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  • View in gallery

    Simulated leaf assimilation responses to internal CO2 concentration (Ci) and photosynthetically active radiation (Qp), at 25°C. As Ci changes, Qp was set equal to 1500 μEinstein m−2 s−1. (a), (b) Temperate and boreal trees, and (c), (d) tropical trees. In (b) and (d), the thin (thick) lines are used for 350 (700) ppm. In all figures, solid lines correspond to model outputs with downregulation while dashed lines correspond to the lack of downregulation. For ease of interpretation and visualization, the vertical dotted line in (a) and (c) delimitates the behavior of the assimilation curves when ambient CO2 concentration (Ca) is either lower or larger than 350 ppm (Ci = 0.7 × Ca). Here Vmax is cited at 15°C (Kucharik et al. 2000). For all simulations, the relative humidity was set to 80%.

  • View in gallery

    Potential effect of the photosynthetic downregulation on predicted NPP at an atmospheric CO2 concentration of 700 ppm: (a) global NPP distribution; (b) difference between a run without (NDR) and one with (WDR) downregulation; (c) relative difference between a run without (NDR) and one with (WDR) downregulation. In (c), the relative difference is larger than 50% in the black pixels located in the “tundra/polar desert” ecotone.

  • View in gallery

    As in Figure 2, but the atmospheric CO2 concentration is 350 ppm.

  • View in gallery

    Simulated global vegetation distribution. (a) The photosynthetic downregulation is not included; (b) the photosynthetic downregulation is included; (c) dark squares indicate areas where simulated vegetation with downregulation is different from the one simulated without downregulation.

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Modeling Global and Regional Net Primary Production under Elevated Atmospheric CO2: On a Potential Source of Uncertainty

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  • 1 Department of Geography, University of Toronto, Toronto, Ontario, Canada
  • | 2 Center for Sustainability and the Global Environment, Gaylord Nelson Institute for Environmental Studies, University of Wisconsin—Madison, Madison, Wisconsin
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Abstract

Terrestrial ecosystem models are built, among several reasons, to explore how the Earth’s biosphere responds to climate change and to the projected continual increase of atmospheric CO2 concentration. Many of these models adopt the Farquhar et al. approach, in which leaf carbon assimilation of C3 plants is regulated by two limitations depending on the rate of Rubisco activity and ribulose-1, 5-bisphosphate regeneration (RuBP). This approach was expanded upon by others to include a third limitation that expresses the occurrence, in some plant species, of a photosynthetic downregulation under high concentrations of ambient CO2. Several ecosystem models, however, constrain leaf photosynthesis using only two limitations according to the original formulation of Farquhar et al. and thus neglect the limitation that represents the downregulation of photosynthesis under elevated atmospheric CO2. In this study, the authors first reviewed the effect of elevated CO2 on photosynthesis of C3 plants, which illustrated that short-term observations are likely to considerably underestimate the number of plant species that exhibit a photosynthetic downregulation. Several recent long-term field observations have shown that such downregulation starts to be effective only after several seasons/years of plant exposure to elevated CO2. Second, an ecosystem model was used to illustrate that neglecting the photosynthetic downregulation may significantly bias predictions of net primary production of the middle and high latitudes under high atmospheric CO2 concentrations. Based on both review of field observations and results of simulations, the authors conclude that a more appropriate representation of plant physiology and choice of plant functional types may be required in ecosystem models in order to accurately simulate plant responses to changing environmental conditions.

* Corresponding author address: Mustapha El Maayar, Department of Geography, University of Toronto, 100 St. Georges Street, Toronto, ON M5S 3G3, Canada. elmaayarm@geog.utoronto.ca

Abstract

Terrestrial ecosystem models are built, among several reasons, to explore how the Earth’s biosphere responds to climate change and to the projected continual increase of atmospheric CO2 concentration. Many of these models adopt the Farquhar et al. approach, in which leaf carbon assimilation of C3 plants is regulated by two limitations depending on the rate of Rubisco activity and ribulose-1, 5-bisphosphate regeneration (RuBP). This approach was expanded upon by others to include a third limitation that expresses the occurrence, in some plant species, of a photosynthetic downregulation under high concentrations of ambient CO2. Several ecosystem models, however, constrain leaf photosynthesis using only two limitations according to the original formulation of Farquhar et al. and thus neglect the limitation that represents the downregulation of photosynthesis under elevated atmospheric CO2. In this study, the authors first reviewed the effect of elevated CO2 on photosynthesis of C3 plants, which illustrated that short-term observations are likely to considerably underestimate the number of plant species that exhibit a photosynthetic downregulation. Several recent long-term field observations have shown that such downregulation starts to be effective only after several seasons/years of plant exposure to elevated CO2. Second, an ecosystem model was used to illustrate that neglecting the photosynthetic downregulation may significantly bias predictions of net primary production of the middle and high latitudes under high atmospheric CO2 concentrations. Based on both review of field observations and results of simulations, the authors conclude that a more appropriate representation of plant physiology and choice of plant functional types may be required in ecosystem models in order to accurately simulate plant responses to changing environmental conditions.

