Forum
Commentary Reducing the gaps in our understanding of the global terrestrial carbon cycle
need for the diversification and differential treatment of plant functional types to better represent carbon uptake by tropical forests.
‘By providing some of the necessary observational data that It has long been recognized that we need to understand how terrestrial primary production, as a major player of the global carbon cycle, responds to changes in the atmospheric CO2 concentration, and associated changes in temperature, if we are to predict the fate of the anthropogenic CO2 emissions and its effect on future climate. Recently, the importance of photosynthetic processes in predicting future atmospheric CO2 concentrations has been highlighted, with plant photosynthetic responses to temperature being identified as a major uncertainty in current land carbon modelling (Booth et al., 2012). The models of photosynthesis and related processes are among the most accurate and mechanistic parts of a modern ecosystem model (Dietze, 2014). Uncertainties of these models mostly arise from uncertainties in the parameters that constrain them, such as Vcmax (the maximum carboxylation rate of Rubisco) and Jmax (the maximum rate of photosynthetic electron transport). While Vcmax and Jmax have among the highest data quantity and quality of the parameters used in the models – and are thus well parameterized in general (Ziehn et al., 2011) – data coverage is largely limited to the high latitudes where most of the research is performed. By contrast, sampling in the tropical biomes, which are responsible for a large proportion of the global carbon flux, as well as species diversity, is vastly underrepresented (Dietze, 2014; Rogers, 2014). The study of V arhammar et al. in this issue of New Phytologist (pp. 1000–1012) is a rare attempt to address this imbalance by providing temperature responses of photosynthetic parameters (namely Vcmax and Jmax) in a comparison of native tropical montane rainforest trees and exotic plantation species in upland, tropical Africa. The report reveals not only big differences between native and exotic plants, but also between the species studied in general: exotic species showed high area-based values of Vcmax and Jmax, while native montane trees on average exhibited lower values of Vcmax and Jmax, but with significant variation between them. The thermal optimum of the assimilation rate was similar between the two groups compared; however, due to differences in stomatal conductance (which influences the rate of transpiration and thereby leaf temperature), Tleaf was generally higher in the native species than in the exotic species. Consequently, in contrast to the exotic species, the native species appear to operate at or above their thermal optimum. This effect will likely be exacerbated under future climatic conditions. Overall, the rigorous study by V arhammar et al. highlights the 886 New Phytologist (2015) 206: 886–888 www.newphytologist.com
can be used to parameterize these models, V arhammar et al. bring us one step closer to this goal.’
The graphical representation of the parameters Vcmax and J1800 (the rate of electron transport at the measurement light intensity) at 25°C found by V arhammar et al. emphasizes the need for greater data coverage of the tropics (Fig. 1), which has also been noted by others (Rogers, 2014). It not only demonstrates the large biochemical differences between the studied species, but also indicates the biochemical limitation at which each species is operating when CO2 supply to the chloroplast via stomatal processes is included. At current ambient CO2 concentration and standard temperature, the native species are all limited by the maximum carboxylation rate of Rubisco (Vcmax), whereas the exotic species are all limited by their photosynthetic electron transport capacity (Jmax). This indicates that, when grown under the conditions used in V arhammar et al., the exotic trees are likely less responsive to an increase in atmospheric CO2 concentrations, as they are already limited by Jmax. It also means that the temperature response of net photosynthesis (An) in the native species at current CO2 concentrations is dominated by the temperature response of Vcmax, while that of the exotic species appears to be governed by the response of Jmax. Stomata not only affect the underlying biochemical limitation of photosynthesis via the CO2 supply, but also influence leaf temperature. This is exemplified in Fig. 4 in V arhammar et al., with higher leaf temperatures additionally shifting the limitation towards Vcmax. Even though plants may behave similarly under certain conditions (e.g. have the same Topt or An), physiological differences therefore warrant a more differentiated treatment of plant functional types than currently exists in ecosystem models, especially within the diverse flora of the tropics. By providing some of the necessary observational data that can be used to parameterize these models, V arhammar et al. bring us one step closer to this goal. The presented results also point towards a more general issue – the accurate representation of parameters other than Vcmax and Jmax, such as stomatal (gs) and boundary layer conductances, in models of the carbon cycle. While the temperature response of photosynthesis has been identified as the factor to which global models are most sensitive (Booth et al., 2012), it is also among the Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust
