P. B. REICH, M. B. WALTERS, M. G. TJOELKER, D. VANDERKLEIN and C. BUSCHENA

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1 Functional Ecology 1998 ORIGINAL ARTICLE OA 000 EN Photosynthesis and respiration rates depend on leaf and root morphology and nitrogen concentration in nine boreal tree species differing in relative growth rate P. B. REICH, M. B. WALTERS, M. G. TJOELKER, D. VANDERKLEIN and C. BUSCHENA Department of Forest Resources, University of Minnesota, 1530 N. Cleveland Avenue, St Paul, MN 55108, USA Summary 1. To test several hypotheses about acclimation and adaptation of photosynthesis and respiration to differing light conditions, we investigated the interspecific relationships between leaf and root metabolism, chemistry and morphology in high and low light conditions for young seedlings of nine boreal tree species that differ in relative growth rate (RGR). 2. Light-saturated net photosynthesis (A sat ), whole-plant nitrogen (N) uptake rates, leaf and root respiration and morphology, and RGR all varied in parallel among the nine species when grown in both 5% and 25% of full sunlight. RGR, A sat, leaf and root respiration (R d ), and N uptake rate per unit root mass or length differed significantly among species, ranking (from high to low): Populus, Betula and Larix spp. (all deciduous) and then to five evergreen conifers (Pinus, Picea and Thuja spp.), which were generally comparable in these measures. 3. A sat, leaf and root R d and N uptake rates were all correlated (r 0 8 to 0 9) with species traits, such as seed mass, leaf life span and shade-tolerance rankings. Massbased A sat was greater in conifer seedlings raised in low than high light. In contrast, area-based A sat was higher for plants grown in high than low light, especially in the deciduous species. Once adjusted for differences in plant mass, leaf or root respiration rates did not differ for plants grown in low vs high light. 4. Interspecific variation in RGR was positively correlated (r 0 9) with rates of photosynthesis, respiration and N uptake. Leaf photosynthesis and respiration rates were correlated to specific leaf area and leaf N concentrations (r 0 9). Root respiration rates, N uptake rates, specific root length (root length per root dry mass) and root N concentrations were all highly correlated with each other (r 0 8 to 0 9). These data suggest a close coupling of tissue-level metabolism, chemistry and structure with whole-plant performance and species ecophysiological and life-history traits. Key-words: Light, net CO 2 exchange, plant traits, relative growth rate, shade tolerance Functional Ecology (1998) Ecological Society Introduction Differences in leaf and root morphology and net carbon dioxide exchange rates may help species to differentially succeed in various habitats and successional niches (Bazzaz 1979; Chapin 1980; Bazzaz & Carlson 1982; Walters & Reich 1996). Several studies have investigated the extent of co-ordination in physiological, structural and/or whole-plant traits of tree species common to regeneration microhabitats varying in resource availability (e.g. Loach 1967, 1970; Pearcy 1987; Walters, Kruger & Reich 1993a; Kitajima 1994; Walters & Reich 1996). A second issue concerns whether a co-ordinated set of leaf, root and plant-level attributes, such as leaf life span, RGR, net photosynthetic rate, specific leaf area and leaf area ratio (Poorter, Remkes & Lambers 1990; Reich, Walters & Ellsworth 1992; Walters et al. 1993a), and plasticity in such traits, vary systematically among species adapted to high- vs low-resource habitats. A third question involves the relationship between leaf and root structure and N status, and the uptake/use potential for carbon and N, respectively, of those tissues (Reich, Kloeppel et al. 1995; Ryan 1995; Reich, Oleksyn & Tjoelker 1996). Photosynthesis, respiration, proportional biomass partitioning and/or tissue morphological properties have been identified as important correlates of RGR 395

2 396 P. B. Reich et al. (e.g. Poorter & Remkes 1990; Poorter et al. 1990; Reich et al. 1992; Walters, Kruger & Reich 1993b) and variation among species in such traits has been related to their differences in niche or habitat. In this paper we contrast leaf and root net CO 2 exchange characteristics in contrasting light environments among seedlings of nine common North American sub-boreal species. In a companion paper (Reich, Tjoelker et al. 1998) we identified differences in leaf and root structure and biomass partitioning that were correlated with growth-rate differences across these species and light environments. In the current paper we examine further the components of plant traits related to RGR and seek to identify the relationships between metabolic rates and plant structural and chemical traits. Based on the strong co-ordination of leaf, root and whole-plant structure and growth, and their relation to species ecological characteristics (Reich, Tjoelker et al. 1998), we predict that resource uptake rates should also be closely co-ordinated with differences among seedlings in habitat, growth rate and tissue structural attributes. In this paper we address the following questions: how do species differ in photosynthesis, leaf and root respiration and N uptake in high and low light; are species differences in leaf and root resource acquisition rates linked to differences in leaf and root structure, respiration rates and N concentrations; do species differences in photosynthesis, respiration and N uptake contribute to their differences in RGR? It is not yet definitely known whether shade-tolerant tree species have lower respiration rates when grown under comparable light environments than intolerants. This is a long-held, oft-cited (Givnish 1988) but inadequately tested hypothesis (especially for