* Corresponding author address: Mustapha El Maayar, Department of Geography, University of Toronto, 100 St. Georges Street, Toronto, ON M5S 3G3, Canada. elmaayarm@geog.utoronto.ca

1. Background

Atmospheric CO2 concentration (Ca) has increased by more than 30% since the preindustrial era, from 280 to about 370 ppm, and is expected to reach about 540–970 ppm by the end of the twenty-first century (Houghton et al. 2001). Both laboratory and field observations indicate that a doubling of Ca would enhance photosynthesis of C3 plants by about 50%, although the response varies considerably among species (e.g., Curtis and Wang 1998; Lüscher et al. 1998; Norby et al. 1999; Kimball et al. 2002). Some observations also show that an enhancement of plant photosynthesis does not necessarily lead to an enhancement of terrestrial carbon storage (Norby et al. 2002) and that the highly variable responses to increased Ca among plant types could alter the process of competition among plant species (e.g., Derner et al. 2003), which ultimately may lead to changes in terrestrial ecosystem composition. An additional effect of Ca increase is the alteration of Earth’s climate and its variability through effects of CO2 on the Earth’s radiative budget (Houghton et al. 2001). Consequently, current scientific knowledge suggests that evaluating the implications of the continual Ca increase requires very sophisticated parameterizations to help understand the complex interactions between the different components of the Earth system.

It is now well recognized that terrestrial ecosystems play a key role in fashioning the Earth’s climate and atmospheric chemistry, through its exchanges of energy, momentum, water vapor, and various trace gases such as CO2 and CH4 with the atmosphere (Schlesinger 1991; Pielke et al. 1998). For that reason, considerable efforts have been made over the last three decades to develop terrestrial ecosystem models that describe the multiple interactions that occur at the land surface, and between the surface and the atmosphere at varying spatiotemporal scales (Foley 1995; Hurtt et al. 1998). For instance, some models are now able to integrate a detailed description of land surface physics, vegetation physiology, biogeochemistry, and vegetation dynamics [e.g., IBIS (Foley et al. 1996); MOSES-TRIFFID (Cox 2001); LSM-DGVM (Bonan et al. 2003)].

The representation of processes that occur at the leaf level is a critical aspect of current biospheric models, because of the control that stomatal conductance exerts on water and carbon budgets of the plant (e.g., Woodward 2002) and ultimately on the whole ecosystem (e.g., Pollard and Thompson 1995). To represent the assimilation of carbon at the leaf level, most current models of the terrestrial biosphere adopt the widely accepted photosynthesis model of Farquhar et al. (Farquhar et al. 1980, hereafter referred to as FM), as expanded upon by others to incorporate the coupled carbon–water exchanges at the leaf–atmosphere interface (Collatz et al. 1991; Leuning et al. 1995). Originally, FM stipulated that leaf assimilation is regulated by two limitations, namely, low activity of the carboxylase–oxygenase enzyme (Rubisco) under low intercellular concentration of CO2 (Ci), and by the rate of regeneration of the ribulose-1, 5-bisphosphate (RuBP) under low irradiance level. This formulation was expanded thereafter by Sharkey (Sharkey 1985) to include a third limitation that represents a reduction of the rate of leaf photosynthesis (or photosynthetic downregulation), related to the inadequate rate of utilization of triose-phosphate (TPU) on carboxylation under high irradiance and Ci levels. This limitation takes effect when, under high Ca, the accumulation of sugars in the leaf is sufficiently high to cause an inadequate release of inorganic phosphorus to sustain photosynthesis (Harley and Sharkey 1991; Krapp et al. 1993; Socias et al. 1993; Van Oosten and Besford 1995; Paul and Driscoll 1997).