1: C. grandiflora 2: E. excelsum 3: H. abyssinica 4: C. serrata 5: E. maidenii 6: E. microcorys
35 30 25
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Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust
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CO2 concentration (Ci or Ca ) (µmol mol )
Intercellular CO2 concentration Ci (µmol mol )
Fig. 2 CO2 response curves for a plant with the fixed values of Vcmax and Jmax for Cedrela serrata, graphed for different values of stomatal conductance (gs; mol CO2 m 2 s 1) in the range of the values obtained from different species by V arhammar et al. (in this issue of New Phytologist, pp. 1000–1012). Black line, An graphed against the intercellular CO2 concentration (Ci; equivalent to infinite gs), representing the biochemical capacity to assimilate CO2 inside the leaves; red lines, the resulting actual CO2 assimilation rates assuming fixed values of gs, plotted against an increasing ambient CO2 concentration (Ca). Dashed lines, the current atmospheric CO2 concentration as well as 50% elevated CO2. 25
CO2 assimilation rate An (µ mol m–2 s–1)
factors that are overall best parameterized by available data (Dietze, 2014). Unlike the mechanistic model of Farquhar et al. (1980) describing photosynthesis on an enzyme kinetic basis, models predicting stomatal conductance are largely empirical and usually couple gs to An (Damour et al., 2010; De Kauwe et al., 2013), despite evidence that gs may not be mechanistically linked to An (discussed by Busch, 2014). While the empirical models generally still produce reasonable results under current conditions, there is a chance that they misrepresent important aspects when environmental conditions change, for example, due to drought or humidity stress (Schaefer et al., 2012). Conductance models have, therefore, been declared as one of the most uncertain aspects of the land photosynthesis contribution (Dietze, 2014). How do differences in gs affect the outcomes of An? The principal effect is outlined in Fig. 2, where the black line denotes the apparent underlying biochemical capacity of the plant (in response to the intercellular CO2 concentration (Ci,); in this case Cedrela serrata), and the red lines indicate the resulting An at any given atmospheric CO2 concentration for four values of gs in the range given for the studied species (V arhammar et al.). While the graphed values of gs are not necessarily all physiologically relevant for C. serrata, the figure demonstrates that small inaccuracies for gs can have quite substantial effects on the estimated parameter An, with the magnitude of the effect changing with CO2 concentration. De Kauwe et al. (2013) have shown that climate models are very sensitive to how stomatal conductance is parameterized. In their study they compared the different implementations of plant wateruse in different ecosystem models to estimate the uncertainties related to plant water-use in response to rising CO2 concentrations. Comparing the models with observations from two FACE sites the
400
–1
–1
Fig. 1 Graphical representation of the biochemical parameters Vcmax (blue) and J1800 (red) in response to the intercellular CO2 concentration (Ci) at 25°C, outlining the physiological differences found in the species measured by V arhammar et al. (in this issue of New Phytologist, pp. 1000–1012). Native species, solid lines; exotic species, dashed lines. Arrows point at the Ci values found at an ambient CO2 concentration of 389 lmol mol 1, and indicate under which biochemical limitation the plants operate at the measurement conditions. Solid arrows, native species; open arrows, exotic species.
gs = 0.04
gs = 0.09
10
0
0
Forum 887
Commentary
CO2 assimilation rate An (µ mol m–2 s–1)
CO2 assimilation rate An (µ mol m–2 s –1)
New Phytologist
shift in Topt 20
15
10
5
0
Plants grown at low temp. Plants grown at high temp. 15
20
25
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35
40
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Leaf temperature (ºC)
Fig. 3 Schematic diagram of the temperature acclimation of the response of An to temperature following the temperature response model used by V arhammar et al. (in this issue of New Phytologist, pp. 1000–1012). Plants grown at higher temperature tend to shift their temperature optimum (Topt) to higher temperatures as compared with plants grown at lower temperatures. Some species can also acclimatize to new growth temperatures by changing the shape of the temperature response without shifting Topt (see Yamori et al. (2014) for a detailed description of the temperature acclimation of photosynthesis).