roots) about ways in which shade-tolerant species may enhance their carbon balance in deep shade, relative to intolerants. It is also not known whether plants of all species have lower respiration rates when grown in low vs high light. This long-held hypothesis suggests that all species acclimate to growth in low light by downregulation of respiration rates, which theoretically would reduce carbon losses in that resource-poor environment. There are extremely few data available in the literature to answer this question. To address the above questions we tested the following specific hypotheses: (1) rates of photosynthesis and respiration are positively correlated with RGR (and associated plant traits); (2) shade-tolerant species have lower leaf and root respiration and photosynthetic rates than intolerants, when compared at similar light levels; (3) plants growing in high-light environments have higher photosynthetic capacity than those growing in the shade (i.e. leaves acclimate to their light environment to match somewhat photosynthetic capacity to light availability); (4) plants have lower respiration rates when grown in low vs high light, thereby reducing carbon losses in a carbon-poor environment. Materials and methods PLANT MATERIAL The nine species used in this study are described in Table 1. More detailed descriptions of the species and seed sources can be found in Reich, Tjoelker et al. (1998). For brevity, species will be denoted by genus hereafter, except where necessary to differentiate among species within a genus. GROWTH CONDITIONS, EXPERIMENTAL DESIGN AND GROWTH MEASUREMENT Seedlings were grown in a 60/40% mixture of silica sand and field soil mix in 2 7 litre plastic pots in a temperature-controlled greenhouse at the University of Minnesota, St Paul, MN, USA. Seedlings were irrigated daily with half-strength Hoagland s solution. A fivefold contrast in light level was produced by comparing unshaded blocks with those covered with Table 1. Study species, seed mass, leaf life span and shade tolerance. Mean seed mass determined from mass of 100 seeds. Leaf life span based on field observations of trees (Reich, Walters et al. 1998, and unpublished data). Shade tolerance rankings based on USDA (1990). Because the combined rankings of seed mass, leaf life span and shade tolerance are comparable for the evergreen conifers except Pinus banksiana, they are listed in alphabetical order Combined seed Leaf mass, leaf life Seed mass life span span and shade- Species Type (mg) (months) Shade tolerance tolerance ranking Populus tremuloides Deciduous hardwood Intolerant 1 Betula papyrifera Deciduous hardwood Intolerant 2 Betula allegheniensis Deciduous hardwood Intermediate 3 Larix laricina Deciduous conifer Intolerant 4 Pinus banksiana Evergreen conifer Intolerant 5 Picea glauca Evergreen conifer Intermediate to tolerant 6 Picea mariana Evergreen conifer Tolerant 6 Pinus strobus Evergreen conifer Intermediate to tolerant 6 Thuja occidentalis Evergreen conifer Very tolerant 6

3 397 Photosynthesis, respiration and nitrogen in boreal trees neutral density woven polypropylene cloth attached to wooden frames. Total daily photosynthetic photon flux density (PPFD) in the unshaded (other than by the greenhouse itself) and shaded treatments were 25% and 5% of full sunlight (based on comparisons of treatment vs outdoor daily integrated PPFD). Over the course of the experiment, day/night temperatures averaged 25/20 C. The experiment was arranged as a randomized complete block design, with four blocks containing the two light levels. The nine species and two light combinations comprised a complete factorial arranged as a split-plot, with species as subplots within light environment whole plots. All plants were harvested and oven-dry (70 C for 48 h) masses of leaves, stems and roots were determined over multiple harvests at approximately day intervals. One randomly chosen plant per species light-block combination was harvested at each harvest, used for gasexchange measurements on the day of harvest, and their leaf, root and whole-plant N concentrations were assessed by gas chromatography with a Perkin-Elmer 2400 Elemental Analyzer. Additional details can be found in Reich, Tjoelker et al. (1998). The mean whole-plant net N uptake rate (mg N g root 1 day 1 ) was calculated using the functional approach (Hunt 1982). Acronyms and units for gas-exchange, N uptake and growth-analysis parameters are summarized in Table 2. GAS-EXCHANGE MEASUREMENTS Net photosynthesis and tissue respiration were measured with ADC LCA-2 and LCA-3 portable gasexchange systems operated in the differential mode (Analytical Development Corporation, Hoddesdon, UK). The CO 2 analysers were calibrated against CO 2 standards. Net photosynthesis was measured under saturating irradiance (>900 µmol m 2 s 1 PPFD) at all harvests from day 36 onwards. Light-saturated net photosynthesis (A sat ) was determined for low-light Table 2. Abbreviation, full name and units for gas-exchange rates, growth analysis and tissue morphology terms used in this paper. Areas presented for projected areas Variable abbreviation Variable name Units RGR Relative growth rate mg g 1 day 1 A sat (mass) Light saturated net photosynthetic rate (mass basis) nmol g 1 s 1 A sat (area) Light saturated net photosynthetic rate (area basis) µmol m 2 s 1 R d leaf Leaf dark respiration rate nmol g 1 s 1 R d root Root dark respiration rate nmol g 1 s 1 NUR mass Nitrogen uptake rate per unit root mass mg N g 1 day 1 NUR length Nitrogen uptake rate per unit root length mg N m 1 day 1 LWR Leaf weight ratio (foliage mass/plant mass) g 1 g 1 LAR Leaf area ratio (foliage area/plant mass) cm 2 g 1 SLA Specific leaf area (foliage area/foliage mass) cm 2 g 1 RLR Root length ratio (root length/plant mass) m g 1 SRL Specific root length (root length/root mass) m g 1 plants after placing them in high light for approximately min. Measurements at earlier harvests were not possible owing to small plant sizes. Within each of the four blocks, one plant per each species in both light levels was measured. During measurements, relative humidity was usually 50 60% and air temperature between 23 and 27 C. Usually the entire shoot (stem and needles) was measured for the conifers, whereas the second fully expanded leaf was measured for Populus and Betula plants. Because the broad-leaved species grew faster they had substantial variation in leaf age on a plant. The conifers, except for Larix, largely produced all their foliage in a single early flush (and we measured Larix when the newest flush was mature). Thus, at the time of measurements all foliage for the conifers was mature foliage similar developmentally to the foliage sampled in broadleaved species. To alleviate concern about selfshading within conifer shoots, we measured photosynthetic rates of isolated (unshaded) foliage vs whole-shoot measures, with no discernible difference; probably largely owing to the small degree of shading of these small shoots and the high total light level and its distribution within the cuvette. Shoot and root respiration were measured separately at harvests on day 48 and after (plants were too small at earlier harvests), on the same plants used for photosynthesis measurements. Root respiration was not measured for the low-light treatment for four species because plants were too small. Roots and shoots were separated during harvest and the roots were rinsed with water to remove the sand medium. Given the fact that prior studies found no significant diel variation in respiration rate of seedlings of five northern temperate tree species (Walters et al. 1993a), plants were measured during the daytime after being placed in the dark under constant temperature for one to several hours. Dark respiration was measured at 23 5 (±0 5) C. Projected leaf area and root length were determined as appropriate (AgVision, Decagon Devices, Inc., Pullman, WA, USA). All plant material was then dried in a forced-air oven at 70 C. These data were used to calculate specific leaf area (SLA, cm 2 projected leaf area g 1 dry foliage) and specific root length (SRL m length g 1 dry root). DATA ANALYSIS To compare A sat data to RGR and tissue structure (measured over the first 61 days of growth) we show A sat means for days 36 and 48 combined (n = 8 per species light combination). To compare R d among species and with RGR, comparisons are made using R d rates at a common plant mass range (to the extent possible). Values at a given mass were estimated from regressions of plant traits (e.g. root R d rate) on ln-dry mass using individual plant data from all blocks and harvests within a treatment. Leaf and root R d were strongly correlated with plant dry mass in

4 398 P. B. Reich et al. the fastest-growing species treatment combinations and, for these, R d rates at plant mass 0 05 to 0 5 g were used; for all other species, where respiration was not significantly related to plant mass, mean values were used. To compare with RGR and mean tissue traits (averaged over a 61-day interval, see Reich, Tjoelker et al. 1998), N uptake rates were averaged over the same 61-day interval. We tested for treatment (species, light) effects and their interactions on growth and physiology with analysis of variance using JMP statistical software (SAS Institute, Cary, NC). Regression analyses were used to examine relationships between continuous variables. Treatment and species effects and relationships were considered to be significant if P Results PHOTOSYNTHESIS AND RESPIRATION In both high and low light, light-saturated net photosynthesis (A sat ) varied considerably among species (fourfold variation) when expressed on a mass basis (Fig. 1). Populus and Betula papyrifera had the greatest mass-based A sat in both light regimes, followed in decreasing order by Betula alleghaniensis and the six Fig. 2. Leaf and root dark respiration rates (nmol g 1 s 1 ) for nine boreal species grown at either 5 or 25% of full sunlight (± one standard error). n.d. = no data. conifers as a group, with Pinus strobus having the lowest rates. When expressed on an area basis, A sat was higher in the deciduous species than in Thuja and both Pinus spp., but only in high-light grown plants. Mass-based A sat was significantly greater in the conifers for plants grown in low than high light. However, because SLA was markedly higher in low than in high light (Reich, Tjoelker et al. 1998), differences in area-based A sat between high and low light were generally reversed, with area-based A sat greater in high light plants, especially for the deciduous species (Fig. 1). Leaf and root R d followed the same pattern among plants as A sat and RGR (Reich, Tjoelker et al. 1998): being greatest in deciduous Populus, Betula and Larix and least in the evergreen conifers (Fig. 2). Within species, plants grown in low and high light generally had similar R d rates. Fig. 1. Light-saturated net photosynthesis (± one standard error of blocks) on a mass (nmol g 1 s 1 ) and area (µmol m 2 s 1 ) basis for nine boreal species at either 5 or 25% of full sunlight. NITROGEN UPTAKE RATES Nitrogen (N) uptake rates per g of root mass followed the same rankings as for root R d, being highest in Populus, the two Betula spp. and Larix, and lowest in evergreen conifers (Fig. 3). Species differed by as much as sevenfold in this measure. Differences among species in N uptake rates were not just a function of species differences in SRL (Reich, Tjoelker et al. 1998); even when N uptake was expressed per unit root length, species differences followed the same ranking, although the total variation was halved.

