The first physiologically meaningful contact between plant and atmosphere takes place at the leaf, and most subsequent effects of increasing CO2 concentration are linked to changes in CO2 assimilation. Because of this, a great deal of attention has focused on leaf-level photosynthetic responses to CO2 enrichment. The undisputed response to increasing [CO2] is an increase in photosynthesis, but a host of questions have arisen concerning longer-term effects after growth and development at a higher CO2 concentration. The key question, relative to global change impacts on forests, is how much photosynthesis will increase as atmospheric CO2 concentrations rise, and what bearing this will have on higher-level processes. The answer may (or may not) be complicated by interactions with other environmental gradients, and may vary within the canopy, seasonally or between species. In addition, because some very early CO2 enrichment experiments reported complete losses of photosynthetic enhancement after extremely short exposure times (e.g. days to weeks; reviewed for crops by Cure & Acock 1986), there has been a particular focus on detecting and explaining possible decreases in photosynthetic stimulation over time.
The first part of the question, the magnitude of photosynthetic response to CO2 that can be sustained over a season or several seasons, can be addressed by comparing assimilation at the growth CO2 concentration, typically measured on single leaves at light saturation. In trees growing outdoors, rooted in the ground, these rates were almost always higher in elevated CO2, regardless of the duration of the study. Photosynthesis was stimulated 40–80% in most of the studies reviewed here, although in several cases the enhancement was substantially greater (Table 2, Fig. 1a). The mean enhancement of 66% (geometric mean 63%) is greater, and the variability is less, than that reported in a previous review of tree responses (44%, Gunderson & Wullschleger 1994), at which time most available data were from experiments with potted material, and encompassed a wider range of [CO2].
Table 2. . Photosynthetic enhancement ratios (elevated/ambient, E/A) observed in field grown trees exposed to CO2 concentrations ≈250–350 p.p.m. above ambient. Ratios in column 6 were calculated from the photosynthetic rates measured at the growth concentration, and those in columns 7 and 8 from rates measured at common Ca. Values in the table represent the mean ratio for an experiment within each species and interacting treatment. Photosynthetic rates were taken from text, tables or estimated from figures in the sources cited. Trends within a treatment (seasonal, with temperature, with decreasing water potential, etc.) are discussed in the text
Figure 1. . Frequency distribution for the relative photosynthetic responses of field-grown trees under CO2 enrichment compared to those at ambient [CO2]. Frequency indicates the number of observations (see Table 2) within each ratio interval for (a) leaves measured at their respective growth [CO2], (b) leaves measured at ambient [CO2] concentrations regardless of growth [CO2], and (c) leaves measured at a common elevated [CO2].
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The field experiments have been useful for describing how other environmental variables could modify the photosynthetic responses to CO2. The photosynthetic response might be reduced by nutrient deficiency (Eamus & Jarvis 1989; Tissue, Thomas & Strain 1993; Curtis et al. 1994; Sage 1994), or conversely, enhanced in combination with other stresses (Long 1991; Idso & Idso 1994), or unaffected by stress (Curtis & Wang 1998). Conflicting interactions between CO2 and nitrogen concentration have also been related to secondary effects of nutrient supply on growth and sink strength (Pettersson & McDonald 1994), which could complicate the interpretation of experimental results. In three field experiments in which nutrients were deliberately manipulated (Table 2), season-long enhancements were greater in the high-nutrient treatments (Curtis et al. 1995; Kubiske et al. 1997) or increased after nutrients were added (Curtis et al. 1994), but photosynthesis was enhanced by 40–62% even in the lower-nutrient treatments, and there was no evidence of a nitrogen reallocation from ribulose bisphosphate carboxylase/oxygenase (rubisco) to other photosynthetic systems (Curtis et al. 1995). In a fourth study (Tissue, Griffin & Ball 1998), annual soil nitrogen fertilization had no significant effect on photosynthetic parameters.
Under some circumstances, responses to CO2 might be reduced if water deficits are severe enough to limit photosynthetic enzymatic activity, but responses are more likely to be enhanced if elevated [CO2] reduces the importance of drought-induced stomatal limitation (Chaves & Pereira 1992). Two experiments with mature trees, in which the response to CO2 enrichment and changes in leaf water potential were tracked during natural droughts, support the latter hypothesis. The relative photosynthetic stimulation increased to 100% enhancement at water potentials of – 4·5 MPa (Scarascia-Mugnozza et al. 1996), particularly at elevated temperatures (Kellomäki & Wang 1996). Enhancement was likewise greater during drought for Picea abies saplings in an unreplicated open-top chamber experiment, although the four Quercus rubra saplings in the same chamber showed the opposite response (Dixon, LeThiec & Garrec 1995). Goodfellow, Eamus & Duff (1997) reported greater impacts of CO2 enrichment during the tropical dry season when stomatal conductance was low, and leaf water potential was maintained.
Many of the differences in CO2 effects within studies and perhaps between studies can be explained by temperature differences. As discussed by Long (1991), the relative affinity of rubisco for CO2 decreases markedly with increasing temperature, but elevated CO2 concentrations increase the competitive inhibition of oxygenation such that the relative stimulation of assimilation by elevated CO2 increases with temperature, and the temperature optimum for assimilation increases with increasing [CO2]. Experiments with temperature manipulation treatments confirm this with higher CO2 enhancement ratios for trees growing in temperatures raised 2–4 °C above the ambient chambers (Table 2; Kellomäki & Wang 1996; C. Gunderson, unpublished results). Idso et al. (1995) compared rates measured at leaf temperatures from 30 to 46 °C over four growing seasons. Relative stimulation by CO2, already higher at these temperatures than at the more moderate conditions of many studies, increased with temperature, and sharply so, as assimilation rates in the ambient CO2 trees approached zero at the highest temperatures. Temperature is also a factor in some seasonal patterns reported for CO2 responses, for example, much of the difference in enhancement of assimilation in Pinus taeda in summer months (60–130% increase) versus winter months (14–44%), is explained by seasonal temperature differences (Tissue, Thomas & Strain 1997; Lewis, Tissue & Strain 1996).
Most of the results discussed above are from single healthy leaves at comparable leaf age and position, measured at light saturation and, in some cases, under idealized conditions, minimizing leaf-to-air vapour pressure difference and controlling temperature. This approach minimizes factors that might confound the interpretation of photosynthetic response per se, but does not address the question of canopy-level effects on assimilation, which will change with plant development. Pertinent experimental techniques include single-leaf measurements at multiple positions in the canopy, measurements of the entire canopy (which is difficult for larger trees), and light–response curves to estimate CO2 effects within a closed canopy. Young saplings of both Liriodendron tulipifera and Quercus alba sustained comparably higher assimilation rates at multiple canopy positions and leaf ages (Gunderson, Norby & Wullschleger 1993), but photosynthetic enhancement in 1-year-old needles of Pinus radiata was lower than in current needles (31% versus 64%; Turnbull et al. 1998). One-year-old Populus tremuloides demonstrated greater photosynthetic enhancement by CO2 in the lower half of the crown, without a change in N distribution within the canopy (Kubiske et al. 1997), although mid-crown leaves (only) exhibited reductions in photosynthetic capacity. Measurements in these studies, however, were made at light saturation, and in young trees without much self shading. Measurements of Pinus eldarica seedlings incorporated self-shading effects by use of a whole-tree cuvette (Garcia et al. 1994), but it is essential with such techniques to separate the CO2 effect on photosynthesis (1·9 times higher in a short-term measurement) from the combined effects of increased canopy leaf area and higher photosynthesis (2·8 times higher). A related approach involved microcosms enclosing small stands of young Fagus sylvatica trees, where whole ecosystem measurements were compared to single leaf measurements via modelling procedures (Overdieck 1993).