study highlighted the difficulties that are still inherent in modelling conductances. Another important physiological aspect has so far been left out of the ecosystem models altogether: once the CO2 has diffused from New Phytologist (2015) 206: 886–888 www.newphytologist.com
888 Forum
New Phytologist
Commentary
the atmosphere into the intercellular air space (determined by stomatal and boundary layer conductances) it still has a long way before it reaches the chloroplast. The last part of the pathway of CO2 diffusion into the chloroplast is determined by mesophyll conductance (gm), which also affects the prediction of CO2 assimilation rates (Flexas et al., 2012). On the leaf-scale, the effects of gm are comparable to those of gs demonstrated in Fig. 2, and are also on the ecosystem scale of importance (Keenan et al., 2010). This was addressed in more detail recently by Sun et al. (2014), who compared the state-of-the-art Community Land Model 4.5 that assumes zero diffusion resistances through the mesophyll with a version of the model that accounts for gm. The model simulations revealed that the effects of gm are significant and could largely correct for the observed underestimation of the long-term fertilization effect of anthropogenic CO2 emissions on global primary productivity (Sun et al., 2014). Therefore, inclusion of gm should not be ignored. In this analysis the tropics were responsible for about half of the global underestimation owing to their high productivity, which further underlines the importance of accurately parameterizing photosynthetic processes at this latitude. Aside from the uncertainties associated with how CO2 diffusion conductances are implemented and parameterized in the models, the other big unknown factors are the temperature response and temperature acclimation of photosynthesis (Ziehn et al., 2011; Booth et al., 2012; Dietze, 2014; Yamori et al., 2014). The temperature response of photosynthesis is a product of a number of different processes, two of which have been thoroughly addressed by V arhammar et al. In addition to the instantaneous temperature responses described here, we also need to better understand longterm acclimation of these responses to a change in temperature with a warming climate (Bagley et al., 2015). Detailed studies in this regard are especially important since plants can acclimate to higher temperatures in unexpected ways, for example, by an increase in biomass in response to night-time warming in experiments on tropical tree seedlings (Cheesman & Winter, 2013; Rogers et al., 2014). Processes affecting photosynthetic performance, such as the heat tolerance of thylakoid membranes, Rubisco activation state, or mitochondrial respiration, show some degree of thermal acclimation when exposed to higher temperatures for extended periods of time (Yamori et al., 2014). In plants grown at higher temperatures the thermal optimum of photosynthesis tends to shift to higher temperatures (Fig. 3). The degree to which this occurs depends on the underlying biochemical limitation of photosynthesis, plant functional type, and photosynthetic type (C3 vs C4 or CAM) of the plant, among other things. On average across species, Topt in C3 plants changes by c. 0.5°C for every °C change in growth temperature, with slightly lower values for C4 and CAM plants (Yamori et al., 2014). Considering that different limitations underlie the observed An in native vs exotic species in the study by V arhammar et al., one might suspect that these species also respond differentially in their ability for thermal acclimation. This indicates that a differentiated treatment of species depending on physiological properties or plant functional types could increase the reliability of ecosystem models (Dietze, 2014). Overall, the ability of models to predict land carbon feedbacks on the atmospheric CO2 concentration is still constrained by the amount of New Phytologist (2015) 206: 886–888 www.newphytologist.com