More than 10 years ago, Wullschleger (Wullschleger 1993) analyzed the response of 109 C3 species to elevated Ca and found that one-fourth of them were TPU limited. The latter analyses were based, however, on short-term observations only. Several recent field experiments have shown that the photosynthetic downregulation process becomes evident only after long-term plant exposure to elevated CO2, and its occurrence may be caused either by an inadequate use of triose-phosphate, as suggested by Sharkey (Sharkey 1985), or by other mechanisms depending on the plant species. For instance, Lewis et al. (Lewis et al. 1996) who observed the response of loblolly pine (Pinus taeda L.) to elevated CO2 over three growing seasons reported a reduction in photosynthetic capacity that started only in the third season. Similarly, Griffin et al. (Griffin et al. 2000) observed that the same phenomenon was triggered only after 4 years of Pinus radiata D.’s exposure to elevated CO2. Besford et al. (Besford et al. 1998) also reported a downregulation of photosynthesis that started only during the third year of exposure of three tree species. Furthermore, while Curtis and Wang (Curtis and Wang 1998) reported that photosynthetic downregulation under high Ca did not occur in many short-term experiments made in indoor pots, Medlyn et al. (Medlyn et al. 1999) reported an 18% downregulation resulting from long-term field observations of 10 different tree species. Lee et al. (Lee et al. 2001) also found that a short-term exposure of C3 prairie grasses to an elevated rate of ambient CO2 caused an increase of photosynthetic rate by about 55%. This enhancement of photosynthesis dropped to about 13% and 8% after one and two complete years of exposure, respectively. Additionally, while most of the observations used in Curtis and Wang (Curtis and Wang 1998) were related to indoor pot experiments, Gunderson and Wullschleger’s (Gunderson and Wullschleger 1994) analysis of field observations of 39 C3 tree species exposed to high Ca exhibited an average of 21% photosynthetic downregulation. Moreover, Rey and Jarvis (Rey and Jarvis 1998) and Tissue et al. (Tissue et al. 1999) observed that the downregulation of photosynthesis of young birch (broadleaf deciduous species) and ponderosa pine (coniferous species) trees was maintained over the 4 years and the 3 last years, respectively, of their exposure to elevated Ca.

Several hypotheses have been suggested to explain the processes underlying the photosynthetic downregulation of C3 plants that occurs under high Ca (also termed acclimation and photosynthetic adjustment; see the review of Gunderson and Wullschleger 1994; Tissue et al. 1999). In addition to the already mentioned inadequate TPU (Sharkey 1985), these processes include a decrease in maximum Rubisco activity and electron transport in relation to leaf nitrogen content (Medlyn et al. 1999); the occurrence of physical damage to the thylakoidal membrane (DeLucia et al. 1985; Sage 1994); possible effects of some environmental factors such as photoperiod and light, and nutrient availability (El Kohen and Mousseau 1994; Lewis et al. 1996; Kubiske et al. 2002); soil nitrogen and phosphorus status (e.g., Oren et al. 2001; Winkler and Herbst 2004); an unclear phloem loading mechanism (Körner et al. 1995); and a possible combination of several mechanisms (Lee et al. 2001). Furthermore, a recent review of the mechanisms involved in the response of C3 plant photosynthesis to elevated CO2 suggested the existence of a complex system in which several feedbacks occur permanently to regulate photosynthesis (Paul and Foyer 2001). Moreover, some long-term observations show a strong seasonal variability of photosynthetic downregulation (e.g., Lewis et al. 1996; Besford et al. 1998; Rey and Jarvis 1998; Medlyn et al. 1999), though the environmental conditions that control that seasonal variability are unclear (Gunderson and Wullschleger 1994; Lee et al. 2001). In summary, in spite of the significant advances in recent years, the mechanisms responsible for photosynthetic downregulation are still poorly understood and more research is needed (e.g., Rey and Jarvis 1998; Saxe et al. 1998; Lee et al. 2001; Loreti et al. 2001; Paul and Foyer 2001; Ainsworth and Long 2005).

The fact that photosynthetic downregulation in C3 plants under elevated Ca has been observed in some species, but not in others, has led some biosphere modelers to consider a third photosynthetic limitation in their models, while others have not. A literature survey shows that several ecosystem models, including equilibrium models of the terrestrial biosphere [e.g., DOLY (Woodward et al. 1995); BIOME3 (Haxeltine and Prentice 1996); BIOME-BGC (Hunt et al. 1996); remote sensing–based models (e.g., BEPS of Liu et al. 1999); InTEC (Chen et al. 2000); Soil–Vegetation–Atmosphere–Transfer (SVAT) models (e.g., LSM of Bonan 1995); CLASS as modified by Arain et al. (2002) and dynamic global vegetation models (DGVM) (IBIS2.0 of Kucharik et al. 2000); HYBRID3.0 (Friend et al. 1997); LPJ (Sitch et al. 2003)] do not include a third limitation on simulated leaf assimilation, while other models such as CARAIB (Warnant et al. 1994), IBIS1.1 (Foley et al. 1996), and TRIFFID (Cox 2001) include the limitation. In this study, we are not trying to prove that ecosystem models must incorporate such a limitation to account for the photosynthetic downregulation at high Ca. Additional long-term field observations of the response of C3 plants to elevated CO2 are needed to further demonstrate the need for inclusion in ecosystem models (e.g., see Besford et al. 1998; Rey and Jarvis 1998; Medlyn et al. 1999; Juurola 2003; Nowak et al. 2004). However, we think that the role of this limitation within these models deserves more attention for the following two main reasons.