Light attenuation within a mature forest canopy and the interactions between [CO2] and leaf acclimation to light environment are important factors in evaluating CO2 responses at the canopy level, but these issues are not easily addressed in open-top chambers. Single-leaf measurements of light–response curves generally reveal an increase in apparent quantum yield (Kubiske & Pregitzer 1996; Goodfellow et al. 1997) and a decrease in light compensation point (Kubiske & Pregitzer 1996) with CO2 enrichment, because elevated [CO2] inhibits photorespiration (Long & Drake 1991). A higher initial slope for assimilation versus light has also been noted at the canopy level (Garcia et al. 1994) for seedlings at elevated [CO2]. Variability in leaf response to CO2 was reported in relation to the light environment and a species’ shade tolerance (Kubiske & Pregitzer 1996), and with seasonal and diel variation in irradiance (Goodfellow et al. 1997). In general, however, higher CO2 concentrations should enhance carbon gain at low light levels, for example, in the lower canopy, in understory plants, and on cloudy days.
As indicated by many single and multiyear studies, sustained photosynthetic responses to elevated CO2 (Table 2, Fig. 1a) have disproved the conjecture that days, weeks or months of exposure to CO2 would result in a loss of most of the enhancement effect. These data do not by themselves, however, indicate whether there may have been a more subtle biochemical or physiological ‘acclimation’ to growth at elevated CO2, a reduction in photosynthetic capacity at equivalent conditions, or a partial loss of enhancement with time. Results of this type have been reported in trees grown in pots, and in other types of plant material (reviewed elsewhere: Gunderson & Wullschleger 1994; Sage 1994; Drake, Gonzàlez-Meler & Long 1997). When reduced stimulation has been found, it has been postulated to arise from either end-product inhibition (i.e. down-regulation by carbohydrate accumulation) or as a result of what may be termed acclimation, a suite of biochemical and physiological adjustments considered to improve plant performance through increased efficiency in use of resources (Sage 1994). These internal changes could be extremely important if they were to have an impact on net assimilation such that photosynthetic stimulation by CO2 was lost over time, or was much lower than predicted from short-term measurements.
Such major losses of enhancement have not been demonstrated for trees rooted in the ground. Nevertheless, there have been attempts to resolve smaller differences in foliage developed under CO2 enrichment. A downward trend in photosynthetic enhancement through time might be revealed by repeated measurements during the course of an experiment. There was no such trend in Acer saccharum: in ambient temperatures the 25% enhancement on the first day of exposure (C. Gunderson, unpublished results) was almost the same as the 4-year mean (Table 2). Enhancement was higher (53%) in Eucalyptus tetrodonta after 2·5 years than in previous years (Eamus et al. 1995). Several studies report seasonal differences in sensitivity to CO2, but these differences cannot be characterized as a general downward trend over time and were often attributed, as indicated above, to other environmental factors, e.g. moisture availability (Dixon et al. 1995; Scarascia-Mugnozza et al. 1996; Kellomäki & Wang 1996; Goodfellow et al. 1997) or temperature (Lewis et al. 1996), or to a seasonal change in source–sink balance (Rey & Jarvis 1998). In some cases, enhancement was greater at the end of a growing season, attributed to effects of N availability on late season dynamics (senescence), either from applied N (Curtis et al. 1994) or from symbiotic N2 fixation (Vogel & Curtis 1995).
A second method of assessing photosynthetic capacity in trees from two CO2 treatments has been ‘reciprocal transfer’, switching the CO2 concentrations, either of the whole chamber (Goodfellow et al. 1997), or more commonly, of only the leaf cuvette. For the nine species-treatment combinations where these data are available (Table 2), the ratio of enriched-grown foliage to ambient (E/A) ranged from 0·68 to 1·15, for a geometric mean of 0·92 — only an 8% decrease in capacity (Table 2, Fig. 1b,c). This is in marked contrast with the 21% decline calculated from 20 studies of pot-grown tree seedlings (Gunderson & Wullschleger 1994) and more in agreement with the nonsignificant 1% decline noted for trees in pots larger than 0·5 dm3 (Curtis & Wang 1998) and the 7% decline for a variety of species in rooting volumes > 10 dm3 (Drake et al. 1997). These types of measurements are designed to represent photosynthesis at equivalent conditions, and therefore a ratio less than one purports to indicate a loss of photosynthetic capacity. However, as pointed out by Goodfellow et al. (1997), stomatal conductance (gs) may remain lower in foliage grown under elevated CO2, even at equivalent cuvette concentrations (Ca), perhaps because of reduced stomatal density (cf. Rey & Jarvis 1998). If a lower gs reduces intercellular CO2 concentrations (Ci) in elevated CO2-grown foliage, as in Mangifera indica (Goodfellow et al. 1997), then E/A ratios at a common Ca would not represent differences in biochemical capacity at equivalent conditions.
Measurement at equivalent Ci can be assured with the development of A/Ci curves, that is, net assimilation measured at multiple CO2 concentrations for which Ci are calculated based on stomatal conductance. These curves can also be used to estimate the carboxylation efficiency [Vcmax, the capacity of rubisco to carboxylate ribulose bisphosphate (RuBP)] and RuBP regeneration capacity mediated by electron transport (Jmax) (Sage 1994; Lewis et al. 1996). Little or no difference was reported between the A/Ci curves of ambient and elevated CO2-grown foliage in four species (Liriodendron tulipifera and Quercus alba: Gunderson et al. 1993; Pinus taeda: Ellsworth et al. 1995; Lewis et al. 1996; Pinus sylvestris at two temperature treatments: Kellomäki & Wang 1996). The A/Ci curves of N2-fixing Alnus glutinosa were identical early in the season, but Vcmax was 16% higher in high-CO2 foliage later in the season (Vogel & Curtis 1995). Reductions in the A/Ci response were seen in high-CO2 foliage of Populus tremuloides, but only in the middle of three canopy positions (Kubiske et al. 1997). Vcmax was 12–20% lower in Populus deltoides×P. nigra in mid-September, but not in early August (Curtis et al. 1995). In contrast, elevated CO2-grown Betula pendula had significantly lower A/Ci curves in August and September of the fourth year, and Vcmax and Jmax were numerically lower even in June (Rey & Jarvis 1998). The reduction in Vcmax increased from 9% to 23% over the course of the season, which is in agreement with a consistently lower and decreasing E/A ratio at equal Ca (Table 2). An even larger reduction was seen in Vcmax and Jmax (36 and 21%, respectively) of Pinus ponderosa in September of the sixth year of CO2 enrichment (Tissue et al. 1998), although photosynthesis at the growth concentration was still stimulated 53%. In Picea abies, A/Ci curves were not affected in June, but in September were lower in foliage from the elevated CO2 treatment (Marek, Kalina & Matous˘kova 1995). The A/Ci curves from current-year needles of Pinus radiata showed no differences even late in the growing season, but were lower in 1-year-old needles at that time (Turnbull et al. 1998).
