Oecologia (2001) 129:31–42 DOI 10.1007/s004420100699
Stephan Hättenschwiler
Tree seedling growth in natural deep shade: functional traits related to interspecific variation in response to elevated CO2 Received: 31 August 2000 / Accepted: 9 March 2001 / Published online: 17 May 2001 © Springer-Verlag 2001
Abstract The mechanisms for species-specific growth responses to changes in atmospheric CO2 concentration within narrow ecological groups of species, such as shade-tolerant, late-successional trees, have rarely been addressed and are not well understood. In this study the underlying functional traits for interspecific variation in the biomass response to elevated CO2 were explored for seedlings of five late-successional temperate forest tree species (Fagus sylvatica, Acer pseudoplatanus, Quercus robur, Taxus baccata, Abies alba). The seedlings were grown in the natural forest understorey in very low and low light microsites (an average of 1.3% and 3.4% full sun in this experiment), and were exposed to either current ambient CO2 concentrations, 500, or 660 µl CO2 l–1 in 36 open-top chambers (OTC) over two growing seasons. Even across the narrow range of successional status and shade tolerance, the study species varied greatly in photosynthesis, light compensation point, leaf dark respiration (Rd), leaf nitrogen concentration, specific leaf area (SLA), leaf area ratio (LAR), and biomass allocation among different plant parts, and showed distinct responses to CO2 in these traits. No single species combined all characteristics traditionally considered as adaptive to low light conditions. At very low light, the CO2 stimulation of seedling biomass was related to increased LAR and decreased Rd, responses that were observed only in Fagus and Taxus. At slightly higher light levels, interspecific differences in the biomass response to elevated CO2 were reversed and correlated best with leaf photosynthesis. The data provided here contribute to a mechanistic process-based understanding of distinct response patterns in co-occurring tree species to elevated CO2 in natural deep shade. I conclude that the high variation in physiological and morphological traits among late-successional species, and the consequences for their responses to slight changes in resource availability, have S. Hättenschwiler (✉) Institute of Botany, University of Basel, Schönbeinstrasse 6, 4056 Basel, Switzerland e-mail:
[email protected] Tel.: +41-61-2673516, Fax: +41-61-2673504
previously been underestimated. The commonly used broad definitions of functional groups of species may not be sufficient for the understanding of recruitment success and dynamic changes in species composition of oldgrowth forests in response to rising concentrations of atmospheric CO2. Keywords Allocation · Functional adjustment · Photosynthesis · Shade tolerance · Temperate forest understorey
Introduction Light availability most strongly limits tree seedling growth in forest understoreys (Pearcy 1983; Canham 1988; Kobe et al. 1995) while other factors, such as nutrient availability, are usually less important in these carbon-limited, low-light environments (Denslow et al. 1990; Pacala et al. 1994). Late-successional tree species adapted to conditions of deep shade respond to low quantum supply by morphological and architectural adjustments (Canham 1988; Coomes and Grubb 1998), differential dry matter allocation (Walters et al. 1993; Poorter 1999), metabolic changes (Björkman 1981; Pearcy 1990; Ellsworth and Reich 1992), and variable investments in defense (cf. Augspurger 1984) which are often species-specific (see review by Givnish 1988). The rising concentration of atmospheric carbon dioxide could partially mitigate carbon limitation in low-light forest understoreys. In fact, large relative CO2 effects may be expected in deep shade because higher CO2 concentrations reduce photorespiration and, as a consequence, increase apparent quantum yield (Ehleringer and Björkman 1977; Long and Drake 1991) and decrease the light compensation point of photosynthetic carbon uptake (Kubiske and Pregitzer 1996; Osborne et al. 1997). High relative CO2 stimulation of biomass production has been found at low levels of light in growth chamber experiments (Bazzaz and Miao 1993; Hättenschwiler and Körner 1996), and in natural forest
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understoreys (Würth et al. 1998; Hättenschwiler and Körner 2000). Because these effects differ among species (Bazzaz et al. 1990; Hättenschwiler and Körner 2000), the regeneration success of tree seedlings, and thus forest species composition, may become affected as CO2 concentration continues to rise (Bolker et al. 1995). The identification of different functional groups with characteristic responses to changes in CO2 concentration such as C3 versus C4 plants, fast growers versus slow growers, competitors versus stress tolerators, or shade intolerant versus tolerant species (e.g. Hunt et al. 1991; Poorter et al. 1996; Kerstiens 1998) contribute to a functional interpretation of observed and expected patterns of community dynamics. The physiological and/or morphological mechanisms that drive variable responses within narrower ecological groups of species, such as late-successional forest tree species, however, have rarely been addressed and are much