5 399 Photosynthesis, respiration and nitrogen in boreal trees and 0 98, respectively). Leaf R d was also well correlated with SLA (r = 0 88 and 0 95, respectively in high and low light). For both A sat and leaf R d there was a tendency for high-light plants to have greater rates at any given SLA. Nitrogen uptake rate was highly correlated with SRL (Fig. 6, Table 3) in both 25% and 5% light treatments (r = 0 90 and 0 98, respectively), as was root R d (r = 0 77 and 0 84, respectively). Rates of leaf and root R d, A sat and N uptake rates were poorly correlated with LWR (Table 3) or other measures of biomass allocation (data not shown). Fig. 3. Nitrogen (N) uptake rates (± one standard error) on root mass and length bases for nine boreal species grown at either 5 or 25% of full sunlight. Populus and Betula have high SRL, but also have high N uptake rates per unit root length; thus both root structure and metabolism probably contribute to species differences in N uptake rates. Low-light grown plants tended to have lower N uptake rates than high-light grown plants in most species. TISSUE N CONCENTRATION IN RELATION TO A sat, AND LEAF AND ROOT R d Among species, rates of net CO 2 exchange were closely correlated with tissue nitrogen concentrations. Mass-based A sat was correlated to leaf N (P < 0 05, r = 0 80 and 0 85 in high and low light) (Fig. 4). Variation among species in leaf respiration was closely coupled with leaf N (r = 0 96, all data pooled) and root respiration was also correlated with root N (r = 0 81, all data pooled). These respiration N relationships were quite similar for plants in both light environments (Fig. 4). RELATIONSHIPS AMONG GAS EXCHANGE AND GROWTH TRAITS RGR was well correlated (P < 0 05) with mass-based A sat, leaf R d, root R d and N uptake rates (r = 0 94 to 0 99 for plants grown in high light, r = 0 90 to 0 96 in low light) (Fig. 5, Table 3). Rates of net CO 2 exchange and N uptake were closely related with tissue structure. Mass-based A sat was highly correlated with SLA (Fig. 6) in both 25% and 5% light treatments (r = 0 88 Fig. 4. Correlations between mass-based A sat (nmol g 1 s 1 ) vs leaf N concentration (mg g 1 ), leaf R d (nmol g 1 s 1 ) vs leaf N concentration and root R d (nmol g 1 s 1 ) vs root N concentration, for seedlings of nine boreal species grown at either 5 or 25% of full sunlight. Correlations and P values, as follows: high light A sat = leaf N, P < 0 001, r = 0 80; low light A sat = leaf N, P < 0 001, r = 0 85; high light leaf R d = leaf N, P < 0 001, r = 0 96; low light leaf R d = leaf N, P < 0 001, r = 0 96; high light root R d = leaf N, P < 0 001, r = 0 83; low light root R d = leaf N, P < 0 001, r = 0 76.

6 400 P. B. Reich et al. Fig. 5. Correlations between relative growth rate (RGR, mg g 1 day 1 ) and mass-based A sat, leaf R d, root R d (all nmol g 1 s 1 ) and N uptake rate [mg N (g root) 1 day 1 ] for seedlings of nine boreal species grown at either 5 or 25% of full sunlight. Correlation and P values, as follows: high light RGR = A sat, P < 0 001, r = 0 97; low light RGR = A sat, P < 0 001, r = 0 93; high light RGR = N uptake rate, P < 0 001, r = 0 99; low light RGR = N uptake rate, P < 0 001, r = 0 93; high light RGR = leaf R d rate, P < 0 001, r = 0 97; low light RGR = leaf R d rate, P < 0 001, r = 0 96; high light RGR = root R d rate, P < 0 001, r = 0 94; low light RGR = root R d rate, P < 0 01, r = Table 3. Correlation matrix (using pairwise correlations) for net CO 2 exchange rates and other measured variables. The upper right half of the matrix shows correlations in high light (25%) and the lower left half the correlations in low light (5%). Correlations significant at P < 0 05 are shown in bold. RGR was measured over a 61-day growth period, while leaf weight ratio (LWR), leaf area ratio (LAR), specific leaf area (SLA) and N uptake rates (per unit root mass and root length, NUR-mass and NUR-length) were for data averaged over four intermediate harvests equally spaced during that period. Light-saturated net photosynthetic rate on mass (A sat-mass ) and area (A sat-area ) bases were averaged over two harvest dates during the 61 day growth period. Leaf and root respiration (Rd leaf and Rd root ), root length ratio (RLR) and specific root length (SRL) were averages at a common plant size. Seed mass is the logarithm of dry seed mass and leaf life span the inverse (i.e. turnover rate) observed in field studies (e.g. Reich, Walters et al. 1998) Leaf A sat- A sat- NUR NUR life Seed Variable mass area Rd leaf Rd root mass length LWR LAR SLA RLR SRL RGR span mass A sat-mass A sat-area Rd leaf Rd root NUR mass NUR length LWR LAR SLA RLR SRL RGR Leaf life span Seed mass