From the range of responses obtained from A/Ci curves, (one increasing, seven no change, one decreasing only at one of three canopy positions, and five decreasing later in the season in at least some foliage), it is apparent that prolonged growth at elevated [CO2] does not result in a consistent down-regulation of photosynthetic parameters. The pattern does suggest a potential decrease in both Vcmax and Jmax, particularly late in the season, concurrent with decreases in measured rubisco content (and thus activity per unit leaf area) (Tissue et al. 1997, 1998; Rey & Jarvis 1998; Turnbull et al. 1998), although decreases in rubisco activity, measured biochemically, can occur with little effect on Vcmax (Lewis et al. 1996; Drake et al. 1997).
In most cases, leaf mass per unit area is higher with growth at elevated [CO2], and, as discussed later, in many cases, leaf nitrogen concentrations decrease while starch, and, less frequently, soluble sugar concentrations increase (cf. Körner & Miglietta 1994). These changes in tissue chemistry form the basis for proposed mechanisms of acclimation based on N reallocation and feedback-driven down-regulation (Drake et al. 1997), but they are not necessarily indicative of either phenomenon. In fact, in many of the studies in Table 1, these changes occur without any evidence of altered photosynthetic response, and conversely, some of the changes in A/Ci curves noted above were not associated with changes in N or sugars. With respect to the N reallocation hypothesis, Drake et al. (1997) point out that at higher temperatures and increasing [CO2], a leaf can sustain a substantial loss in rubisco content (which accounts for a significant fraction of foliar N) without an effect on assimilation rate. A model of Pinus sylvestris trees in open-top chambers indicated that crown photosynthesis increased 22–27% in elevated CO2 with only marginal effects of the observed adjustment in leaf biochemistry (Kellomäki & Wang 1997a). Thus, although there are some consistent changes in leaf properties with growth in elevated CO2, many of the previously reported changes in leaf biochemistry are less pronounced in trees planted in the ground and appear to have minimal impact on photosynthetic enhancement. Seasonal changes in carbohydrate status associated with the cessation of above-ground growth and a reduction in sink strength may explain some of the observations of late-season reductions in photosynthetic response (e.g. Epron, Liozon & Mousseau 1996). Nevertheless, it is important to emphasize that changes in leaf biochemistry, including seasonal declines in Vcmax or rubisco, do not eliminate a photosynthetic response to elevated CO2.
All of the evidence from field-grown trees suggests a continued, and surprisingly consistent, stimulation of photosynthesis, ≈ 60% for a 300 p.p.m. increase in [CO2]. There is, at present, little reason to expect a long-term loss of sensitivity to CO2 as suggested by earlier pot studies of trees. Research on the response of photosynthesis to rising CO2 will continue, of course, to extend our understanding beyond 6-year exposures and to resolve questions about seasonal changes in photosynthetic biochemistry.
The carbon uptake of a tree or a forest stand cannot be calculated simply from the rates of net photosynthesis of individual leaves. These rates must be integrated over the entire canopy and over the growing season. Tree and forest models accomplish this through calculation of the light extinction within a canopy for a given leaf area index (LAI), coupled with information on the light response of photosynthesis and seasonal trends in temperature, water, and other environmental factors that influence net carbon uptake (Kellomäki & Wang 1997a). Tree growth in elevated CO2 has the potential to alter many of these relationships. Any effect of CO2 on maximum LAI, the seasonal development or structure of the canopy, or the single-leaf response to gradients within the canopy will change the relationship between instantaneous net carbon uptake of individual leaves and annual carbon uptake of the whole canopy.
Although canopy structure and processes are clearly critical components of tree response to increasing atmospheric CO2, there are very few data from CO2 enrichment studies that are relevant to our scale of interest. Consider first the central question of whether the LAI of a forest stand will be different in a high-CO2 world. The leaf area of the seedlings and saplings grown in open-top experiments has usually increased with CO2 enrichment. Leaf area of Pinus taeda was 41% greater in elevated versus ambient CO2 after 4 years (Tissue et al. 1997), and it increased 8–18% in Populus clones (Ceulemans, Jiang & Shao 1995). An increase in CO2 concentration resulted in a higher leaf area via an increase in flush length and number of fascicles in P. sylvestris (Kellomäki & Wang 1997a). Leaf area of Citrus aurantium trees was increased primarily because CO2-enriched trees had 78% more leaves than trees in ambient CO2, but average leaf size also increased by 13% (Idso, Kimball & Hendrix 1993a). The increase in leaf area of Quercus alba saplings in elevated CO2 (Fig. 2) also can be attributed to increased leaf number; leaf size and shape changed little (Gregory 1996).
Figure 2. . Leaf area and leaf area ratio (leaf area divided by above-ground plant dry mass) of Quercus alba trees grown in ambient and elevated CO2 (Norby et al. 1995). The plants were grown in open-top chambers with two replicates for each of the three CO2 concentrations from April 1989 until September 1992. Leaf area was determined from collections made at the end of each growing season as the leaves abscissed. Above-ground plant dry mass was estimated from height and diameter measurements in 1989–91 and was measured directly when the plants were harvested in September 1992.
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These observations of increased leaf area in elevated CO2 do not indicate a specific stimulatory effect of CO2 on leaf production. In the Q. alba experiment, for example, leaf area increased with CO2 enrichment less than whole-plant mass did; hence leaf area ratio (LAR) was lower in elevated CO2 (Fig. 2). LAR was reduced in Pinus taeda as well (Tissue et al. 1997). In a compilation of all CO2 experiments with trees (including growth chamber experiments with seedlings in pots), LAR was on average 15% less in elevated CO2 (Wullschleger et al. 1997a). Hence, we can conclude that the data from open-top chambers mostly show that larger plants had more leaf area.
Unfortunately, these observations tell us little about the potential CO2 effect on LAI in a closed-canopy forest where LAI is constrained by nutrients, water or light. There have been no manipulative studies in which the experimental trees grew long enough to maintain a closed canopy for several years. Elevated CO2 might be expected to increase LAI if the light-compensation point for photosynthesis is higher such that leaves are retained deeper in the canopy. Alternatively, if elevated CO2 exacerbates nutrient constraints, LAI could be reduced. The observation that LAR is reduced in CO2-enriched trees might also suggest that LAI will be reduced. The only direct measure of a CO2 effect on LAI comes from unreplicated observations of two coppice forests near CO2 vents in Italy, where the trees have been exposed to elevated CO2 concentrations throughout their 35–40 years. There was no difference in LAI between the CO2-enriched sites and nearby control sites, although LAR was lower in the CO2-enriched sites (Hättenschwiler et al. 1997b).