less well understood. Moreover, data on CO2 responses of tree seedlings growing in real forest understoreys with naturally fluctuating light are very limited. This may be a serious gap in the understanding of how understorey tree seedlings respond to CO2 enrichment, because temporal variability in light availability has been shown to significantly influence growth and morphology in tree seedlings, independent of total photon flux density (Wayne and Bazzaz 1993). As far as I know, there are only two published studies that compare photosynthetic responses to CO2 among cooccuring trees in a natural forest understorey (DeLucia and Thomas 2000; Naumburg and Ellsworth 2000), and only three studies that compare in situ growth responses to CO2 among more than one species (Würth et al. 1998; Naidu and DeLucia 1999; Hättenschwiler and Körner 2000). While all of these studies reported marked differences among species, there has so far been no attempt to link the observed interspecific variation in growth responses to species-specific traits in seedling physiology and morphology. Hättenschwiler and Körner (2000) have previously shown that only minor increases in light availability at the forest floor can reverse interspecific differences in biomass response to elevated CO2 in five late-successional co-occurring temperate forest tree species. In very low light microsites (an average of 1.3% of full sun) the CO2 response of total seedling biomass was greatest in Fagus sylvatica (+73%) and decreased in the order Fagus>Taxus baccata (+37%)>Quercus robur (+17%)> Acer pseudoplatanus (+6%)>Abies alba (–2%). At a slightly higher light availability (an average of 3.4% of
Table 1 Characteristics of the tree species studied. The shade tolerance rankings indicate the presumed relative differences within the five species considered here (5 indicates the highest shade tolerance)
full sun), in contrast, the CO2 response was greatest in Abies (+55%) and decreased in the order Abies>Acer (+39%)>Quercus (+25%)>Fagus (+18%)>Taxus (+7%). In this study I explore the underlying physiological and morphological mechanisms for interspecific differences in the biomass response to elevated CO2, and I address the question of why these differences change with a comparatively small increase in light availability.
Materials and methods Study site A 120-year-old mixed deciduous broadleaf-coniferous forest with a canopy height of about 35 m was chosen for the experiment (Hättenschwiler and Körner 2000). The forest is located 12 km southwest of Basel (47°28′N, 7°30′E) at an elevation of 550 m and with a long-term mean annual precipitation of 885 mm. Long-term average daily mean temperatures are –2.0°C in January and 18°C in July. The soil is a loamy rendzina with a 12-cm-deep, rock-free topsoil (pH=5.8) of a relatively low nutrient availability (see Hättenschwiler and Körner 2000 for soil nutrient analyses), and a 17- to 24-cm-deep rocky subsoil (~40% of the subsoil volume consists of rocks) underlain by limestone bedrock. Thirteen different tree species are found in the forest canopy with Fagus sylvatica and Quercus robur as the dominant species. Plant materials Five co-occuring, late-successional tree species were studied. Three of them were broadleaf deciduous (F. sylvatica L., Acer pseudoplatanus L., Q. robur L.), and two of them were evergreen conifers (Abies alba Miller, T. baccata L.). Although different in some aspects (Table 1) all of these tree species can be considered as shade tolerant and capable of growing beneath a closed overstorey canopy. Seeds of Fagus, Acer, Quercus and Abies were collected at or nearby the study site in autumn 1995. Taxus seeds were obtained from a forest tree nursery that uses native seed sources from forest sites with elevations and soils similar to our study site. All seeds were stratified in a 1:1 mix of sand and peat in the Botanical Garden of the University of Basel from late autumn 1995 to spring 1996 (Taxus seeds were already pre-stratified). Germinating seedlings were inserted directly into the top 2 cm of the undisturbed forest soil within each of the 36 open-top chambers (OTCs) in April 1996. Twenty-four (5.5×5.5 cm) seed positions were defined per OTC using a grid, and species were randomly allocated to the positions. A total of four individuals of six species were grown in each OTC. Pinus sylvestris was also included in the study initially, but is not considered here because of very low survival (see Hättenschwiler and Körner 2000). The few natural seedlings and forbs within the OTCs were removed before the start of the experiment, but the natural litter layer remained in place. From each OTC a total of three to four individuals of the conifers and one to two individuals of deciduous species (two individuals of deciduous species were removed at the end of the first
Species
Canopy position
Successional status
Shade tolerance
Leaf age
Cotyledon age
Fagus sylvatica Acer pseudoplatanus Quercus robur Taxus baccata Abies alba
Dominant Co-dominant Co-dominant Sub-canopy Co-dominant
Late Early to late Late Late Late
5 4 3 5 4
5–6 months 5–6 months 5–6 months >3 years >3 years
3–5 months 2–4 months Hypogeic 10–15 months 10–15 months