  • First, several models among those cited above use the concept of plant functional types (PFTs) to represent the Earth’s major biomes based on a combination of climatic criteria (e.g., tropical versus temperate) and some key ecological characteristics (broadleaf versus needleleaf, and deciduous versus evergreen) (e.g., Foley et al. 1996; Bonan et al. 2003). This concept groups an ensemble of species together within a single PFT (e.g., temperate broadleaf deciduous trees), and assumes a common physiology for all species that belong to that PFT. However, under high concentrations of CO2, photosynthetic downregulation may occur in some species and not in others, even though they belong to the same PFT. As an illustration, El Kohen et al. (El Kohen et al. 1993) observed that sweet chestnut (Castanea sativa), a temperate broadleaf deciduous tree, exhibited photosynthetic downregulation under elevated Ca, while beech (Fagus sylvatica), which is also a temperate broadleaf deciduous tree, did not. Li et al. (Li et al. 1999) also reported that when exposed to high Ca, Quercus geminata, an scrub-oak species grown in Florida downregulated its photosynthesis while Quercus myrtifolia, another scrub-oak species grown in the same region, did not. Consequently, ecosystem models that incorporate a third limitation (but assume that all species within a PFT downregulate) may systematically underestimate net primary production (NPP), while models that ignore downregulation may systematically overestimate NPP.

  • Second, leaf photosynthetic downregulation is likely to have some ramifications at the canopy scale (e.g., Marek et al. 2001; Kubiske et al. 2002) and at the ecosystem scale. For instance, a reduction in NPP of some PFTs is likely to affect their ability to capture environmental resources, such as water and light, which may alter their competitive ability and ultimately lead to changes in ecosystem composition. Subsequently, changes in ecosystem composition may lead to changes in ecosystem energy and water budgets (e.g., Aguiar et al. 1996; Cramer et al. 2001; Twine et al. 2004), which may in turn affect NPP. Such complex interactive processes, especially long-term feedbacks, can be investigated effectively only through modeling studies or long-term observational studies.

Thus, we used an ecosystem model to examine the sensitivity of the simulated NPP and other components of the carbon cycle of the Earth’s major biomes to leaf photosynthetic downregulation that occurs under high Ca (700 ppm), by comparing results of a simulation where the FM is used in its original form (i.e., leaf photosynthesis is constrained by only two limitations; see above) and results of a second simulation in which we incorporate downregulation by adding a third limitation to photosynthesis. Our results will illustrate the influence of photosynthesis downregulation on the predictions of ecosystem models and highlight a possible source of error that may affect projections of regional and global carbon budgets under elevated Ca.

2. Method

We used version 2.6 of the Integrated Biosphere Simulator (IBIS) (Kucharik et al. 2000). This DGVM simulates the presence/absence of 12 PFTs, where each PFT represents a group of plants with similar ecological and physiological characteristics (e.g., temperate deciduous forest, boreal conifer forest, grasses, and shrubs). Plant competition for light and water, which is influenced by interannual climatic variations, leads to changes in vegetation structure and distribution. Trees (which form the simulated upper canopy vegetation) compete with grasses and shrubs (lower canopy) for water and light. Within each vegetation canopy, competition is driven by differences in the annual carbon balance resulting from different ecological strategies, including differences in phenology (evergreen versus deciduous), leaf form (needle versus broadleaf), and photosynthetic pathway (C3 versus C4).

The model simulates processes at multiple time scales. Fluxes of energy, water, and carbon between the vegetation and atmosphere are simulated on an hourly time step using the land-surface-transfer scheme (LSX) of Pollard and Thompson (Pollard and Thompson 1995), modified to account for the simultaneous transfer of carbon and water based on the leaf physiology models of Farquhar et al. (Farquhar et al. 1980) and Collatz et al. (Collatz et al. 1991, 1992). Leaf-level photosynthesis is further scaled to the canopy level by assuming that photosynthesis is proportional to the absorbed photosynthetically active radiation (APAR) within the canopy, and stomatal conductance is scaled using a big-leaf assumption (Amthor 1994). Leaf biomass accumulation and leaf area index are computed daily in the phenology module. The remaining model processes are computed using an annual time step and include estimation of annual carbon balance; spatial changes in vegetation biomass and species composition; and flows of carbon between vegetation, detritus, and soil organic matter (Kucharik et al. 2000).