Changes in canopy architecture could be important even if LAI is not changed, especially if the photosynthetic responses to CO2 change with light or canopy position. Arnone & Körner (1993) suggested that changes in the vertical leaf display and crown structure might alter the red/far red ratio of light reaching understory tree seedlings, thereby affecting their pattern of growth. Pinus sylvestris trees not only had more leaf area in elevated CO2, but there was also a shift in foliage distribution with relatively more leaves toward the base of the crown in CO2-enriched trees (Kellomäki & Wang 1997a). These adjustments might be important for maximizing light harvesting and minimizing self-shading (Kellomäki & Wang 1997a). Increased secondary branching in elevated CO2 was indicated by Idso, Kimball & Allen (1991) and Ceulemans et al. (1995). Norby et al. (1996), however, saw no change in any index of canopy structure in Quercus alba or Liriodendron tulipifera. Increasing our understanding of branch morphology and crown characteristics will aid in efforts to scale results of physiological studies to the tree or stand level, as large-scale canopy function is an integration of physiological processes and structure at smaller scales.
Recent observations of large-scale carbon fluxes by the eddy covariance approach have demonstrated that canopy phenology, the duration of leaf display, is an important determinant of year-to-year variation in annual net carbon flux (Goulden et al. 1996). The possibility of changes in phenology in response to elevated CO2 was an important reason to conduct experiments in the field over several growing seasons. Most of the observations of phenology in these studies, however, have been somewhat casual, and it is difficult to determine if there are any general patterns. Gunderson et al. (1993) quantified the timing of fall senescence in Liriodendron tulipifera and Quercus alba by measuring the decline in chlorophyll content and the time course of leaf abscission. There were no effects of elevated CO2 in either species. One clone of Populus exhibited delayed bud burst in elevated CO2, whereas another clone exhibited advanced bud set (Ceulemans et al. 1995). Picea sitchensis and Castanea sativa, growing in pots in field chambers, exhibited both delayed bud burst and advanced bud set (El Kohen et al. 1993; Murray et al. 1994), but there were no effects of CO2 on the bud phenology of four other tree species (Murray & Ceulemans 1998). Elevated temperature accelerated bud burst in Pseudotsuga menziesii, but elevated CO2 counteracted this effect; elevated CO2 also decreased bud hardiness during cold hardening and dehardening (Guak et al. 1998). Increased temperature had important effects on the timing of spring bud break and autumn leaf senescence in Acer saccharum and A. rubrum, but there were no important or consistent effects of elevated CO2 (Norby et al. 1998). There is at yet no basis for ascribing this variation in phenological response to increased CO2 to inherent differences between species in their ability to optimize the timing of developmental events. Nevertheless, competitiveness and survival of trees can depend on the ability to avoid having periods of growth coincide with periods of subzero temperatures, and a differential response to elevated CO2 could alter competitive relationships and stand structure.
The supposition that trees will maintain higher rates of leaf and canopy photosynthesis when grown at elevated CO2 appears to be supported by many field experiments. Photosynthesis is, however, only one determinant of a tree's carbon balance, and researchers have in recent years expanded their focus to consider also the respiratory loss of carbon by woody plants exposed to atmospheric CO2 enrichment. These studies have provided periodic estimates of respiration for both seedlings and saplings grown at ambient and elevated CO2 (Idso & Kimball 1992a; Wullschleger, Norby & Hendrix 1992b; Vogel & Curtis 1995; Curtis et al. 1995; Ceulemans et al. 1997) and have attempted to identify the sensitivity of growth and maintenance respiration to elevated CO2 in leaves (Wullschleger & Norby 1992; Wullschleger, Norby & Gunderson 1992a; Will & Ceulemans 1997) and stems (Wullschleger, Norby & Hanson 1995b; Carey, DeLucia & Ball 1996; Dvorak & Oplustilova 1997). The energetic costs of tissue construction have similarly been examined in leaves, stems, and roots for field-grown trees exposed to elevated CO2 (Carey et al. 1996; Wullschleger et al. 1997b), and these effects have, in potted Pinus ponderosa and P. taeda seedlings, been attributed to CO2-induced changes in the biochemical composition of leaves (Griffin, Winner & Strain 1996b).
While these studies have advanced to some extent our understanding of the potential response of woody plant respiration to CO2 enrichment, it is unfortunate that no strong scientific consensus has yet emerged from these observations. Single-leaf rates of respiration are often reported to be lower for field-grown trees exposed to elevated CO2 (Idso & Kimball 1992a; Wullschleger et al. 1992a; Teskey 1995; Ceulemans et al. 1997). These effects range from a 14% suppression of respiration for needles of Pinus taeda in branch bags (Teskey 1995) to 60% or more for one clone of hybrid poplar (Ceulemans et al. 1997). There are, however, equally compelling observations that respiration is unresponsive to CO2 enrichment (Vogel & Curtis 1995; Curtis et al. 1995; Ceulemans et al. 1997; Will & Ceulemans 1997). This inconsistency of response has been observed both within individual experiments and between studies conducted by different investigators. Ceulemans et al. (1997), for example, studied the respiratory response of two contrasting Populus hybrids grown at ambient and elevated CO2 in open-top field chambers. Elevated CO2 had no long-term effect on leaf respiration for the slow-growing clone Robusta (P. deltoides×P. nigra), but rates of respiration for the fast-growing clone Beaupré (P. trichocarpa×P. deltoides) were more than 60% lower at elevated CO2 concentrations. Genotypic variation such as this, if substantiated, could be used to explore mechanisms whereby respiration changes in response to CO2 enrichment. Unfortunately, these clonal differences were not observed in a subsequent study conducted on coppice regrowth of the original plant material (Will & Ceulemans 1997), so there is some question as to whether the clone-specific response observed by Ceulemans et al. (1997) represents true genetic variation or instead reflects variability attributable to experimental protocol.
There are, of course, other possibilities that could be invoked to explain the highly variable and inconsistent response of respiration to CO2 enrichment: complications caused by expressing respiration on a leaf mass or area basis, stages of plant development, leaf age and carbohydrate composition, chamber leaks and artifacts resulting from methodology, and interacting factors such as temperature or nutrient status of the measured tissues. These confounding factors have seldom been considered in measurements of leaf respiration at either ambient or elevated CO2, and such uncertainties are currently hindering progress in this area. Steps must be taken to resolve these issues by conducting field-based studies that systematically address the short-term direct effects and long-term acclimation effects of elevated CO2 on leaf respiration. A direct effect is defined here as an immediate response in which rates of respiration are altered by a change in CO2 surrounding a leaf or whole plant; it is a reversible effect and occurs within minutes of a step change in CO2 (Drake et al. 1999). An acclimation effect, by comparison, occurs when rates of respiration for trees grown in elevated CO2 differ from those grown in ambient CO2, with the stipulation that all measurements are made at a common CO2 partial pressure. This latter definition implies that the acclimation effect is persistent and thus reflects an intrinsic change in tissue chemistry (e.g. N or protein content) or in some whole-plant process (e.g. growth or biomass allocation) that is subsequently reflected in measurements of respiration.