33 growing season) were excavated at the end of the second growing season. I dug out the entire soil block covered by the OTC to a depth of 20 cm. Each individual seedling was then carefully washed out of the soil block using a hose. Seedlings were divided in roots, stems and leaves, leaf area was measured, and then all plant parts were oven dried at 80°C for 36 h and weighed. Experimental design Thirty-six OTCs were positioned on the forest floor in such a way that half of them represented very low light microsites with an average light availability of 1.3% of full sun (range 0.8–2.1%), and the other half represented low light microsites with an average light availability of 3.4% of full sun (range 2.6–4.8%). Each of 12 OTCs were supplied with either ambient or two elevated CO2 concentrations (500 and 660 µl l–1) for 24 h. The mean daytime natural CO2 concentration at a height of 0.05 m above the forest floor measured outside the OTCs was 363 µl l–1 between March and November and 371 µl l–1 between June and August (Hättenschwiler and Körner 2000). Except for the restriction that all three CO2 concentrations are equally represented in low and very low light microsites, CO2 concentrations were randomly assigned to each OTC. CO2 enrichment started on 28 April 1996 and continued during winter, except for 8 weeks between early January and end of February when snow covered the ground, and ended on 10 September 1997 with the final plant harvest. For more details about CO2 enrichment and control see Hättenschwiler and Körner (2000). The cylindrical OTCs had a diameter and height of 38 cm and were constructed from 2-mm-thick, UV-transmissive Plexiglas (gs 2458, Röhm GMBH, Germany). I decided to use many but rather small OTCs (0.113 m2 of forest floor covered by each OTC) so as to have well defined microsites in terms of light availability. Tree seedlings remained small throughout the experiment and chamber height exceeded maximum seedling height at the end of the 2-year experiment by a factor of 2–8. Continuous microclimatic measurements inside and outside the OTCs showed no significant differences resulting from the use of OTCs and no differences among the CO2 treatments (Hättenschwiler and Körner 2000). Microsite-specific photosynthetically active photon flux densities (PPFD, see Table 2 for further abbreviations) were measured with one or two permanent light sensors 15 cm above the ground in each OTC (a total of 62 sensors, GaAsP photo diodes, G1115, Hamamtsu Photonics, Hamamtsu City, Japan). Two additional sensors were placed outside the forest as a reference. All sensors were calibrated at the start of the experiment and a second time in February 1997 with a quantum sensor (LI-189, LI-COR Inc., Lincoln, Neb., USA). Readings were logged at 10-s intervals (CR 10 with relay multiTable 2 List of abbreviations used and their description
plexer, Campbell Scientific Ltd., Loughborough, UK) and means of 5 min of measurements were stored. Light data were continuously collected for 10–20 days of each month (May 1996–October 1996, March 1997–October 1997). During overcast summer days that predominated at the study site, the total daily PPFD in the forest understorey ranged from 0.36 to 1 mol m–2 day–1 with an average of 0.58 mol m–2 day–1 (1.3% of full sun) in very low light microsites, and from 1.2 to 2.16 mol m–2 day–1 with an average of 1.52 mol m–2 day–1 (3.4% of full sun) in low light microsites (means of the months from June to August, see Hättenschwiler and Körner 2000 for further details). Leaf CO2 exchange For all deciduous species I measured the rate of leaf net photosynthesis at treatment CO2 concentrations with a LICOR-6400 photosynthesis system (Licor Inc., Lincoln, Neb., U.S.A.) and the standard leaf chamber in June, July and August 1997. In June a fully developed leaf from the upper part of seedlings was selected and the same leaf was then used for all consecutive measurements in deciduous tree seedlings. Photosynthesis in the coniferous species was measured only once, during the final harvest. The top 3 cm of the shoot of an individual coniferous seedling was enclosed in the leaf chamber and measured in the same way as deciduous leaves. Shoots were unbranched and commonly less than 6 cm long and bore needles along 3–5 cm of the shoot. The projected area of all needles within the leaf cuvette was determined after photosynthesis measurements and used for the calculation of CO2 assimilation rates. For all measurements the internal LICOR-6400 CO2 control was used to achieve treatment CO2 concentrations and the LICOR LED light source (LI 6400–02) was used to simulate different levels of PPFD. Leaf cuvette temperature (20±0.5°C) and air humidity (75±5%) were always kept constant and leaf temperature during measurements were never below 19°C or above 22°C. In July and August photosynthesis was measured in fully lightinduced leaves at five different levels of PPFD (800, 200, 80, 40, 20 µmol m–2 s–1) starting at the highest level. In June I measured only light-saturated net photosynthesis at 800 µmol m–2 s–1. Prior to photosynthesis measurements leaves were allowed to adjust at the highest light level until the rate of CO2 assimilation did not increase further or at least for 3 min. At each following lower PPFD level photosynthetic rate was recorded when stable or at least after a 2-min adjustment. Light-response curve data for individual leaves (shoots in conifers) were fitted to a rectangular hyperbolic model (Givnish 1988; Long and Hällgren 1993): A=(Amax×PPFD)/(K+PPFD)-Rd where A is the photosynthetic CO2 assimilation rate, Amax is the maximum CO2 assimilation rate at light saturation, K is PPFD at