The model was forced with observed, gridded climate data (New et al. 1999), consisting of monthly normals (1961–90) of mean temperature and diurnal temperature range, total precipitation, relative humidity, cloud cover fraction, wind speed, and number of wet days. The International Geosphere Biosphere Program global soil database (IGBP-DIS 1999) was used to represent soil texture conditions in terms of fraction of sand, silt, and clay. The simulation was made at 2° × 2° spatial resolution. Moreover, to ensure that all ecosystem carbon pools had equilibrated, the simulation was performed for a 300-yr period. Results reported here represent the state of the simulated carbon pools at year 300. It should be noted that in reality, the soil carbon pool takes thousands of years to equilibrate; IBIS therefore uses a numerical acceleration technique that allows for the simulation of about 5000 yr of soil carbon dynamics in only 150 yr (Kucharik et al. 2000).

In version 2.0 of IBIS described in Kucharik et al. (Kucharik et al. 2000), the leaf photosynthesis of C3 plants is constrained by only two limitations. Here, in simulations where we introduced a third limitation to account for the photosynthetic downregulation under high Ca, we simply expressed it as half of the maximum catalytic capacity of Rubisco (Vmax), according to Collatz et al. (Collatz et al. 1991). This formulation (Vmax/2) is a general expression of the effect of the photosynthetic downregulation under elevated Ca, in contrast to other formulations that express the downregulation due to one particular mechanism, such as the inadequate use of triose-phosphate (Sharkey 1985). In this paper, we are interested in exploring how a third limitation might affect global terrestrial carbon budgets, rather than finding the best formulation for simulating the third limitation.

3. Results

We first briefly examined the leaf-level simulations of gross assimilation of temperate/boreal (Figures 1a,b) and tropical (Figures 1c,d) trees using the coupled leaf photosynthesis (FM) stomatal conductance model (Collatz et al. 1991), as described in Kucharik et al. (Kucharik et al. 2000). The leaf photosynthesis model equations are presented in the appendix while its parameter values are shown in Table 1. The simulations were performed under unstressed water conditions in order to verify that the leaf model used in IBIS reproduces the ratio (0.7) of intercellular to ambient CO2, in agreement with reported experimental observations (Poorter and Farquhar 1994; Ehleringer and Cerling 1995; Kull and Niinemets 1998). At 350 ppm, the simulated leaf assimilation remained almost unchanged for both tree categories, regardless of whether a third limitation was considered or not (Figures 1a–d). At 700 ppm, however, the effect of this limitation became substantial. Furthermore, for tropical trees that have a higher photosynthetic capacity than temperate trees, as parameterized in IBIS (Kucharik et al. 2000), the third limitation became effective under a much higher irradiation level than temperate trees (Figures 1b,d).

Ecosystem-level simulations show that while the photosynthetic downregulation caused only a small reduction (6%) in the predicted global NPP, its effect varies largely across the different biomes, particularly along the north–south latitudinal transect (Figures 2a–c; Table 2). The simulated NPP was reduced, for example, by more than 0.2 kg C m−2 yr−1 (∼15%) in central Europe (Figures 2b,c), which is more than the observed annual average NPP of boreal deciduous forests and about one-third of the observed annual average NPP of temperate deciduous forests (see Kucharik et al. 2000). However, downregulation caused only a very slight effect on NPP of tropical forests (Figures 2b,c). Moreover, it is known that on an annual time scale, the amount of solar radiation received at low and high latitudes is approximately the same. Only about 50 W m−2 more is received at latitude 0° than at latitude 45° (Hartmann 1994). Ecosystem-level results are therefore in agreement with those of leaf-level simulations, in which it is illustrated that for similar irradiance levels, the effect of downregulation decreases with the increase of the photosynthetic capacity of the leaf (Figures 1b,d). Ecosystem-level simulations made at 350 ppm indicate that the difference between NPP as obtained with and without downregulation is very small (Figures 3a–c), which is also in good agreement with leaf-level simulations. Additionally, we should note that under current Ca conditions, IBIS predictions of NPP compare favorably with observations (Kucharik et al. 2000), and with predictions of models of similar degree of complexity [e.g., the LSM-DGVM (Bonan et al. 2003)]. Results of our simulations as performed with and without downregulation are thus being gauged against reasonable baseline simulations.