The utility of separating direct from acclimation effects has been nicely demonstrated in the branch-bag studies of Teskey (1995) and in the whole-shoot investigations of Griffin, Ball & Strain (1996a). Each of these studies observed that short-term increases in CO2 could elicit an immediate and apparently reversible suppression of respiration. This direct effect ranged from a 6–14% suppression of respiration as [CO2] surrounding branches of 21-year-old Pinus taeda was raised from ambient to ambient + 330 p.p.m. (Teskey 1995) and from a 3–13% inhibition of respiration as [CO2] was increased from 350 to 700 p.p.m. around whole-shoots of Pinus ponderosa seedlings (Griffin et al. 1996a). Although this latter study was conducted on potted seedlings, it nonetheless illustrates an experimental approach whereby the direct and acclimation effects of elevated CO2 can be separately addressed. This is an important consideration, as Griffin et al. (1996a) demonstrated that the magnitude of a direct suppression of needle respiration was correlated in P. ponderosa with longer-term changes in tissue C/N ratios; the direct effect of elevated CO2 on respiration was greatest in shoots with a higher C/N ratio. These findings are particularly relevant given the often reported observation that leaf [N] is lower in woody plants exposed to long-term CO2 enrichment (Curtis & Wang 1998). Thus, barring unforeseen changes in leaf carbon content, a decrease in tissue C/N ratios may strengthen any direct response of leaf respiration to elevated CO2 concentration.
A mechanistic explanation and a series of testable hypotheses are urgently needed for the direct and, to a lesser extent, the acclimation effects of elevated CO2 on respiration. It is likely that without such an explanation future measurements of leaf respiration at ambient and elevated CO2 will be viewed cautiously. Once a cause-and-effect relationship is proposed, however, there will still be a critical need to integrate this information within the context of whole-tree responses to CO2 enrichment. Wang, Rey & Jarvis (1998) conducted such a prototype analysis for young Betula pendula trees and not only considered the effects of elevated CO2 on biomass growth, but integrated this information with known or suspected effects of atmospheric CO2 on photosynthesis and tissue-specific rates of respiration. Trees in their fourth year of growth at elevated CO2 were 48% larger than those grown at ambient CO2, and during the growing season trees in the ambient and elevated CO2 treatments increased their biomass by 4–5-fold. The annual loss of carbon (g C tree–1 year–1) for all plant tissues combined (leaves, stems, and roots) was about equally divided between growth (45%) and maintenance (55%) respiration, and accounted for 31–38% of the total CO2 assimilated in gross photosynthesis (Fig. 3). One surprising finding from this analysis was that a 23% reduction in leaf respiration in elevated CO2 had little impact on the overall carbon budget of these rapidly growing trees (Wang et al. 1998). However, if Wang et al. (1998) assumed that both growth and maintenance respiration were reduced in elevated CO2, then CO2-enriched trees were simulated to produce and maintain ≈ 60% more leaf biomass (and 43% more leaf area) per tree with an additional respiratory cost of less than 10% (332 versus 361 g C tree–1 year–1). A similar conclusion was reached by Norby et al. (unpublished results) in their carbon budget analysis of Quercus alba where CO2-induced reductions in growth and maintenance respiration enabled trees at elevated CO2 to produce and maintain throughout the season more than 90% more leaf biomass at an additional respiratory cost of less than 15% (160 versus 181 g C tree–1 year–1). These analyses suggest that while the effects of elevated CO2 on leaf growth and maintenance respiration may play only a limited role in whole-plant carbon budgets, these effects could nonetheless be of some ‘local’ significance to the carbon balance of tree canopies.
Figure 3. . Annual carbon fluxes (g C tree–1 year–1) for young birch trees during their fourth year of growth at ambient and elevated CO2 concentration. Pg, gross photosynthesis; L, S and R designate leaves, woody stems and roots; the subscripts g and m designate either growth or maintenance respiration. Data were adapted from Table 6 of Wang et al. (1998) with the permission of Y.-P. Wang.
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The carbon budget analysis of Wang et al. (1998) admittedly lacks explicit treatment of root turnover and the energy costs of carbohydrate translocation and nutrient uptake, although these issues are critical unknowns for the carbon balance of CO2-enriched trees. Wang et al. (1998) emphasized that much uncertainty surrounds the large respiratory losses associated with fine-root production and growth of the root-associated mycorrhiza at ambient and elevated CO2 conditions. These topics have received little attention in field-grown trees. Pregitzer et al. (1995) suggested that the respiratory costs associated with fine-root turnover (growth and maintenance costs) may account for at least a portion of the carbon that is otherwise missing from comparisons of rates of photosynthesis and estimates of net assimilation made by destructively harvesting plants. At a more refined scale, there was a small but significant reduction in specific respiration rates of fine roots of Fraxinus excelsior, Quercus petraea, and Pinus sylvestris in elevated CO2 (Crookshanks, Taylor & Broadmeadow 1998). Further uncertainty surrounds the respiratory costs of nutrient uptake in trees exposed to elevated CO2 conditions. This point was emphasized by BassiriRad et al. (1996), who reported that the differential response of root uptake kinetics for NH4+ and NO3– in field-grown Pinus taeda may have important implications for the energy requirements of nutrient acquisition by future forests. Finally, respiration is more than a process whereby carbon is lost from terrestrial vegetation; it provides carbon skeletons and energy for biosynthesis and maintenance of existing biomass, and contributes fundamentally to plant vigour. Studies that focus on the potential effects of elevated CO2 on respiration must therefore consider also the significance of respiration for forest health and productivity.
Above-ground growth is perhaps the most obvious manifestation of the effect of CO2 on trees in many experiments. It would also appear to be the most important and relevant measure for projecting the response of forests to global change, for it is through growth and standing biomass that the health and functioning of a forest ecosystem is first evaluated. Above-ground growth is relatively easy to measure in comparison to root growth or the more subtle changes in gas exchange or biochemical constituents. Nevertheless, there has been a wide range of responses of tree growth reported from field experiments (Table 3), and a great deal of uncertainty on how to apply the results to the larger questions at hand.
Table 3. . CO2 enrichment ratio (E/A) of above-ground dry mass of trees grown in elevated CO2 compared to trees grown in ambient CO2 in field experiments
The variety of results was apparent from the first two reports from field experiments on tree growth response to elevated CO2. Citrus aurantium trees were reported to have more than doubled in size in response to CO2 enrichment (Idso & Kimball 1992a), and that size advantage has continued for 7 years (Idso & Kimball 1997). But Liriodendron tulipifera trees, grown for 2·5 growing seasons in elevated CO2, had only 27% more dry mass than trees grown in ambient CO2, an increment that was not statistically significant (Norby et al. 1992). Subsequent reports have shown intermediate responses. Additional experiments in which there was no significant growth response to CO2 are known to exist but have not been published in detail (Karnosky et al. 1998; D. Olszyk, personal communication). This wide range in response immediately gives rise to numerous questions: Why do the results vary? What is the ‘average’ response? Is there any meaning to an ‘average’ response? And perhaps most important, what are the implications of these results for forest response?