observational data and more effort should be targeted towards filling the existing gaps of modelling carbon–climate interactions. Florian A. Busch Research School of Biology, The Australian National University, Canberra, ACT 2601, Australia (tel +61 2 6125 0123; email [email protected])
References Bagley J, Rosenthal DM, Ruiz-Vera UM, Siebers MH, Kumar P, Ort DR, Bernacchi CJ. 2015. The influence of photosynthetic acclimation to rising CO2 and warmer temperatures on leaf and canopy photosynthesis models. Global Biogeochemical Cycles 29: 194–206. Booth BBB, Jones CD, Collins M, Totterdell IJ, Cox PM, Sitch S, Huntingford C, Betts RA, Harris GR, Lloyd J. 2012. High sensitivity of future global warming to land carbon cycle processes. Environmental Research Letters 7: 024002. Busch FA. 2014. Opinion: the red-light response of stomatal movement is sensed by the redox state of the photosynthetic electron transport chain. Photosynthesis Research 119: 131–140. Cheesman AW, Winter K. 2013. Elevated night-time temperatures increase growth in seedlings of two tropical pioneer tree species. New Phytologist 197: 1185–1192. Damour G, Simonneau T, Cochard H, Urban L. 2010. An overview of models of stomatal conductance at the leaf level. Plant, Cell & Environment 33: 1419–1438. De Kauwe MG, Medlyn BE, Zaehle S, Walker AP, Dietze MC, Hickler T, Jain AK, Luo Y, Parton WJ, Prentice IC et al. 2013. Forest water use and water use efficiency at elevated CO2: a model–data intercomparison at two contrasting temperate forest FACE sites. Global Change Biology 19: 1759–1779. Dietze M. 2014. Gaps in knowledge and data driving uncertainty in models of photosynthesis. Photosynthesis Research 119: 3–14. Farquhar GD, von Caemmerer S, Berry JA. 1980. A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species. Planta 149: 78–90. Flexas J, Barbour MM, Brendel O, Cabrera HM, Carriquı M, Dıaz-Espejo A, Douthe C, Dreyer E, Ferrio JP, Gago J et al. 2012. Mesophyll diffusion conductance to CO2: an unappreciated central player in photosynthesis. Plant Science 193–194: 70–84. Keenan T, Sabate S, Gracia C. 2010. The importance of mesophyll conductance in regulating forest ecosystem productivity during drought periods. Global Change Biology 16: 1019–1034. Rogers A. 2014. The use and misuse of Vc,max in Earth System Models. Photosynthesis Research 119: 15–29. Rogers A, Medlyn BE, Dukes JS. 2014. Improving representation of photosynthesis in Earth System Models. New Phytologist 204: 12–14. Schaefer K, Schwalm CR, Williams C, Arain MA, Barr A, Chen JM, Davis KJ, Dimitrov D, Hilton TW, Hollinger DY et al. 2012. A model–data comparison of gross primary productivity: results from the North American Carbon Program site synthesis. Journal of Geophysical Research: Biogeosciences 117: G03010. Sun Y, Gu L, Dickinson RE, Norby RJ, Pallardy SG, Hoffman FM. 2014. Impact of mesophyll diffusion on estimated global land CO2 fertilization. Proceedings of the National Academy of Sciences, USA 111: 15774–15779. V arhammar A, Wallin G, McLean CM, Dusenge ME, Medlyn BE, Hasper TB, Nsabimana D, Uddling J. 2015. Photosynthetic temperature responses of tree species in Rwanda: evidence of pronounced negative effects of high temperature in montane rainforest climax species. New Phytologist 206: 1000–1012. Yamori W, Hikosaka K, Way D. 2014. Temperature response of photosynthesis in C3, C4, and CAM plants: temperature acclimation and temperature adaptation. Photosynthesis Research 119: 101–117. Ziehn T, Kattge J, Knorr W, Scholze M. 2011. Improving the predictability of global CO2 assimilation rates under climate change. Geophysical Research Letters 38: L10404. Key words: anthropogenic CO2 emissions, carbon feedbacks, Jmax, modelling carbon–climate interactions, temperature responses, tropical biomes, Vcmax.
Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust
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Commentary Reducing the gaps in our understanding of the global terrestrial carbon cycle
need for the diversification and differential treatment of plant functional types to better represent carbon uptake by tropical forests.