Abbreviation Description
Unit
PPFD RMF SMF LMF LAR LARMR SLA Amax (mass) Amax (area) Alow (mass) Alow (area) Rd (mass) Rd (area) Qlcp φ Nmass Narea
µmol m–2 s–1 g g–1 g g–1 g g–1 cm2 g–1 cm2 g–1 cm2 g–1 µmol g–1 s–1 µmol m–2 s–1 µmol g–1 s–1 µmol m–2 s–1 µmol g–1 s–1 µmol m–2 s–1 µmol m–2 s–1 mol mol–1 % of dry mass g m–2
Photosynthetically active photon flux density Root mass fraction (root mass/total seedling mass) Stem mass fraction (stem+petiole mass/total seedling mass) Leaf mass fraction (leaf mass/total seedling mass) Leaf area ratio (total leaf area/total seedling mass) Leaf area root mass ratio (total leaf area/root mass) Specific leaf area (leaf area/leaf mass) Maximum CO2 assimilation rate at light saturation per unit leaf mass Maximum CO2 assimilation rate at light saturation per unit leaf area CO2 assimilation rate at 20 µmol photons m–2 s–1 per unit leaf mass CO2 assimilation rate at 20 µmol photons m–2 s–1 per unit leaf area Leaf dark respiration (predawn) per unit leaf mass Leaf dark respiration (predawn) per unit leaf area Light compensation point for net photosynthesis Apparent quantum efficiency (CO2 uptake per incident photon) Leaf nitrogen concentration per unit leaf mass Leaf nitrogen concentration per unit leaf area
34 half of Amax, and Rd is the rate of dark respiration. The transformation of this equation was then used to calculate the light compensation point (i.e. A=0): Qlcp=(Rd×K)/(Amax-Rd) The apparent quantum efficiency (φ) was calculated as the slope of photosynthesis versus incident irradiance between 20 and 80 µmol m–2 s–1 PPFD. In order to detect possible photosynthetic down-regulation in response to elevated CO2, the CO2 concentrations of the 365 and 660 µl CO2 l–1 OTCs were reversed for 2 h and Amax was measured in all deciduous species in July 1997. Rd was measured with the same LICOR-6400 for all deciduous species between 1.00 and 5.00 a.m. in July 1997 during three consecutive nights. For the Rd measurements the same temperature and air humidity conditions were chosen as were used for photosynthesis measurements. Because I was interested in long term acclimation effects rather than short term direct effects of elevated CO2 on Rd, all Rd measurements were conducted at a common ambient night-time CO2 concentration which varied from about 410 to 440 µl CO2 l–1. Leaf area of the leaves used for photosynthesis measurements was determined after the last measurements were completed (15 August for deciduous trees and 8–10 September for conifers) and then the leaves were immediately dried at 80°C for 24 h, weighed, ground and analysed for N concentration (CHN-analyser, Model 932, LECO Instruments, St. Joseph, Mich., USA). Statistical analysis Data for individual seedlings within each OTC were averaged for each species and used as the sample units (n=6 replicates per CO2 concentration and light level). Analysis of variance (ANOVA) was used to test for treatment effects on all measured morphological and physiological seedling traits. According to the split-plot design (species within OTCs), the effects of CO2 and light and their interaction were tested against the OTC mean squares, while the effect of species and its interactions with the other treatments were tested against the species mean squares. Tests for effects of CO2 and light within species were performed with 2-factorial ANOVAs. Differences between individual levels within factors were tested using Fisher LSD post hoc tests. All biomass-related variables were ln-transformed prior to statistical analyses to meet the requirement of normal distribution and to avoid incorrectly identifying significant treatment×species interactions in analyses of non-transformed data (Poorter and Garnier 1996). Percentage data (N concentration) were arcsine transformed. The proportional allocation of biomass among different plant parts and morphological traits usually change considerably over the course of plant growth and development. To distinguish between these plant-size-related differences (ontogenetic drift) and real functional adjustments to differing environmental factors I additionally determined allometric relationships of the general form: ln y=m ln x+a where x is total seedling biomass, y represents biomass, area, or length of different plant parts, and m is the slope (allometric coefficient) of the allometric equation (Coleman et al. 1994; Gebauer et al. 1996). The ln-transformed data were centered around the origin by subtracting the grand mean (i.e. the mean values of all treatments within species pooled) from the individual plant values. Differences among allometric coefficients were tested using analysis of covariance (ANCOVA). Stepwise multiple linear regressions were used to determine the morphological and physiological traits that best predict total seedling biomass. For these analyses, total biomass was regressed against morphological and physiological traits by removing the least of the non-significant variables step by step (Zar 1984). The final multiple regression model contained only the significant (PFagus (13×)>Abies (10×)>Taxus (7×)>Quercus (3×) in low light, and in the order (Acer 10×)>Abies (6×)>Taxus (4×)>Fagus (3.5×)>Quercus (1.5×) in very low light (average across all CO2 treatments). In addition to the large interspecific differences in total biomass, species varied considerably in biomass allocation and morphology, irrespective of any CO2 and light effects (Tables 3, 4). Pooled across all CO2 and light treatments, the leaf mass fraction (LMF) ranged from 0.42 to 0.19 g g–1 in the order Abiesa>Taxusb>Acerc>Quercusd>Fagusd (in order of decreasing mean values with different letters indicating significant differences). The root mass fraction decreased from 0.57 to 0.29 g g–1 in the order Quercusa>Acerb>Fagusb>Taxusc>Abiesd (Table 3). The average leaf area ratio (LAR) ranged from 62 to 40 cm2 g–1 and decreased in the order Acera> Fagusb>Taxusc>Quercusd>Abiese. The specific leaf area (SLA) was between 299 and 107 cm2 g–1 and decreased in the order Fagusa>Acerb>Quercusc>Taxusd>Abiese (Fig. 1). CO2 enrichment and increased light availability both significantly stimulated seedling biomass growth and altered the allocation of biomass in all species studied (Tables 3, 4). Some of the CO2 effects depended on light and differed among species as indicated by the significant species×light×CO2 interactions (Table 4). For example, LMF was not significantly affected by elevated CO2 in Quercus and Abies, but slightly decreased with increasing CO2 in Acer. In Fagus LMF increased with increasing CO2 at 1.3% light, whereas it decreased at 3.4% light (Table 3). Taxus showed the same, though only marginally significant pattern of CO2-induced changes in LMF as Fagus did. As a consequence of these light-specific CO2 effects on LMF, Fagus and Taxus seedlings had a higher LAR at elevated CO2 in the 1.3% light microsites (Fig. 1). The LAR in all other species at 1.3% light and in all the species at 3.4% light was either lower or not significantly changed at elevated CO2 (Fig. 1). With the exception of Fagus and Taxus, the observed changes in LAR in response to elevated CO2 and/or light were mainly driven by corresponding changes in SLA rather than LMF. Altered biomass allocation with increasing availability of CO2 and/or light were mainly the result of larger seedlings. Root mass fraction (RMF) increased and LMF decreased as a logarithmic function of total seedling biomass, while SMF remained fairly constant with increasing seedling size (Fig. 2). These relationships were found across all five species (Fig. 2) as well as within each species. Using the coefficients of various allometric
35 Table 3 Total seedling biomass and biomass allocation among leaves, stems and roots at different CO2 concentrations in very low light (1.3% of full sun) and low light (3.4% of full sun) microsites (abbreviations as in Table 2). Means (SEs in parentheses) of a 1.3% of full sun 365 µl l–1 Total seedling biomass (g) Fagus 0.38 (0.04)a Acer 0.30 (0.04)a Quercus 0.74 (0.07)b Taxus 0.044 (0.005)c Abies 0.073 (0.001)d –1 LMF (g g ) Fagus 0.19 (0.01)bc Acer 0.27 (0.02)a Quercus 0.19 (0.02)c Taxus 0.34 (0.03)d Abies 0.43 (0.01)e SMF (g g–1) Fagus 0.50 (0.02)a Acer 0.33 (0.01)be Quercus 0.24 (0.03)bc Taxus 0.36 (0.03)e Abies 0.28 (0.01)bd –1 RMF (g g ) Fagus 0.31 (0.02)a Acer 0.40 (0.02)b Quercus 0.57 (0.03)g Taxus 0.30 (0.01)ac Abies 0.29 (0.01)a
parameter within a row (within species) or column (across species within one treatment) with different letters are significantly different (PQuercusc>Taxusd>Abiese. Alow was positively affected by CO2 (Table 4), but tested within each species separately, the CO2 effect on Alow was significant in Acer, Abies and Taxus, but not in Fagus and Quercus (Fig. 4). In contrast to Amax there was a highly significant light effect on Alow, with higher rates observed in 1.3% light in all species (Fig. 4).
In addition to the photosynthesis measurements made at growth CO2 concentrations I switched the 365 and 660 µl l–1 CO2 concentrations and measured Amax of the same leaves in all deciduous tree seedlings at contrasting CO2 concentrations in July (data not shown). Amax differed only slightly and not significantly between seedlings grown at different CO2 concentrations but measured at equal CO2 concentrations of either 365 or 660 µl CO2 l–1. On average, Amax tended to be 13% lower in Fagus, 5% lower in Acer, and 10% higher in Quercus seedlings grown at 660 compared to those grown at 365 µl CO2 l–1 when measured at equal CO2 concentrations, showing that there was little physiological acclimation to elevated CO2. Leaf area versus mass based assimilation rates According to species and treatment specific differences in SLA (Fig. 1), the patterns described above for leaf mass based net photosynthesis differed from those on a leaf area basis (Table 5). The species effect was less pronounced for area than for mass based assimilation rates (Table 4). The stimulatory CO2 effect on leaf area based Amax and Alow was greater than that observed per unit leaf mass, but was similar in its direction (Table 5). In contrast to Amax per unit leaf mass, leaf area based Amax was significantly higher in seedlings grown at 3.4% light compared to those grown at 1.3% light, while the negative light effect observed for Alow per unit leaf mass was less pronounced when expressed on a leaf area basis.