7 401 Photosynthesis, respiration and nitrogen in boreal trees Fig. 6. Relationship between mass-based A sat and leaf R d (both nmol g 1 s 1 ) vs SLA (cm 2 g 1 ); and N uptake rate [mg N (g root) 1 day 1 ] and root R d (nmol g 1 s 1 ) vs specific root length (m g 1 ) for seedlings of nine boreal species grown at either 5 or 25% of full sunlight. Correlations and P values, as follows: high light A sat = SLA, P < 0 001, r = 0 88; low light A sat = SLA, P < 0 001, r = 0 98; high light leaf R d rate = SLA, P < 0 001, r = 0 88; low light leaf R d rate = SLA, P < 0 001, r = 0 95; high light N uptake rate = SRL, P < 0 001, r = 0 90; low light N uptake rate = SRL, P < 0 001, r = 0 97; high light root R d rate = SRL, P < 0 001, r = 0 70; low light leaf R d rate = SRL, P < 0 001, r = Leaf and root structure and function were also well correlated with seed mass and leaf life span (Table 3), and with shade tolerance rankings using the Spearman rank association test (data not shown). Mass-based A sat, leaf R d, root R d and N uptake rate were all significantly correlated with seed mass in both high (r = 0 74 to 0 86) and low light (r = 0 88 to 0 94), respectively (Table 3). A sat, leaf R d, root R d and N uptake rate were also well correlated in high (r = 0 90 to 95) and low light (r = 0 76 to 0 87), respectively, with the leaf turnover rate (inverse of leaf life span) of the species as assessed under field conditions. Discussion There was fourfold variation in A sat, leaf R d, root R d and N uptake rate among seedlings of tree species that co-occur in boreal ecotone sites in North America. Variation in these traits was closely associated with species differences in RGR, leaf and root structure, successional niche, seed size, and leaf life span [supporting hypothesis (1)]. Results of this study were consistent with trends for greater massbased A sat, leaf R d, root R d and/or N uptake in species that are shade-intolerant rather than tolerant (Bazzaz 1979; Walters et al. 1993) [supporting hypothesis (2)], early rather than late successional (Bazzar 1979; Reich, Ellsworth & Uhl 1995) and of short rather than long leaf life span (Reich et al. 1992; Reich, Kloeppel et al. 1995). Comparison with work by F. S. Chapin and colleagues may be useful, given that five of the same species were studied and their growth rate ranking was similar to the present study. During early and mid-summer in the field of Alaska, leaf N and R d rates were higher in Populus and Betula than in Picea glauca or Picea mariana (Chapin & Tryon 1983) and our greenhouse results for these species contrasts are comparable. However, fine root R d rates in Alaska measured over the season at a common temperature were highest for Larix, comparable for Picea sp. and Betula, and lowest for Populus (Chapin & Tryon 1983). Thus, our data, in which root R d rates closely mirror leaf R d rates and were highest in Populus, are not consistent with these earlier reports. In a laboratory study with seedlings, phosphate was absorbed more rapidly per unit excised fine root mass by Populus tremuloides and B. papyrifera than by P. mariana (Chapin, Van Cleve & Tryon 1986). No data were presented for ammonium and nitrate absorption for the conifers but it was stated that nitrate absorption was low and indistinguishable in all species, apparently including the conifers. In our study, the evergreen conifers had lower total N uptake rates per unit root mass. Given the differences in measurements (our results provide an estimate of N

8 402 P. B. Reich et al. accumulation by an intact whole system root, compared with direct measurements of nutrient uptake rates by excised fine roots), these contrasting results cannot be easily reconciled. NET CO 2 EXCHANGE RATES IN RELATION TO SHADE TOLERANCE AND RGR Data from our study of nine boreal species also differ from results of other prior studies. First, in either 23% or 2% of full light, maximum net photosynthesis and leaf R d rates of 13 tropical tree species were generally unrelated to species variation in shade tolerance (Kitajima 1994). In contrast, there was correlation between shade tolerance and both mass-based A sat and R d for the nine boreal species, consistent with results across five sympatric temperate deciduous species that vary in shade tolerance (Walters et al. 1993a). Mass-based A sat was positively related to RGR for Kitajima s (1994) 13 tropical species when grown in 23% light (r = 0 71) but not in 2% light, whereas in our boreal species group this relationship was observed in both 25% (r = 0 97) and in 5% light (r = 0 86). In our study, area-based A sat was better correlated with RGR for high-light grown plants than for those in low light (Table 3) similar to results of both Kitajima (1994) and Walters et al. (1993a). The consistent finding of a stronger overall relationship between mass-based A sat and RGR than areabased A sat and RGR in this study and others (Poorter et al. 1990; Walters et al. 1993a; Kitajima 1994) suggests a more direct relationship between RGR and CO 2 gained per unit mass invested than per unit area displayed. The better overall relationship between mass- than area-based A sat and RGR also supports the idea that mass-based gas-exchange rates are more relevant to measures of plant energetics than areabased ones (Reich et al. 1992; Walters et al. 1993). The smaller differences among species in area-based than mass-based A sat results from the large gradient in a co-ordinated set of leaf traits (SLA, leaf life span, nutrient concentrations) among the nine boreal species, as has been demonstrated in other species groups (Reich et al. 1992, 1994). It has been shown that SLA decreases with increasing leaf life span and decreasing mass-based N and A sat ; and that decreasing SLA offsets these to result in a smaller gradient in area-based than mass-based A sat among species (Reich et al. 1992; Reich, Ellsworth & Uhl 1995; Reich, Kloeppel et al. 1995). Several authors have stated that there is no relationship between photosynthetic capacity and plant productivity or growth rate (e.g. Gifford et al. 1984; Teskey, Whitehead & Linder 1994). Clearly, the validity of this statement depends on the contrasts being made but such statements have been used perhaps out of context to suggest