Some potential feedback effects that resulted from the inclusion of the photosynthetic downregulation on individual biomes are also illustrated in Table 2. NPP of tundra and boreal evergreen forest (BEF) decreased for example by 21% and 16%, respectively. At the same time, interestingly, the area of tundra decreased by 5% while area of BEF increased by 15%. Photosynthetic downregulation decreased the capacity of tundra vegetation to assimilate carbon and to grow further. This reduced this vegetation’s capacity to utilize available environmental resources for maintenance. Consequently, a portion of this vegetation, which was able to grow under some extreme climatic conditions in the polar region when photosynthetic downregulation is not applied, was not able to do so when photosynthetic downregulation is applied. Thus, a part of the tundra is replaced by the polar desert biome, which explains the southern shift of the polar desert biome and the increase in its area (Table 2; Figures 4a–c). The case of BEF is different from the case of tundra because trees of the BEF are in ecophysiological competition with trees of the boreal deciduous forest (BDF). Although NPP of both BEF and BDF decreased in response to photosynthetic downregulation, area of BEF increased while area of BDF decreased. The increase in BEF area is due to an enhancement of the competitive ability of BEF trees over BDF trees, which resulted from an increase in the amounts of canopy light penetration and photosynthetically active radiation (PAR) that reach BEF trees. The increase of light penetration within the mixed BEF–BDF canopies resulted from a decrease in leaf area index (LAI) (Table 2). The overall increase in BEF area did not, however, totally compensate for the important decrease in its total NPP. This is because NPP per unit area is lower in the simulation with downregulation than in the simulation without downregulation. Another interesting case is the small increase in NPP of grasses though their area increased considerably (Table 2). In fact, area of grasses increased substantially (36%) at the expense of shrubs following the inclusion of the photosynthetic downregulation (Table 2; Figures 4a–c). That increase in area contributed only to a minor increase in total NPP (3%). Similar to the BEF case, the NPP per unit area is lower in the simulation with downregulation than in the simulation without downregulation. For all biomes, soil carbon changes were fairly similar to NPP changes (Table 2).

4. Discussion and conclusions

Our main objective was to highlight a potential source of uncertainty related to ecosystem model representation of photosynthesis downregulation in C3 plants under elevated Ca. The study was stimulated by recent long-term field observations that revealed that photosynthetic downregulation occurs in a much higher number of C3 plant species compared to earlier knowledge based on indoor short-term experiments. Because such downregulation does not occur in all C3 plants, predictions of ecosystem models that both include photosynthetic downregulation for all PFTs or ignore it completely might be systematically biased. We used an ecosystem model (IBIS), which is widely used to predict the effects of land–climate interactions, to perform sensitivity experiments including and excluding photosynthetic downregulation.

This study found that while photosynthetic downregulation at high atmospheric CO2 concentrations would be responsible for only a small decrease in global average NPP, its effect would vary considerably from one biome to the other. Our model predicted that NPP of middle- and high-latitude forests are sensitive to downregulation, whereas NPP of tropical forests remained mostly almost unaffected. Therefore, the ecosystem modeling community needs to pay more careful attention to the parameterization of photosynthesis in their models.

Our study was limited by the lack of a consensus formulation for parameterizing the effects of photosynthetic downregulation; we used a simple formulation proposed by Collatz et al. (Collatz et al. 1991, see last paragraph in the method section). Medlyn et al. (Medlyn et al. 1999) suggested another simple approach to account for the photosynthetic downregulation through a linear adjustment of the maximum capacity of Rubisco to leaf nitrogen concentration, based on a synthesis of reported observations made in European forests. However, before models can accurately represent photosynthesis, further observational studies are required to understand the mechanisms responsible for the response of plants to elevated Ca, and the effect of the surrounding environment on that response.