The simple arithmetic mean of the enrichment response for above-ground woody dry mass of the experiments in Table 3 is 1·73, the log-adjusted mean is 1·64, and the median value is 1·55. These values are higher than but still within the range of values from previous data compilations, which were dominated by seedling studies: 1·40 (Eamus & Jarvis 1989), 1·38 for conifers and 1·68 for broadleaved trees (Ceulemans & Mousseau 1994), 1·40 (Poorter, Roumet & Campbell 1996), 1·30 (Wullschleger et al. 1997a), and 1·29 (Curtis & Wang 1998). Although the summary presented here ignores the important principles of meta-analysis (Curtis & Wang 1998), no degree of sophistication in calculating a mean value will circumvent the dubious value of a mean over such a wide range for understanding the response or predicting future responses. These are our most important challenges. Can the diversity of results be explained by the growth rate or growth potential of the different species, effects of environmental interactions, or differences in experimental protocol? Is there a better expression of growth that would be more informative and useful for longer-term predictions?
One of the most commonly invoked explanations for the differences in response illustrated in Table 3 (as well as for differences in photosynthesis, allocation, or almost any other measured response to elevated CO2) is that species respond differently. On the surface this statement is almost a truism — several different species have been tested and their responses to CO2 are different — but the conclusion is not supported by rigorous analysis. Clearly, the potential effect of species is completely confounded by many other factors, including soil conditions, weather, length of growing season, duration of the experiment, plant culture, chamber conditions and biases (which we hope do not exist!) of the experimenter. Although variation between species under identical site conditions (Liriodendron tulipifera versus Quercus alba) is large, so too is the variation within a species attributable to environmental factors (N or temperature) and the variation within a genus (Pinus, Populus) in different studies. A coherent description of differential responses to CO2 enrichment, based on species characteristics or functional groupings of species, could be a useful input for ecosystem models, and several such schemes have been proposed (Poorter et al. 1996). However, without a rigorous demonstration that species characteristics were responsible for differences in the observed CO2 response in a controlled experiment, this common reliance on ‘species differences’ to account for disparate responses should be avoided.
Increases in atmospheric CO2 will be accompanied by changes in temperature, precipitation, N deposition, and tropospheric ozone. Any of these factors can be expected to modify the response of trees to CO2, and likewise, elevated CO2 could exacerbate or ameliorate the responses to the other factors. Some of the experiments in Table 1 have addressed these critical questions. There was no effect of elevated CO2 on stem mass of Populus tremuloides grown in twice-ambient ozone, which imposed a significant stress (Karnosky et al. 1998). Elevated CO2 compensated for the negative effects of increased temperature in Acer saccharum and A. rubrum ( Norby et al. 1998). There were no CO2–temperature interactions in Psuedotsuga menziesii (D. Olszyk, personal communication). Interactions between CO2 and N additions varied between experiments (Table 3), but it is questionable whether these results are a good model for interactions with deposition of N from the atmosphere (Norby 1998). These data sets from field experiments on interactions between CO2 and other global change factors are too limited to allow general conclusions to be drawn, but this is clearly a research area that needs to be pursued. Responses to temperature increases in particular have many points of intersection with CO2 responses and this interaction deserves more attention in future studies (Ceulemans 1997).
The largest difficulty in interpreting the data in Table 3, and a probable cause of the wide range of values, is the dominant effect of tree developmental patterns (ontogeny) on the attainment of dry matter. Tree and forest stand development must be a primary consideration in the interpretation of field experimental results and their application to longer-term predictions. In all of the experiments represented in Table 3, the trees were undergoing exponential growth for all or most of the exposure period. Larger plants have more leaf area, which increases their capacity to take up CO2 and make more stem and leaf tissue, which further increases their capacity to take up CO2 and grow. The effect of any factor that increases leaf area early in an experiment, such as random variation between individuals, differences in how seedlings were raised or planted, or specific effects of CO2 enrichment, will be magnified over time by the principle of compound interest (Ceulemans & Mousseau 1994; Norby et al. 1996). As long as there are no constraints on leaf area production, spectacularly large CO2 responses can occur. But in a forest stand there are always constraints to leaf area development — depending on the site, the constraint may be low nutrient availability, dry conditions, or ultimately not enough light to support the deepest leaves of a dense canopy. A CO2 stimulation that depends on an ever-increasing leaf area index cannot be expected to be sustained, and projections that ignore this critical determinant of tree growth (Idso 1991) are certain to be false or misleading.
The large increase in final dry mass of Quercus alba (Norby et al. 1995) was shown to be a result of an early stimulation by CO2; subsequent responses to elevated CO2 included photosynthetic enhancement compensated by a downward adjustment in leaf area development from the expected exponential increase. The net result was a large difference in final dry mass without any increase in relative growth rate (RGR) over the last 3 years of the 4-year study. One interpretation of the growth trends in that experiment was that trees in elevated CO2 would reach canopy closure 1 year earlier than those in ambient CO2 (accelerated ontogeny), and at that point the relative CO2 effect would decline. But as in other experiments, the trees were harvested while they were still in an exponential growth phase, so the projections about future responses are only speculation. Ultimately, we are interested in absolute growth rate, not relative growth, and RGR (a difficult term to apply to trees in which much of the biomass is dead) is useful only to the extent that it guides long-term predictions from experimental data.
Leaf area constraints have probably come into play in some of the longer open-top chamber studies. The group of Pinus ponderosa trees in chambers had a closed canopy in the sixth and final year of the experiment, and the final increase in above-ground growth was less than that shown in Table 3 (J.T. Ball, personal communication). Citrus aurantium trees were grown individually, so there was not mutual shading by adjacent trees, but leaf area development was nevertheless constrained by the walls of the chamber, and the relative enhancement of above-ground growth (including fruit rinds) began to decline steadily in the third year of exposure (Idso & Kimball 1997). A decline in growth response with time, as has been observed in these experiments as well as in experiments (Bazzaz, Miao & Wayne 1993) with potted tree seedlings (where the constraint is on root development), is frequently cited as evidence that CO2 fertilization is transitory and not likely to have a long-term influence on forest productivity. Actually, however, a decline in relative enhancement of woody biomass is expected and consistent with the patterns of tree development. Long-term predictions should not be based on the biomass enrichment ratio at the end of an experiment of only several years’ duration.
If the biomass enrichment ratio is not an appropriate parameter on which to base long-term predictions, is there another expression of growth that accounts for developmental patterns and could be more robust? Norby (1996) proposed a ‘canopy productivity index’ (CPI) to normalize growth responses to equal leaf area. It is calculated as the annual increment in stem mass per unit leaf area. A better expression might include woody root increment as well, but such data are rare, and an index is useful only if there are data to support it. The CPI was used by R. H. Waring (Waring & Schlesinger 1985) as ‘growth efficiency’ (although the term does not properly meet the definition of an efficiency), as an indication of a tree's responses to environmental stresses. The CPI is relevant only on an annual time step. It should not be confused with net assimilation rate (NAR), an instantaneous expression of growth that can be integrated over time under certain conditions. NAR has been a useful analytical tool in short-term CO2 enrichment experiments (Norby & O’Neill 1989, 1991), but there rarely are sufficient data to support its use in longer-term experiments.