there is little overall relationship between photosynthetic rate and growth. Frequently, studies of populations or treatment contrasts within species do find little support for any such relationship. However, in broad species comparisons, it has been clearly demonstrated that RGR of young seedlings under controlled conditions (this study, also Poorter et al. 1990; Reich et al. 1992; Walters et al. 1993a; Kitajima 1994; Walters & Reich 1996) and height growth of young individually grown field trees (Reich et al. 1992) are highly correlated with photosynthetic rate, especially on a mass basis. Area-based photosynthesis may also be significantly correlated with RGR (plants in high but not low light in this study), although this relationship may often not hold (Walters et al. 1993a; Kitajima 1994). We found that shade-tolerant species do have lower shoot and root respiration rates at a common plant size than intolerant species [supporting hypothesis (2)], consistent with a long-held, oft-cited and poorly tested hypothesis (Givnish 1988) that already borders on paradigm despite the lack of empirical evidence (but see Fredeen & Field 1991; Ellsworth & Reich 1992). It is also not known whether plants of all species have lower respiration rates when grown in low vs high light [hypothesis (4)]. This long-held hypothesis suggests that all species acclimate to growth in low light by down-regulation of respiration rates, which theoretically would reduce carbon losses in that resource-poor environment. There are extremely few data available in the literature to answer this question. In this study, data did not support this hypothesis: for every species, respiration rates for plants of comparable size were similar whether grown in high or low light. LINKAGES OF PLANT TRAITS It is worth considering the issues of direct and indirect cause and effect related to the strong correlations across species in so many tissue and whole-plant traits (Table 3). Several likely are indirectly related: for instance, seed mass variation among species may be associated with root, leaf and whole-plant net CO 2 exchange, morphology and growth as the result of parallel selection. Having small or large seeds and high or low SLA, NUR, A sat and RGR, respectively, may be advantageous for species adapted to certain types of regeneration microhabitats. Other correlations are the direct result of aggregation of tissue traits from organs to the whole plant: LAR is a direct function of SLA LWR, RLR is a direct function of SRL RWR and whole-plant net CO 2 exchange rates are a direct function of tissue net CO 2 exchange rates proportional biomass allocation to each tissue type. Yet a third type of correlation is functional, with either direct one- or two-way cause effect relationships, or indirect relationships mediated at the organ system or whole-plant level. An example of the former would involve the relationships of tissue N with leaf and root CO 2 exchange rates. It is well established

9 403 Photosynthesis, respiration and nitrogen in boreal trees that these relationships are fundamental in nature because N-containing proteins play a major biochemical role in photosynthesis and respiration. Relations between A sat, leaf R d, SLA and leaf life span are less direct, but no less fundamental or widespread, and involve both the physico-chemical and evolutionary constraints on photosynthesis and respiration related to leaf structure and longevity (Reich et al. 1992). Finally, the close correlation between structure, chemistry and function of leaves and roots seen among the nine species suggests that selection for leaf and root traits is parallel in boreal tree species: trees have evolved to have both leaves and roots with finer structure and higher %N concentration, respiration rates and resource uptake rates (A sat and N uptake), or the reverse set of traits. Whether this pattern is widespread among trees or other plant types is unknown at present, because insufficient characterization of root traits exists across large species numbers to make such comparisons. NET CO 2 EXCHANGE RATE AS INFLUENCED BY LIGHT LEVELS, N CONCENTRATIONS AND MODE OF EXPRESSION On a mass basis, N and A sat were equal or slightly greater at low than high light in the nine boreal species, similar to slightly greater rates of mass-based A sat for Sugar Maple and hybrid Poplar grown experimentally at 7% vs 45% of full sunlight (Volin et al. 1993) and for five hardwood tree species grown at 15% vs 75% of full sunlight (Walters et al. 1993a). For mature field-grown Sugar Maple, leaves grown on branches in deep shade (c. 2 10% of full light) had similar leaf %N and A sat as leaves growing on branches in higher light (25 60% of full light) (Ellsworth & Reich 1993). In contrast, natural Sugar Maple seedlings growing in a gap (15% of full light) had higher leaf %N and mass-based A sat than seedlings in very deep shade (2% of full light) (Ellsworth & Reich 1992) and tropical tree seedlings, whether shade-tolerant or intolerant, had greater mass-based A sat when grown at 23 than 2% light (Kitajima 1994). In addition to the present paper, several crossspecies studies have found significant correlations between foliar respiration and N concentrations, including with field-grown trees (Ryan 1995; Reich et al. 1996; Reich, Walters et al. 1997). Poorter et al. (1990) reported higher tissue %N and R d for fastgrowing than slow-growing herbs, but it was not clear whether R d and %N were