Existing studies suggest that the response of plants to elevated Ca is varied and complex. For example, trees are more responsive than grasses (e.g., Ainsworth and Long 2005), and the magnitude of photosynthetic downregulation differs among plant types (e.g., Ellsworth et al. 2004). Furthermore, for many plant species, the photosynthetic downregulation is triggered only after several years of exposure to elevated Ca. This time-lagged response was not accounted for in our study either. There is also incomplete understanding of how the magnitude of downregulation varies according to the environmental conditions. For example, although some studies have clearly demonstrated a real effect of the availability of nutrients in the soil on the magnitude of the photosynthetic downregulation (e.g., Oren et al. 2001; Winkler and Herbst 2004), Körner (Körner 2003) and Thürig et al. (Thürig et al. 2003) argued that there is still insufficient data to derive any statistically significant conclusions on the response of plants to elevated CO2 under limited nutrient conditions. Other authors argued that while Free-Air CO2 Enrichment (FACE) experiments significantly improved our understanding of plant responses to elevated Ca, a very large proportion of these experiments focused on plant types grown in temperate regions and mostly involved rapidly growing young trees. There is obviously a need to focus more substantively on vegetation of the tropical and boreal regions, and on mature trees (e.g., Saxe et al. 1998; Körner 2003; Ainsworth and Long 2005). In addition to the age criteria, a synthesis study by Osborne et al. (Osborne et al. 1998) reported that the magnitude of photosynthetic downregulation depends on leaf position within the canopy as well as on its developmental stage. Furthermore, Poorter and Navas (Poorter and Navas 2003) and Nowak et al. (Nowak et al. 2004) suggested that for a better assessment of the overall response of plants to CO2, future efforts must also focus on studying the effects of elevated CO2 on individual plants under conditions where they are in competition with other plants. Interestingly, Ellsworth et al. (Ellsworth et al. 2004) have also raised the question of whether the photosynthetic downregulation, once triggered, would be maintained a long period of time or not.

Further limitations of this study are directly related to IBIS structure. In fact, although the biochemical mechanisms responsible for photosynthetic downregulation are far from being completely understood, this downregulation is triggered to increase plant N use efficiency through a reorganization of the process of allocation of carbon and nitrogen to the different parts of the plant (e.g., see Norby et al. 2002). Currently, the plant C–N cycle is not simulated in IBIS, and a constant fraction of the assimilated carbon is allocated to the different plant pools (stem, root, leaf), independent of environmental conditions and plant age. Inhibitory effects of pollutants, such as ozone (O3) (e.g., Fowler et al. 1999; Fumagalli et al. 2001), and their interactions with atmospheric CO2 on photosynthesis are additional processes that are not simulated in IBIS (as in most current global ecosystem models). Our results are likely to be different if O3 effects on plant growth were simulated, especially in regions where O3 concentrations are high, such as the northeastern United States.

In spite of the various uncertainties in scientific understanding of photosynthetic downregulation, we hope this study has sufficiently highlighted an important issue related to ecosystem model parameterizations of plant physiology. We hope these results will stimulate the emergence of new insights toward improving model simulations of carbon cycle under changing atmospheric CO2 conditions. Finally, based on both recent long-term field observations and results of our simulations, we concur with Smith et al. (Smith et al. 1993) that a more adequate parameterization of PFTs in ecosystem models must necessarily take into account the physiological characteristics that govern their response to environmental conditions.

Acknowledgments

This research was supported by a grant through the Office of Science, Biological and Environmental Research Program (BER), U.S. Department of Energy, through the South Central Regional Center of the National Institute for Global Environmental Change (NIGEC) under Cooperative Agreement DE-FC03-90ER61010. We thank Dr. Sharon Cowling and Dr. Christine Delire for their review of a previous version of this manuscript. We thank two anonymous reviewers for insightful comments.

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Appendix

Description of the C3 Plants Leaf Photosynthesis Model Used in IBIS, as Described in Kucharik et al. (Kucharik et al. 2000)

Following Farquhar et al. (Farquhar et al. 1980) and Farquhar and Sharkey (Farquhar and Sharkey 1982), gross assimilation of C3 plants, Ag (mol CO2 m−2 s−1), is expressed as the minimum of two potential capacities to fix carbon as follows:
i1087-3562-10-2-1-ea1
where Je and Jc are light and Rubisco limited rates of photosynthesis, respectively.
The rate Je is given as
i1087-3562-10-2-1-ea2
where α is the intrinsic quantum efficiency for CO2 uptake (mol CO2 Einstein−1), Qp is the flux density of photosynthetically active radiation absorbed by the leaf (Einstein m−2 s−1), Ci is the concentration of CO2 in leaf’s intercellular air spaces (mol mol−1), and Γ* is the compensation point for gross photosynthesis (mol mol−1), given as
i1087-3562-10-2-1-ea3
where O2 is the atmospheric oxygen concentration (0.209 mol mol−1), and τ is the ratio of kinetic parameters describing the partitioning of enzyme activity to carboxylase or oxygenase function.
The rate Jc is given as
i1087-3562-10-2-1-ea4
where Vmax is the maximum catalytic capacity of Rubisco per unit leaf area (mol CO2 m−2 s−1), and Kc and Ko are the Michaelis–Menten coefficients (mol mol−1) for CO2 and O2, respectively.
The temperature dependence of Vmax, Kc, Ko, and τ is described using a modified Arrhenius function of Lloyd and Taylor (Lloyd and Taylor 1994):
i1087-3562-10-2-1-ea5
where To is the absolute zero temperature (−273.16°C), while Eo is the temperature sensitivity factor set to 3000, 1400, 6000, and −5000 for Vmax, Kc, Ko, and τ, respectively.
Stomatal conductance, gs,h2o (mol H2O m−2 s−1), is simulated according to Collatz et al. (Collatz et al. 1991) as follows:
i1087-3562-10-2-1-ea6
where An, hs, and Cs are net assimilation (mol CO2 m−2 s−1), air relative humidity (fraction), and CO2 concentration at the leaf surface (mol mol−1), respectively, and m and b are the slope and the intercept of the equation.
To account for the gradual transition between Je and Jc, the quadratic equation of Collatz et al. (Collatz et al. 1991) is solved for. Leaf net assimilation, An (mol CO2 m−2 s−1), is thereafter obtained as
i1087-3562-10-2-1-ea7
where γ is the leaf respiration cost of Rubisco activity.