‘By providing some of the necessary observational data that It has long been recognized that we need to understand how terrestrial primary production, as a major player of the global carbon cycle, responds to changes in the atmospheric CO2 concentration, and associated changes in temperature, if we are to predict the fate of the anthropogenic CO2 emissions and its effect on future climate. Recently, the importance of photosynthetic processes in predicting future atmospheric CO2 concentrations has been highlighted, with plant photosynthetic responses to temperature being identified as a major uncertainty in current land carbon modelling (Booth et al., 2012). The models of photosynthesis and related processes are among the most accurate and mechanistic parts of a modern ecosystem model (Dietze, 2014). Uncertainties of these models mostly arise from uncertainties in the parameters that constrain them, such as Vcmax (the maximum carboxylation rate of Rubisco) and Jmax (the maximum rate of photosynthetic electron transport). While Vcmax and Jmax have among the highest data quantity and quality of the parameters used in the models – and are thus well parameterized in general (Ziehn et al., 2011) – data coverage is largely limited to the high latitudes where most of the research is performed. By contrast, sampling in the tropical biomes, which are responsible for a large proportion of the global carbon flux, as well as species diversity, is vastly underrepresented (Dietze, 2014; Rogers, 2014). The study of V arhammar et al. in this issue of New Phytologist (pp. 1000–1012) is a rare attempt to address this imbalance by providing temperature responses of photosynthetic parameters (namely Vcmax and Jmax) in a comparison of native tropical montane rainforest trees and exotic plantation species in upland, tropical Africa. The report reveals not only big differences between native and exotic plants, but also between the species studied in general: exotic species showed high area-based values of Vcmax and Jmax, while native montane trees on average exhibited lower values of Vcmax and Jmax, but with significant variation between them. The thermal optimum of the assimilation rate was similar between the two groups compared; however, due to differences in stomatal conductance (which influences the rate of transpiration and thereby leaf temperature), Tleaf was generally higher in the native species than in the exotic species. Consequently, in contrast to the exotic species, the native species appear to operate at or above their thermal optimum. This effect will likely be exacerbated under future climatic conditions. Overall, the rigorous study by V arhammar et al. highlights the 886 New Phytologist (2015) 206: 886–888 www.newphytologist.com
can be used to parameterize these models, V arhammar et al. bring us one step closer to this goal.’
The graphical representation of the parameters Vcmax and J1800 (the rate of electron transport at the measurement light intensity) at 25°C found by V arhammar et al. emphasizes the need for greater data coverage of the tropics (Fig. 1), which has also been noted by others (Rogers, 2014). It not only demonstrates the large biochemical differences between the studied species, but also indicates the biochemical limitation at which each species is operating when CO2 supply to the chloroplast via stomatal processes is included. At current ambient CO2 concentration and standard temperature, the native species are all limited by the maximum carboxylation rate of Rubisco (Vcmax), whereas the exotic species are all limited by their photosynthetic electron transport capacity (Jmax). This indicates that, when grown under the conditions used in V arhammar et al., the exotic trees are likely less responsive to an increase in atmospheric CO2 concentrations, as they are already limited by Jmax. It also means that the temperature response of net photosynthesis (An) in the native species at current CO2 concentrations is dominated by the temperature response of Vcmax, while that of the exotic species appears to be governed by the response of Jmax. Stomata not only affect the underlying biochemical limitation of photosynthesis via the CO2 supply, but also influence leaf temperature. This is exemplified in Fig. 4 in V arhammar et al., with higher leaf temperatures additionally shifting the limitation towards Vcmax. Even though plants may behave similarly under certain conditions (e.g. have the same Topt or