38 Table 5 Leaf area-based net photosynthesis, Qlcp, and leaf N concentration in tree seedlings exposed to three different CO2 concentrations in very low light (1.3% of full sun) and low light (3.4% of
full sun) microsites. Means (SEs in parentheses) of a parameter within a row (within species) or column (across species within one treatment) with different letters are significantly different (PFagusb>Acerc and was lower at the two elevated CO2 concentrations than at ambient CO2 (not significantly different between 500 and 660 µl CO2 l–1). Fagus showed the largest CO2-induced reduction in Rd of –40% compared with Acer with –17% and Quercus with –14% (mean across both elevated CO2 concentrations and both light levels). The photosynthetic light compensation point (Qlcp) that was calculated from rectangular hyperbolic functions fitted for individual light response curves, was most strongly affected by species and decreased in the order of Abiesa>Taxusb>Quercusb>Fagusc>Acerc. Increasing CO2 concentrations caused a significant species-specific decrease of Qlcp, irrespective of light availability (Tables 4, 5). Averaged across light levels and
both elevated CO2 concentrations, Qlcp decreased by 22% in Fagus, 27% in Acer, 28% in Quercus, 32% in Taxus, and 25% in Abies compared to Qlcp calculated for seedlings grown at 365 µl CO2 l–1. The light compensation point was lower in seedlings grown at 1.3% light than in those grown at 3.4% light (Tables 4, 5). There was also a significant species and CO2 effect on apparent quantum efficiency (φ, data not shown), but light availability had no influence (Table 4). On average, φ decreased from 0.106 to 0.047 mol mol–1 in the order Abiesa>Taxusb>Fagusc>Acerc>Quercusd. Averaged across all species and both light levels φ was 39% higher at 500 µl CO2 l–1 and 59% higher at 660 µl CO2 l–1 compared to ambient CO2 concentration. Leaf nitrogen concentration Leaf nitrogen concentration per unit leaf mass varied significantly among species (Tables 4, 5) decreasing in the order Fagusa>Quercusa>Taxusb>Abiesc>Acerc. Increasing CO2 concentration had a slight overall negative
39 Table 6 Morphological and physiological traits as predictors of total seedling biomass determined by linear stepwise multiple regression analysis. F-values and significance levels are shown for traits with a significant effect. Mean squares for the final regression equations (containing only the “significant variables”), and proportion of variance explained by the model (r2) are indicated for each species *P Fagusd>Acere. Due to the lower SLA at low compared to very low light, area based N concentration was 21% higher at 3.4% light (mean across all species and CO2 levels), but was not different among CO2 concentrations. Functional traits as predictors of seedling biomass growth The correlation between the various physiological and morphological traits and total seedling biomass production was evaluated with multiple linear regressions within each species (Table 6). Species showed a characteristic set of functional traits that were significantly correlated with biomass. Even though none of the species shared the same set of variables in the final regression equations (Table 6), some common patterns among species were identified. In Fagus and Taxus, the highest proportion of variation in seedling biomass was explained by intraspecific variation in either LAR (Fagus) or leaf area ratio per unit root mass (Taxus). In contrast, photosynthesis per unit leaf area correlated most significantly with total biomass in the other three species. Compared across species, the relative change in total seedling biomass in response to CO2 enrichment correlated well with the relative change in LAR in very low light microsites, but not in microsites of an average light availability of 3.4% of full sun (Fig. 5). At 3.4% light, biomass stimulation by elevated CO2 correlated well with CO2-induced changes in leaf photosynthesis, whereas there was no such effect at very low light.