Considering all of the field experiments with broadleaf trees for which growth increment and leaf area data were available, the effect of CO2 on CPI varied over a much smaller range than the CO2 effect on final dry mass (Norby 1996). The average of the eight values was a 29 ± 7% enhancement (range 19–37%). We can extend this analysis to include several new studies, which slightly lowers the mean value and expands the range of observed values Table 4). Nevertheless, the increase in CPI is still seen to be a consistent response of trees to elevated CO2. Pinus taeda is the only conifer included in Table 4. Calculating a CPI for a tree with several cohorts of leaves contributing to annual stem growth, and each cohort contributing to 2 or more years of stem growth, is computationally difficult while the leaf area is still increasing. Tissue et al. (1997) were able to calculate the CPI in their study because of their extensive data set on leaf area.
Table 4. . Response of annual stem production per unit leaf area (canopy productivity index, CPI) of field-grown trees to elevated CO2. Table modified from table in Norby (1996)
The value of this index is that it provides a simple, measurable CO2 response parameter from experimental studies that might be independent of tree and stand development. Badeck et al. (1997) criticized its use because the CPI could be highly sensitive to differences in LAI between ambient and elevated treatments. As LAI increases, the fraction of less productive shade leaves increases, and therefore CPI should decrease even while productivity per unit ground area might still increase (Badeck et al. 1997). The CPI declines with age and in response to environmental stress (Waring & Schlesinger 1985); hence, its absolute value at the end of an experiment should not be extrapolated into the future. But there is no obvious reason to assume that its relative response to CO2 will change as LAI increases, although this is clearly a conjecture that must be tested. The index is also useful because it separates structural responses to elevated CO2, such as changes in canopy structure discussed in the previous section, from functional responses — the physiological reactions of photosynthesis, respiration, carbon allocation, and so on. Structural and functional responses can be considered separately in ecosystem or global models (Woodward, Smith & Emanuel 1995), and separating them experimentally can help to focus research toward meaningful, testable hypotheses about tree response to elevated CO2. The observation that the CPI response to CO2 is remarkably similar across so many very different experiments under different conditions improves the prospects for success in projecting future response to atmospheric CO2 enrichment and belies the general statement that ‘species differ in their response to CO2’.
This analysis emphasizes the point that short-term tree growth responses cannot be extrapolated outside of the context of stand development. The very large growth responses observed in some experiments are unlikely to be sustained for many years under forest conditions. Much of the variation among experimental results can be explained by differences in leaf area development. On the basis of an analysis of growth per unit leaf area, the predicted long-term response to CO2 (in the absence of interacting factors and environmental feedbacks) is only slightly less than that indicated by seedlings experiments: an increase of about 27% with a 300 p.p.m. increase in [CO2]. This analysis gives rise to several questions. Is the short-term stimulation of leaf area development and tree growth an experimental artifact or an indication of an important effect of CO2 on seedling establishment? Is the enhancement of growth per unit leaf area (or LAI at the stand scale) a robust response; that is, will this response persist after canopy closure? Alternatively, will the response to CO2 continue to decline such that there ultimately is no difference in annual increment, and the only effect of CO2 is the gain from the initial stimulation of growth increment? Or, will there be no gain from CO2 at all in the end, the only effect being to shorten by several years the time over which maximum biomass is attained? These questions cannot be answered from the current database of open-top chamber experiments. Nevertheless, the observations from those experiments have enabled us to ask better questions, and they should be an important guide to interpreting long-term data sets as they become available.
Although decades-long records of response cannot yet come from any manipulative experiments, the vegetation growing in the vicinity of the surface vents of deep geothermal springs, such as those in central Italy (Miglietta et al. 1993), can be a useful alternative source of data on long-term responses of trees to an atmosphere enriched in CO2. Naturally elevated CO2 concentrations can be assumed to have occurred for hundreds of years in these areas, and the vegetation has been subject to a concentration gradient determined by distance from the vent (Miglietta et al. 1993). But the CO2 springs are not ideal experimental systems (Amthor 1995) — the exposure history and dynamics are uncertain, there are no true controls, and environmental conditions may be atypical — and the data must be interpreted with caution. Hättenschwiler et al. (1997a) described the tree ring record of Quercus ilex trees at two natural CO2 springs in Italy. The trees have been exposed continuously to high CO2 since they were seedlings (31–36 years), and throughout that time they have been larger than equal-aged trees in adjacent sites away from the CO2 emissions. An analysis of the relative difference in tree ring width, however, indicated that the response to CO2 was declining with time and had disappeared by the time the trees were 25–30 years old (Hättenschwiler et al. 1997a). Stem basal area of trees in elevated and ambient CO2 was reconstructed from the tree ring records, and we can analyse this record with the assumptions that basal area is a good correlate of above-ground biomass, that the relationship between basal area and biomass is the same for trees in ambient and CO2-enriched trees, and that the relationship has been constant through time. Figure 4(a) shows the relative CO2 stimulation of basal area as a function of tree age at the Rapolano site, and there clearly was a steep decline in response from year 3 to year 13, but the record then levelled off at about 1·26, or a 26% increase in basal area in elevated CO2. Annual basal area increment (Fig. 4a), which is presented as a 3-year running average to smooth out large year-to-year fluctuations, was always higher in the CO2-enriched site, except for the last several years. Starting at year 9, the slope of BAI versus age was not significantly different from zero and centered on an enrichment ratio of 1·19. The record from Laiatico (not shown) was similar except for a sharp rise in BAI only in the control site in 4 of the last 5 years. The BAI record at Rapolano is consistent with predictions from the open-top chamber experiments. The approximate doubling of growth during the earliest years was not sustained, possibly declining as LAI reached maximum values for the sites. (There is no record of leaf area development for these stands, but it is reasonable to assume that as a coppice stand, they reached their maximum LAI fairly early; S. Hättenschwiler, personal communication). Since LAI was the same at enriched and control sites (4·0 for Rapolano and 3·5 for Laiatico; Hättenschwiler et al. 1997a), the data support the premise that enhancement of annual growth per unit leaf area is a sustained response to CO2 enrichment, albeit at somewhat less than the average value in Table 4. As a result of this sustained response, the cumulative gain in basal area (biomass) attributable to CO2 enrichment increased with age and was not simply the result of the early stimulation of growth (Fig. 4b). Whether the unexplained decline in response in the last several years of the record at both Rapolano and Laiatico is the result of some aspect of stand development that will eventually lead to a complete loss of the CO2 response, or a relatively short-term environmental fluctuation that will average out over time, cannot be determined. Hence, even with this much longer record of CO2 response than has been available before, it remains difficult to predict the response in future decades. Nevertheless, these important data sets from the CO2 springs substantially extend the observation that the stimulation of tree growth by elevated CO2 can be sustained over time under field conditions.