An), physiological differences therefore warrant a more differentiated treatment of plant functional types than currently exists in ecosystem models, especially within the diverse flora of the tropics. By providing some of the necessary observational data that can be used to parameterize these models, V arhammar et al. bring us one step closer to this goal. The presented results also point towards a more general issue – the accurate representation of parameters other than Vcmax and Jmax, such as stomatal (gs) and boundary layer conductances, in models of the carbon cycle. While the temperature response of photosynthesis has been identified as the factor to which global models are most sensitive (Booth et al., 2012), it is also among the Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust
1: C. grandiflora 2: E. excelsum 3: H. abyssinica 4: C. serrata 5: E. maidenii 6: E. microcorys
35 30 25
native
3 6
exotic
5 4
20 15
1
10
2
5 0
25
100
200
300
400
500
600
gs = 0.66
20
gs = 0.15
15
5
700
0
200
Ó 2015 The Authors New Phytologist Ó 2015 New Phytologist Trust
600
800
1000
CO2 concentration (Ci or Ca ) (µmol mol )
Intercellular CO2 concentration Ci (µmol mol )
Fig. 2 CO2 response curves for a plant with the fixed values of Vcmax and Jmax for Cedrela serrata, graphed for different values of stomatal conductance (gs; mol CO2 m 2 s 1) in the range of the values obtained from different species by V arhammar et al. (in this issue of New Phytologist, pp. 1000–1012). Black line, An graphed against the intercellular CO2 concentration (Ci; equivalent to infinite gs), representing the biochemical capacity to assimilate CO2 inside the leaves; red lines, the resulting actual CO2 assimilation rates assuming fixed values of gs, plotted against an increasing ambient CO2 concentration (Ca). Dashed lines, the current atmospheric CO2 concentration as well as 50% elevated CO2. 25
CO2 assimilation rate An (µ mol m–2 s–1)
factors that are overall best parameterized by available data (Dietze, 2014). Unlike the mechanistic model of Farquhar et al. (1980) describing photosynthesis on an enzyme kinetic basis, models predicting stomatal conductance are largely empirical and usually couple gs to An (Damour et al., 2010; De Kauwe et al., 2013), despite evidence that gs may not be mechanistically linked to An (discussed by Busch, 2014). While the empirical models generally still produce reasonable results under current conditions, there is a chance that they misrepresent important aspects when environmental conditions change, for example, due to drought or humidity stress (Schaefer et al., 2012). Conductance models have, therefore, been declared as one of the most uncertain aspects of the land photosynthesis contribution (Dietze, 2014). How do differences in gs affect the outcomes of An? The principal effect is outlined in Fig. 2, where the black line denotes the apparent underlying biochemical capacity of the plant (in response to the intercellular CO2 concentration (Ci,); in this case Cedrela serrata), and the red lines indicate the resulting An at any given atmospheric CO2 concentration for four values of gs in the range given for the studied species (V arhammar et al.). While the graphed values of gs are not necessarily all physiologically relevant for C. serrata, the figure demonstrates that small inaccuracies for gs can have quite substantial effects on the estimated parameter An, with the magnitude of the effect changing with CO2 concentration. De Kauwe et al. (2013) have shown that climate models are very sensitive to how stomatal conductance is parameterized. In their study they compared the different implementations of plant wateruse in different ecosystem models to estimate the uncertainties related to plant water-use in response to rising CO2 concentrations. Comparing the models with observations from two FACE sites the
400
–1
–1
Fig. 1 Graphical representation of the biochemical parameters Vcmax (blue) and J1800 (red) in response to the intercellular CO2 concentration (Ci) at 25°C, outlining the physiological differences found in the species measured by V arhammar et al. (in this issue of New Phytologist, pp. 1000–1012). Native species, solid lines; exotic species, dashed lines. Arrows point at the Ci values found at an ambient CO2 concentration of 389 lmol mol 1, and indicate under which biochemical limitation the plants operate at the measurement conditions. Solid arrows, native species; open arrows, exotic species.