Figure 1.
Figure 1.

Simulated leaf assimilation responses to internal CO2 concentration (Ci) and photosynthetically active radiation (Qp), at 25°C. As Ci changes, Qp was set equal to 1500 μEinstein m−2 s−1. (a), (b) Temperate and boreal trees, and (c), (d) tropical trees. In (b) and (d), the thin (thick) lines are used for 350 (700) ppm. In all figures, solid lines correspond to model outputs with downregulation while dashed lines correspond to the lack of downregulation. For ease of interpretation and visualization, the vertical dotted line in (a) and (c) delimitates the behavior of the assimilation curves when ambient CO2 concentration (Ca) is either lower or larger than 350 ppm (Ci = 0.7 × Ca). Here Vmax is cited at 15°C (Kucharik et al. 2000). For all simulations, the relative humidity was set to 80%.

Citation: Earth Interactions 10, 2; 10.1175/EI159.1

Figure 2.
Figure 2.

Potential effect of the photosynthetic downregulation on predicted NPP at an atmospheric CO2 concentration of 700 ppm: (a) global NPP distribution; (b) difference between a run without (NDR) and one with (WDR) downregulation; (c) relative difference between a run without (NDR) and one with (WDR) downregulation. In (c), the relative difference is larger than 50% in the black pixels located in the “tundra/polar desert” ecotone.

Citation: Earth Interactions 10, 2; 10.1175/EI159.1

Figure 3.
Figure 3.

As in Figure 2, but the atmospheric CO2 concentration is 350 ppm.

Citation: Earth Interactions 10, 2; 10.1175/EI159.1

Figure 4.
Figure 4.

Simulated global vegetation distribution. (a) The photosynthetic downregulation is not included; (b) the photosynthetic downregulation is included; (c) dark squares indicate areas where simulated vegetation with downregulation is different from the one simulated without downregulation.

Citation: Earth Interactions 10, 2; 10.1175/EI159.1

Table 1.

Values of the physiological parameters used for the 12 PFTs (see expansions below table) simulated in IBIS. Values of τ, Kc, Ko, and Vmax are cited at 15°C; Kc and Ko are the Michaelis–Menten coefficients (mol mol−1) for CO2 and O2, respectively; Vmax (mol CO2 m−2 s−1) is the maximum catalytic capacity of Rubisco per unit leaf area; α (mol CO2 Einstein−1) is the intrinsic quantum efficiency for CO2 uptake; τ (dimensionless) is the ratio of kinetic parameters describing the partitioning of enzyme activity to carboxylase or oxygenase function; and γ is the leaf respiration cost of Rubisco activity. Here m (dimensionless) and b (mol H2O m−2 s−1) are the slope and the intercept of the stomatal conductance–photosynthesis relationship, and β and θ are empirical constants that govern the sharpness of the transition between the potential photosynthesis rates (Collatz et al. 1991).

Table 1.
Table 2.

Potential effects of the photosynthetic downregulation on predicted total NPP, leaf area index, total soil carbon, and the total area of each of the 15 vegetation categories (see expansions below table) simulated in IBIS. The simulations were made using a prescribed ambient CO2 concentration of 700 ppm. WDR and NDR refer to the simulations made with and without the inclusion of the photosynthetic downregulation, respectively. Results are averages of the simulated values obtained over the last 10 yr of the runs.

Table 2.
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