Fig. 5 Mean relative change in LAR (circles) and Amax (triangles) in response to CO2 enrichment as a function of mean relative biomass stimulation under elevated CO2. Each symbol represents the mean relative change observed at the two elevated CO2 treatments (i.e. 500 and 660 µl l–1) compared to the ambient CO2 concentration in the five studied species grown at either 1.3% of full sun (filled symbols) or at 3.4% of full sun (open symbols). The regression equations for LAR as a function of total biomass are y=0.39x12.0 (very low light) and y=0.07x-12.0 (low light), and those for Amax as a function of total biomass are y=-0.13x+51.3 (very low light) and y=0.32x+43.2 (low light)
Discussion Growth and CO2 exchange in relation to shade tolerance Most of the variability in morphological and physiological traits was explained by species identity. Acer pseudoplatanus was the fastest growing species, followed by Abies alba, T. baccata, F. sylvatica, and Q. robur in microsites of a mean light availability of 1.3% full sun. This ranking was largely independent of CO2 and light treatments, except of Fagus becoming the second most fast growing species at a higher light availability of 3.4% full sun. Fast growth is generally associated with high LARs and a relatively low carbon allocation to roots (Poorter and Remkes 1990; Lambers and Poorter 1992), a relationship that also correlates often (Walters et al. 1993; Kitajima 1994; Huante et al. 1995),
40
but not always (Reich et al. 1998), with decreasing shade tolerance in trees. Despite the large differences in growth, LAR and RMF among the five species studied here, no clear relation to shade tolerance could be identified. Quercus was the most slowly growing species with the highest RMF, but at the same time has a lower shade tolerance compared to all other species, and had higher LAR than the considerably faster growing Abies. According to its presumed lower shade tolerance, Acer grew faster than Fagus and Taxus, but had rather higher RMF than these two species and a similar LAR compared to Fagus at very low light. Furthermore, Taxus showed a 33% higher LAR than Abies, despite its slower growth and higher shade tolerance than Abies. At the leaf level, Fagus had the highest SLA, massbased leaf nitrogen concentration and mass-based net photosynthesis at either saturating or low light compared to the other species. All these traits are generally associated with relatively faster growth (Bazzaz 1979; Lambers and Poorter 1992; Reich et al. 1992) and comparatively low shade tolerance in tree seedlings (Walters et al. 1993; Kitajima 1994), which is not consistent with the presumed highest shade tolerance of Fagus. On the other hand, the fast growing Acer seedlings had a lower leaf area based Amax than Quercus or Abies, and its leaf N concentration was the lowest measured in all species, independently of CO2 and light availability. Acer also had the lowest rates of Rd and the lowest Qlcp compared to the other species; both the traits indicate greater shade tolerance (e.g. Givnish 1988), and thus were expected to occur in Fagus rather than Acer. Taxus had lower Qlcp, Amax, and leaf area based N concentration than Abies, which is in accordance with its slower growth and greater shade tolerance compared to Abies. Light compensation points estimated for the coniferous species cannot be directly compared to those of the deciduous species, because they refer to the upper part of the whole shoot. Although the rectangular hyperbolic function provided excellent fits for measured rates of photosynthesis (coefficients of determination higher than 0.95), the calculated Qlcps in the study species appear to be rather low and compare to a range of 2–11 µmol m–2 s–1 reported for 13 tropical tree species grown at 2% light (Kitajima 1994). Some of the marked differences in growth attributes and physiological traits among species reported here were in line with their presumed position along a comparatively narrow gradient of relative shade tolerance, but others were not. No single species combined all characteristics traditionally classified as adaptive to low light conditions, which suggests that there is not a single suite of interconnected “shade tolerance traits”. According to a recent study of leaf-level photosynthesis in late-successional tropical tree species by Thomas and Bazzaz (1999), some evolved traits may rather be related to later stages in the tree's life history and may not be explained by growth conditions encountered in the seedling stage.
Interspecific variation in response to CO2 availability Among the physiological mechanisms for increasing relative CO2 effects with decreasing light are the theoretically predicted and empirically observed higher apparent quantum efficiency (φ) and lower Qlcp under elevated CO2 (Osborne et al. 1997). At much higher light levels (26% of full sun) than were used here, Kubiske and Pregitzer (1996) additionally observed greater CO2 effects on φ and Qlcp with increasing shade tolerance in three deciduous tree species, which they interpreted as a possible mechanism for a greater CO2 responsiveness in more shade tolerant species. In a natural forest understorey of about 10% light, however, DeLucia and Thomas (2000) did not find any consistent difference in Qlcp between ambient and elevated CO2-grown plants, and the species-specific CO2 effect on φ did not correlate with shade tolerance among the four tree species they studied. In line with this latter study, there was no clear correlation between CO2 effects on φ and Qlcp and the relative shade tolerance in the species studied here. More importantly, there was also no correlation between CO2induced shifts in φ and Qlcp and the species-specific biomass responses to elevated CO2. These results suggest that CO2-induced changes in leaf-level Qlcps may have a limited importance for the understanding of interspecific variation in the biomass response to different CO2 concentrations. Which other traits may better explain the species-specific biomass responses to elevated CO2? Besides photosynthetic carbon uptake, respiratory carbon loss has a major influence on the leaf carbon balance at very low light (Givnish 1988), and was found to be significantly reduced at