugar maple in either phase of the drought cycle (Tables 2,3).

8 70 T. y. Tschapliki and others droughted sweetgum grown in 1000 //mol mol ' CO2 exhibiting a net COj assimilation rate similar to wellwatered plants grown at ambient CO2. Sionit et al. (1985) reported only a partial compeation for drought in sweetgum seedlings grown at 675 //mol Effects of water stress in ambient mor' CO2. These results suggest that the degree of Given that each species received differing degrees of compeation of net CO,^ assimilation rate to drought drought, a species comparison of their drought may depend on the degree of drought experienced resistance capabilities, based on their growth reand the concentration of COj. Although drought spoe to water stress, would not be valid. Reinhibited a photosynthetic respoe to elevated CO2 ductio in RGR due to drought, under ambient during the 'dry' phase of the drought cycle, it did CO2, were greatest in sycamore, followed by sweetnot preclude a growth respoe. Thesefindingsare gum and not evident in sugar maple. The magnitude similar to those reported for sweetgum (Tolley & of respoes were proportional to the degree of Stain, 1984/;) and Pinus radiata (Conroy et al., drought e.xperienced by each species. Neither RGR 1988). of sugar maple nor any of the gas exchange variables There is coiderable risk in extrapolation of was reduced by drought under ambient CO,,. short-term respoes of tree seedlings growing in -induced reductio in RGR of sycamore pots to long-term respoes of mature trees growing and sweetgum were coupled with reductio in net in a forest, but results of such short-term studies CO^ assimilation rate and stomatal conductance. may suggest which factors change and, thereby, alter Sugar maple seedlings in our study were never as the competitive relatiohip between tree species severely droughted (soil water potential did not under elevated CO2. The varied respoes of species decline below 0-7 MPa) as in the study of Davies & to elevated COj may dictate the nature of the Kozlowski (1977) (plant water potential reduced to competition between species, which may be different 2-5 MPa in a single drying cycle), where ofthe six on mesic vs. xeric sites. The short-term growth tree species studied, sugar maple was ranked as the stimulation of sugar maple by elevated CO2 was far most drought seitive. Furthermore, they reported in excess of that of sycamore and sweetgum when that stomata of Acer saccharum did not open to pre-drought levels following rewatering, possibly a plants were well-watered, but this advantage was result of damaged stomatal function during extreme eliminated by drought. While withstanding drought drought. Similarly, Martin, Pallardy & Bahari (1987) of greater severity, both American sycamore and found sugar maple to have little dehydration tol- sweetgum displayed a greater relative growth reerance when compared with Quercus alba, Q. rubra spoe to elevated CO^ than did sugar maple. These results substantiate the hypothesis presented, that a and Q. velutina. species' seitivity to drought determines the extent a growth respoe to elevated CO2 is limited by drought. It should be noted that the three species in CO2 X drought interactio our study would rarely (if ever) be found together in A number of significant interactio between atmos- the same forest. While such short-term studies can pheric CO2 concentration and drought were evident. indicate the possibility of changes in forest sucthe elevated CO2-induced increase of RGR, and LA cession resulting from increases in atmospheric CO2 of maple was greatly reduced by drought, but concentratio and drought, longer-term studies, drought did not preclude an effect of CO2 on net COg with mature trees of species that typically coexist, are assimilation rate and stomatal conductance. The essential for accurately assessing the dynamics of primary effect of the drought-induced reduction in forest ecosystem succession. growth of maple under elevated CO2 was on the reduction of leaf area production rather than on gas exchange. The COj-induced reduction in root/shoot ACKNOWLEDGEMENTS ratio of sugar maple was only evident under well- Research spoored, in part, by the U.S. Environmental watered conditio. Elevated COg had no effect on Protection Agency and by The Global Change Research the growth respoe of sycamore under drought, but Program of the Environmental Sciences Division, U.S. drought inhibited the effects of elevated CO2 on Department of Ivnergy under contract no. DE-AC05stomatal conductance. In sweetgum, the only signifi- 84OR21400 with Martin Marietta Energy Systems, Inc. cant interaction between CO2 and drought resulted Although the research described in this article has been in drought reducing the photosynthetic respoe to funded, in part, by U.S, Environmental Protection Agency elevated CO2. inhibited a photosynthetic Interagency Agreement DOE no, 1824-C150-A1 to R, J, Norby, it has not been subjected to the Agency's review respoe to elevated CO2 in both sweetgum and and therefore does not necessarily reflect the views of the sycamore, which contrasts the results of Tolley & Agency, and no official endorsement should be inferred. Strain (19846, 1985), who reported that elevated Publication No, 4362, Environmental Sciences Division, CO2 resulted in full compeation for drought, with Oak Ridge National Laboratory, Oak Ridge, TN, under elevated CO,,, but only at measurement temperatures above 33 C.

9 Effects of drought and elevated CO., on trees 71 The authors would like to thank S. D. Wullschleger, T. Grizzard and W. K. Roy for their technical assistance during the study, and C. A. Gunderson and E. G. O'Neill for their reviews of the manuscript. REFERENCES Bazzaz FA, Coleman JS, Morse SR Growth respoes of seven major co-occurring tree species of tlie nortlieastern United States to elevated CO,,. Canadian Jouinat of Forest 7?«r«rW; 20: Bunce JA Stomatal conductance, photosynthesis and respiration of temperate deciduous tree seedlings grown outdoors at an elevated concentration of carbon dioxide. Plant, Ce/I and Environment 15: , Conroy JP, Kuppers B, Virgona J, Barlow EWR The influence of COj enrichment, phosphorus deficiency and water stress on the growth, conductance and water use of Pinus radiata D. Don. Platit, CeU and Environment 11: Conroy JP, Milham PJ, Barlow EWR Effect of nitrogen and phosphorous availability on the growth respoe of Eucalyptus grandis to high CO,,. 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