significantly related. Within species, relationships between %N and R d have often, but not always, been found (Byrd, Sage & Brown 1992; Lambers & Poorter 1992; Poorter et al. 1995; Reich et al. 1996). Given the greater range of both %N and R d among than within species, it should not be surprising that a stronger relationship between the two also is found on that basis. How do these seedling results compare with data from mature trees? Rankings among species of A sat and R d were similar for greenhouse-grown seedlings as field-grown trees (Reich, Kloeppel et al. 1995, unpublished data), and the ratio of A sat to leaf R d (based on the slope of the regression between the two) is roughly 10:1 for both seedlings (this study) and mature trees (Reich, Walters et al. 1997). Leaf N concentrations, SLA, A sat and R d were all higher in greenhouse-raised seedlings than in field-grown trees, though. The slopes of the mass-based A sat -leaf N and R d -N relationships were also higher for seedlings than field-grown trees (Reich et al. 1991, 1996; Reich, Kloeppel et al. 1995; Ryan 1995), consistent with data showing a steeper slope of such relationships for plants with higher mean SLA and %N (Reich et al. 1994; Reich, Kloeppel et al. 1995). Sometimes different results have been reported when comparisons were made of mass- vs area-based photosynthetic rates. In this study, among the nine boreal species, the faster-growing species had higher mass-based photosynthetic rate, but similar areabased photosynthetic rates, than slow growers, similar to some other multiple species comparisons (Poorter et al. 1990; Reich et al. 1992). However, in other data sets it has been found that even on an area-basis, A sat is higher in fast-growing, early successional species in either the laboratory (Walters et al. 1993a) or field (Reich, Ellsworth & Uhl 1995). How can these differences be reconciled? It appears that interspecific differences in SLA (which is highly plastic within species as a function of plant age and microenvironment) always reduce the proportional differences among species in large group contrasts when comparisons of A sat or R d are made on area rather than mass bases (Poorter et al. 1990; Reich et al. 1992; Walters et al. 1993a; Reich, Ellsworth & Uhl 1995; Reich, Kloeppel et al. 1995; Reich, Walters et al. 1997; this study). Whether or not marked areabased species differences occur depends on the specifics of each study. For instance, for hardwood seedlings (Walters et al. 1993a) and tropical trees (Reich, Ellsworth & Uhl 1995), similar patterns of interspecific differences in A sat were observed on both area and mass bases, because SLA differences were small enough they reduced but did not eliminate species differences in area-based A sat. In contrast, for herbaceous seedlings (Poorter et al. 1990), woody seedlings (low light, this study), or temperate trees (Reich, Kloeppel et al. 1995), interspecific differences in SLA completely offset differences in massbased A sat, such that no differences in area-based A sat across species occurred. The conclusions that can be reached are: (1) mass-based A sat varies among species with respect to habitat affinity and growth rate differences, (2) area-based A sat differences are always less along such species gradients and (3) the occurrence of area-based patterns among species can be considered a weak general phenomenon.

10 404 P. B. Reich et al. RESULTS IN RELATION TO VARIATION IN SPECIES ECOPHYSIOLOGY AND LEAF MORPHOLOGY In general, data from this study are consistent with theories and empirical data about habitat-related ecophysiological traits, which suggest that species adapted to high-resource environments have high RGR, high potential rates of resource capture and high tissue metabolic rates (and associated morphological) relative to species characteristic of low-resource environments (e.g. Bazzaz 1979; Chapin 1980; Reich et al. 1992; Reich, Ellsworth & Uhl 1995; Reich, Kloeppel et al. 1995). Small-seeded Populus and Betula colonize high-light early-successional sites where a high potential RGR can be realized and as seedlings display high RGR and associated traits, such as high photosynthetic, respiration and N uptake rates, as well as leaf and root morphology (i.e. high SLA and SRL) associated with enhanced resource acquisition (i.e. high LAR and RLR) rather than durability. In contrast, a low potential rate of resource capture is presumed to be an adaptation to habitats characterized by chronically low resource availability (Chapin 1980; Reich et al. 1992; Walters et al. 1993a), such as the shaded and often nutrient-poor environments in which Pinus, Picea and Thuja are common. The latter prediction was supported by the low growth and mass-based photosynthetic rates of these five evergreen species. Overall, the data from this study suggest a close coupling of tissue structural, chemical and metabolic activity both within and among leaves and roots, that, in combination, is strongly associated with RGR, and is also strongly linked to other species attributes and life-history traits. References Bazzaz, F.A. (1979) The physiological ecology of plant succession. Annual Review of Ecology and Systematics 110, Bazzaz, F.A. & Carlson, R.W. (1982) Photosynthetic acclimation to variability in the light environment of early and late successional plants. Oecologia 54, Byrd, G.T., Sage, R.F. & Brown, R.H. (1992) A comparison of dark respiration between C 3 and C 4 plants. Plant