elevated CO2 in the deciduous species studied here. Because leaf dark respiration was measured at a common CO2 partial pressure, these reductions reflect an acclimation response due to intrinsic changes in leaf biochemistry, rather than an immediate CO2 effect. Similar acclimation responses have been observed in previous studies, but were absent in others (see Norby et al. 1999 and references therein). The 40% reduction measured here for Fagus was more than twice that seen in Acer or Quercus, which may be more significant for the overall leaf carbon balance than a reduced Qlcp – in particular at very low light. Although leaf respiration rates may dominate whole plant respiratory carbon loss in small seedlings with relatively large LMF (Lehto and Grace 1994), differential CO2 effects on stem or root respiration, which were not measured here, may further modify the whole plant net carbon balance (Reid and Strain 1994; Tjoelker et al. 1999). At the whole-plant level, Fagus and Taxus showed an increased LAR with increasing CO2 at very low light which corresponds to their greater relative biomass production under these conditions compared to the other species. The allometric shift towards the maximization of LAR is a true, size-independent adjustment to an elevated CO2 concentration (Fig. 3), and as such provides an additional mechanism for the greater CO2 responsive-
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ness at very low light. In all other species, the CO2 effects on seedling morphology and biomass allocation were entirely driven by differences in seedling size, and therefore, can be explained by ontogenetic drift rather than functional adjustments of allocation patterns to an altered environment (Evans 1972; Coleman et al. 1994; Gebauer et al. 1996). The lower relative CO2 stimulation of seedling biomass in Fagus and Taxus at 3.4% light, might be the result of a diminishing relative effect of reduced respiration rates at an overall improved seedling carbon balance, combined with allometric changes, i.e. lower LAR, in faster and bigger growing seedlings. Accordingly, there was no correlation between the relative changes in biomass and LAR in response to CO2 enrichment in seedlings grown at 3.4% light, but a clear positive correlation in seedlings grown at 1.3% light (Fig. 5). The greater CO2 response in Acer, Quercus and Abies at 3.4% light compared to 1.3% light suggests an alternative suite of traits leading to an increased carbon acquisition under elevated CO2 at higher but not at very low light levels. Because biomass production in these three species correlates with leaf photosynthesis better than with any other measured traits, it may be argued that the CO2 stimulation of photosynthesis most importantly contributed to the growth stimulation in these species when grown at 3.4% light. In the very low light microsites, in contrast, light limitation might have prevented any significant contribution of CO2-stimulated photosynthetic carbon uptake. In line with this interpretation, the relative CO2 stimulation of biomass correlated well with the CO2 stimulation of Amax in seedlings grown at 3.4% light, but there was no such correlation in seedlings grown at 1.3% light (Fig. 5). The quite similar increase in Amax among all the species (including Fagus and Taxus) in response to CO2 enrichment, however, indicates that the CO2 stimulation of biomass production was not only the result of a higher steady-state CO2 assimilation rate at relatively high PPFD. It rather might have resulted from species-specific differences in the ability to respond to highly fluctuating light, as has recently been observed for understorey saplings of four different species (Naumburg and Ellsworth 2000). They reported an increase in sunfleck utilization efficiency through longer maintenance of a high induction state under elevated CO2 that differed among the studied species. Conclusions The data provided here contribute to a mechanistic, process-based understanding of distinct response patterns among late-successional tree species to different CO2 concentrations in deep shade. At very low light (1.3% of full sun), a combination of functional adjustments in morphology (higher leaf area ratio) and physiology (lower leaf dark respiration), which were associated with a relatively higher species-specific shade tolerance, are part of the underlying mechanisms for the CO2 stimula-
tion of seedling biomass. At slightly higher average light levels (3.4% of full sun), interspecific differences in the biomass response to elevated CO2 are reversed and correlated with photosynthetic responses. Depending on light and CO2 concentrations, the relative importance of species-specific traits changes, and therefore, no single species or group of species is generally responsive to a higher CO2 availability in the forest understorey. The high variation of the observed responses to relatively small changes in resource availability among cooccurring tree species, received only little attention, because most previous studies included tree seedlings of a wide range in successional status and/or high levels and steep gradients of light availability. I conclude that the broad definitions of functional groups such as early and late-successional, shade tolerant and intolerant, or coniferous and deciduous species, are not sufficient for the understanding of dynamic changes in species composition of understorey tree seedling communities, and the structure of old-growth forests in response to rising concentrations of atmospheric CO2. Acknowledgements I thank L. Zimmermann for the development and maintenance of technical field installations and the collection of microclimatic and CO2 data, S. Pelaez-Riedl for help with seedling harvest, and C. Körner, H. Poorter, and two anonymous reviewers for critically reading earlier versions of the manuscript and helpful comments. Part of this research was funded through the Swiss National Science Foundation grant 31-45822.95 to C. Körner. I also gratefully acknowledge the financial support of The Swiss National Science Foundation, The Novartis Foundation, and The Holderbank Foundation during the writing of this paper.
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