Figure 4. . (a) CO2 enrichment ratios (E/A) for basal area and basal area increment (BAI) of Quercus ilex trees in the vicinity of the Rapolano spring, Italy, and an adjacent control site. Basal area increments are presented as the 3-year running average. The regression line for BAI beginning at year 9 is:
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Allocation below ground
The allocation of carbon to below-ground tissues, and the growth, physiological activity and death of roots that results, are key points of intersection between the carbon cycle and the water and nutrient cycles. If experiments on tree responses to elevated CO2 are to have relevance to forest ecosystem responses, there must be consideration given to the responses of root systems and associated below-ground processes. Unfortunately, of course, root responses are most difficult to study, and the inherent limitations in experimental approaches have meant that most of the observations are single observations at the end of an experiment, which is clearly problematic for such a dynamic system. The increasing use of minirhizotron systems has enabled more frequent observations, but the data can be difficult to quantify.
Earlier studies based on the responses of potted tree seedlings generally concluded that the ratio of root mass to shoot mass increases in elevated CO2 (Oechel & Strain 1985), although perhaps only in low nutrient conditions (Eamus & Jarvis 1989; Bazzaz 1990). There are many problems with the measurement and interpretation of root-to-shoot ratio (Stulen & den Hertog 1993; Norby 1994), and past generalizations probably have little relevance to the issues of tree responses. It is especially important to separate the response of woody root mass from that of fine roots (Norby 1994). On the one hand, an increase in woody root mass implies storage of carbon just as an increase in bole wood does, but this cannot be surmised from young seedlings in which all of the roots are small. On the other hand, changes in whole-root system mass of older saplings or trees will tell us little about fine-root mass or turnover. Because of their much higher turnover rate, a large amount of carbon may be allocated to the production of fine roots, but the standing crop of fine roots can be a small percentage of the whole root mass. Nevertheless, the production and turnover of fine roots are critical processes linking plant response to soil response. Fine roots are the mechanism for nutrient uptake from the soil, the platform for microbial activity related to nutrient turnover, and the source of much of the carbon influx to soil (Norby 1994). Hence, we shall consider the experimental evidence for woody roots and fine roots separately.
Only a few multiyear studies of trees in elevated CO2 have ended with a complete destructive harvest, so the data set on woody root response to elevated CO2 is small. There was no significant effect on root-to-shoot ratio in Liriodendron tulipifera (Norby et al. 1992), Quercus alba (Norby et al. 1995), Betula pendula (Rey & Jarvis 1997), Pinus taeda (Tissue et al. 1997), Pinus ponderosa (Walker et al. 1997), or Fraxinus excelsior, Quercus petraea and Pinus sylvestris (Crookshanks et al. 1998). Static measures of root-to-shoot ratio may mask important treatment effects on allocation that are confounded with developmental changes in allocation. Allometric analyses can be a more powerful method for examining allocation shifts. Tissue et al. (1997) found no effects of CO2 on any allometric coefficients, including those describing root-shoot relations. Norby (1994) saw no effect of CO2 on root–shoot allometry in L. tulipifera, but the allometric coefficient for Q. alba increased with increasing CO2. Given the large root mass of many trees, such a shift could lead to underestimates of a CO2 effect on total carbon storage based only on above-ground mass. For example, if the CPI for B. pendula is calculated to include the biomass increment for the stump and coarse root in addition to stem and branch production, the CO2 effect on CPI increases from 9% (Table 4) to 21%.
In most field studies in which fine-root density (mass of roots per unit ground area) has been measured, fine roots have been shown to be especially responsive to CO2. In the six studies represented in Fig. 5, fine-root density increased from 60 to 140% in elevated CO2. Fine-root mass production also increased by 135% in 3-year-old Pinus sylvestris (Janssens et al. 1998), and fine root length density increased 63% in an oak-palmetto ecosystem (Day et al. 1996). Fine-root length production in Fraxinus excelsior, Quercus petraea, and Pinus sylvestris was increased by 95–240% in elevated CO2 (Crookshanks et al. 1998). Although the direct impact of an increase in fine-root mass on whole-plant mass is small, it could nevertheless be important to longer-term ecosystem response. Increased fine-root density could, for example, support increased rates of nutrient uptake or stimulate increased rhizosphere activity. Although these static measures of fine root density tell us nothing about the total carbon flux to fine roots, there is a presumption that increased fine root density indicates increased turnover as well, and root turnover is a mechanism for additional carbon to enter long-lived soil pools.
The large percentage increase in density of small roots (< 7 mm diameter) in Liriodendron tulipifera relative to the nonsignificant increase in whole-plant dry mass and decrease in leaf area (Norby et al. 1992) apparently confirmed the suggestion from a previous growth-chamber experiment (Norby & O’Neill 1991) that an important CO2 response in field-grown trees could be a shift from leaf production to fine-root production. Such a mechanism could imply a shift in the tree's functional balance between carbon acquisition versus water and nutrient acquisition. In all of the studies represented in Fig. 5, the stimulation of fine-root density exceeded that of leaf area, and in all but Citrus aurantium, the relative response of fine roots also exceeded that of the whole plant. These observations suggest that stimulation of fine-root production may be a specific response to elevated CO2, not simply a proportionate component of larger plants. Generally, the disparity between fine-root and leaf area response was smaller in those experiments in which leaf area showed the greatest response (the right end of the x-axis).
As discussed previously, the increase in LAI observed when open-grown trees are exposed to elevated CO2 cannot be expected to persist indefinitely as a tree grows into a forest canopy. Likewise, the increase in fine-root density can be assumed to saturate as the soil volume becomes fully occupied. These static measures of fine-root density and leaf area do not predict whether a sustained increase in fine root to leaf area ratio is likely. It should, then, be important to look at the effect of CO2 on fine roots in relation to the dynamics of the response of the rest of the plants. The use of minirhizotrons has allowed such analyses. Pregitzer et al. (1995) found that fine-root growth and mortality were more responsive to CO2 than was leaf growth throughout their 1-year study, and data from a single destructive harvest would have been very misleading. Tingey et al. (1996) related fine-root dynamics of Pinus ponderosa to shoot growth dynamics over three growing seasons. Fine-root area density initially increased one to two-fold in elevated CO2, but did not continue to increase as shoot growth continued. The ratio of fine roots to leaf area declined with time, and there was no effect of CO2 on this ratio, although N fertilization did initially decrease the ratio.
Although there may well be differences between species or sites in the relative response of fine roots, the more rigorous observations afforded by periodic observations through minirhizotrons do not support the premise that there is a specific stimulation by elevated CO2 of fine-root density or a shift in the functional balance between roots and foliage that is sustained over time. Nevertheless, it is important that fine-root production is enhanced at least to the same extent as that of the rest of the tree. A greater emphasis on fine-root turnover, instead of static measures of fine-root density, will help to reveal the potential importance of fine-root responses to whole-system function and carbon budget. Observations on the horizontal (Thomas et al. 1996) and vertical distribution of fine roots and root carbon in soil through minirhizotron observation and quantification of mycorrhizal colonization (Rygiewicz et al. 1997; Runion et al. 1997) may make additional links to biogeochemical cycling.