Physiology 100, Chapin III, F.S. (1980) The mineral nutrition of wild plants. Annual Review Ecology and Systematics 11, Chapin III, F.S. & Tryon, P.R. (1983) Habitat and leaf habit as determinants of growth, nutrient absorption, and nutrient use by Alaskan taiga forest species. Canadian Journal of Forest Research 13, Chapin III, F.S., Van Cleve, K. & Tryon, P.R. (1986) Relationship of ion absorption to growth rate in taiga trees. Oecologia 69, Ellsworth, D.S. & Reich, P.B. (1992) Leaf mass per area, nitrogen content and photosynthetic carbon gain in Acer saccharum seedlings in contrasting forest light environments. Functional Ecology 6, Ellsworth, D.S. & Reich, P.B. (1993) Canopy structure and vertical patterns of photosynthesis and related leaf traits in a deciduous forest. Oecologia 96, Fredeen, A.L. & Field, C.B. (1991) Leaf respiration in Piper species native to Mexican rainforest. Physiologia Plantarum 82, Gifford, R.M., Thorne, J.H., Hitz, W.D. & Giaquinta, R.T. (1984) Crop productivity and photoassimilate partitioning. Science 225, Givnish, T.J. (1988) Adaptation to sun and shade: a whole plant perspective. Australian Journal of Plant Physiology 15, Hunt, R. (1982) Plant Growth Curves. The Functional Approach to Growth Analysis. Edward Arnold, London. Kitajima, K. (1994) Relative importance of photosynthetic traits and allocation patterns as correlates of seedling shade tolerance of 13 tropical trees. Oecologia 98, Lambers, H. & Poorter, H. (1992) Inherent variation in growth rate between higher plants: a search for physiological causes and ecological consequences. Advances in Ecological Research 23, Loach, K. (1967) Shade tolerance in tree seedlings. I. Leaf photosynthesis and respiration in plants raised under artificial shade. New Phytologist 66, Loach, K. (1970) Shade tolerance in tree seedlings. II. Growth analysis of tree seedlings raised under artificial shade. New Phytologist 69, Pearcy, R.W. (1987) Photosynthetic gas-exchange responses of Australian tropical forest trees in canopy, gap and understory micro-environments. Functional Ecology 1, Poorter, H. & Remkes, C. (1990) Leaf area ratio and net assimilation rate of 24 wild species differing in relative growth rate. Oecologia 83, Poorter, H., Remkes, C. & Lambers, H. (1990) Carbon and nitrogen economy of 24 wild species differing in relative growth rate. Plant Physiology 94, Poorter, H., van de Vijver, C., Boot, R.G. & Lambers, H. (1995) Growth and carbon economy of a fast-growing and a slow-growing grass species as dependent on nitrate supply. Plant and Soil 171, Reich, P.B., Ellsworth, D.S. & Walters, M.B. (1991) Leaf development and season influence the relationships between leaf nitrogen, leaf mass per area, and photosynthesis in maple and oak trees. Plant, Cell and Environment 14, Reich, P.B., Walters, M.B. & Ellsworth, D.S. (1992) Leaf lifespan in relation to leaf, plant and stand characteristics among diverse ecosystems. Ecological Monographs 62, Reich, P.B., Walters, M.B., Ellsworth, D.S. & Uhl, C. (1994) Photosynthesis nitrogen relations in Amazonian tree species. I. Patterns among species and communities. Oecologia 97, Reich, P.B., Ellsworth, D.S. & Uhl, C. (1995) Leaf carbon and nutrient assimilation and conservation in species of differing successional status in an oligotrophic Amazonian forest. Functional Ecology 9, Reich, P.B., Kloeppel, B.D., Ellsworth, D.S. & Walters, M.B. (1995) Different photosynthesis nitrogen relations in deciduous hardwood and evergreen coniferous tree species. Oecologia 104, Reich, P.B., Oleksyn, J. & Tjoelker, M.G. (1996) Needle respiration and nitrogen concentration in Scots pine populations from a broad latitudinal range: a common garden test with field-grown trees. Functional Ecology 10, Reich, P.B., Tjoelker, M.G., Walters, M.B., Vanderklein, D.W. & Buschena, C. (1998) Close association of RGR, leaf and root morphology, seed mass and shade tolerance in seedlings of nine boreal tree species grown in high and low light. Functional Ecology 12, Reich, P.B., Walters, M.B., Ellsworth, D.S., Vose, J., Volin, J., Gresham, C. & Bowman, W. (1998) Relationships of

11 405 Photosynthesis, respiration and nitrogen in boreal trees leaf dark respiration to leaf N, SLA, and life-span: a test across biomes and functional groups. Oecologia (in press). Ryan, M.G. (1995) Foliar maintenance respiration of subalpine and boreal trees and shrubs in relation to nitrogen content. Plant, Cell and Environment 18, Teskey, R.O., Whitehead, D. & Linder, S. (1994) Photosynthesis and carbon gain by pines. Ecological Bulletin 43, Volin, J.C., Tjoelker, M.G., Oleksyn, J. & Reich, P.B. (1993) Light environment alters response to ozone stress in Acer saccharum Marsh. and hydrid Populus L. seedlings. II. Diagnostic gas exchange and leaf chemistry. New Phytologist 124, Walters, M.B. & Reich, P.B. (1996) Are shade tolerance, survival, and growth linked? Light and nitrogen effects on hardwood seedlings. Ecology 77, Walters, M.B., Kruger, E.L. & Reich, P.B. (1993a) Growth, biomass distribution and CO 2 exchange of northern hardwood seedlings in high and low light: relationships with successional status and shade tolerance. Oecologia 94, Walters, M.B., Kruger, E.L. & Reich, P.B. (1993b). Relative growth rate in relation to physiological and morphological traits for northern hardwood seedlings: species, light environment and ontogenetic considerations. Oecologia 96, Received 15 January 1997; revised 28 July 1997; accepted 7 August 1997