gs = 0.04
gs = 0.09
10
0
0
Forum 887
Commentary
CO2 assimilation rate An (µ mol m–2 s–1)
CO2 assimilation rate An (µ mol m–2 s –1)
New Phytologist
shift in Topt 20
15
10
5
0
Plants grown at low temp. Plants grown at high temp. 15
20
25
30
35
40
45
Leaf temperature (ºC)
Fig. 3 Schematic diagram of the temperature acclimation of the response of An to temperature following the temperature response model used by V arhammar et al. (in this issue of New Phytologist, pp. 1000–1012). Plants grown at higher temperature tend to shift their temperature optimum (Topt) to higher temperatures as compared with plants grown at lower temperatures. Some species can also acclimatize to new growth temperatures by changing the shape of the temperature response without shifting Topt (see Yamori et al. (2014) for a detailed description of the temperature acclimation of photosynthesis).
study highlighted the difficulties that are still inherent in modelling conductances. Another important physiological aspect has so far been left out of the ecosystem models altogether: once the CO2 has diffused from New Phytologist (2015) 206: 886–888 www.newphytologist.com
888 Forum
New Phytologist
Commentary
the atmosphere into the intercellular air space (determined by stomatal and boundary layer conductances) it still has a long way before it reaches the chloroplast. The last part of the pathway of CO2 diffusion into the chloroplast is determined by mesophyll conductance (gm), which also affects the prediction of CO2 assimilation rates (Flexas et al., 2012). On the leaf-scale, the effects of gm are comparable to those of gs demonstrated in Fig. 2, and are also on the ecosystem scale of importance (Keenan et al., 2010). This was addressed in more detail recently by Sun et al. (2014), who compared the state-of-the-art Community Land Model 4.5 that assumes zero diffusion resistances through the mesophyll with a version of the model that accounts for gm. The model simulations revealed that the effects of gm are significant and could largely correct for the observed underestimation of the long-term fertilization effect of anthropogenic CO2 emissions on global primary productivity (Sun et al., 2014). Therefore, inclusion of gm should not be ignored. In this analysis the tropics were responsible for about half of the global underestimation owing to their high productivity, which further underlines the importance of accurately parameterizing photosynthetic processes at this latitude. Aside from the uncertainties associated with how CO2 diffusion conductances are implemented and parameterized in the models, the other big unknown factors are the temperature response and temperature acclimation of photosynthesis (Ziehn et al., 2011; Booth et al., 2012; Dietze, 2014; Yamori et al., 2014). The temperature response of photosynthesis is a product of a number of different processes, two of which have been thoroughly addressed by V arhammar et al. In addition to the instantaneous temperature responses described here, we also need to better understand longterm acclimation of these responses to a change in temperature with a warming climate (Bagley et al., 2015). Detailed studies in this regard are especially important since plants can acclimate to higher temperatures in unexpected ways, for example, by an increase in biomass in response to night-time warming in experiments on tropical tree seedlings (Cheesman & Winter, 2013; Rogers et al., 2014). Processes affecting photosynthetic performance, such as the heat tolerance of thylakoid membranes, Rubisco activation state, or mitochondrial respiration, show some degree of thermal acclimation when exposed to higher temperatures for extended periods of time (Yamori et al., 2014). In plants grown at higher temperatures the thermal optimum of photosynthesis tends to shift to higher temperatures (Fig. 3). The degree to which this occurs depends on the underlying biochemical limitation of photosynthesis, plant functional type, and photosynthetic type (C3 vs C4 or CAM) of the plant, among other things. On average across species, Topt in C3 plants changes by c. 0.5°C for every °C change in growth temperature, with slightly lower values for C4 and CAM plants (Yamori et al., 2014). Considering that different limitations underlie the observed An in native vs exotic species in the study by V arhammar et al., one might suspect that these species also respond differentially in their ability for thermal acclimation. This indicates that a differentiated treatment of species depending on physiological properties or plant functional types could increase the reliability of ecosystem models (Dietze, 2014). Overall, the ability of models to predict land carbon feedbacks on the atmospheric CO2 concentration is still constrained by the amount of New Phytologist (2015) 206: 886–888 www.newphytologist.com
observational data and more effort should be targeted towards filling the existing gaps of modelling carbon–climate interactions. Florian A. Busch Research School of Biology, The Australian National University, Canberra, ACT 2601, Australia (tel +61 2 6125 0123; email [email protected])
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