Does the response of leaf photosynthetic productivity to rising atmospheric temperature and CO2 scale up to the canopy?

Background/Question/Methods

Theory predicts that interacting increases in temperature and CO₂ will work synergistically to enhance leaf photosynthesis. How this interaction will scale up to affect canopy and ecosystem productivity in the future is less clear.  Numerous factors contribute to this uncertainty including higher canopy temperatures from lower stomatal conductance (g_s) driven by elevated [CO₂] and the response and possible acclimation of respiration to increasing temperature and [CO₂].   The goal of this research was first to quantify the effect of indirect temperature increases on canopy productivity and second to estimate the response of soybean canopy assimilation and productivity to simulated increases in temperature and [CO₂]. To resolve direct and indirect effects of prolonged growth at elevated CO₂ and temperature on respiration, mechanistic responses were simulated using WIMOVAC to control respiratory or photosynthetic acclimation to temperature while assuming biochemical acclimation of photosynthesis to elevated [CO₂].  Canopy assimilation was estimated using empirically derived photosynthetic parameters, leaf area index, and meteorological data collected at 10 minute intervals from canopy closure to peak biomass over four years at SoyFACE.  Leaf microclimate was estimated from LAI and meteorological data using an energy balance approach and input into a canopy model to estimate microclimate for each leaf layer at ten minute intervals.  Photosynthesis for each leaf layer was then calculated using a biochemical model of photosynthesis, and canopy assimilation was estimated by summing all leaf layers at a given time.  

Results/Conclusions

Model output was highly correlated with biomass collected for each of the four years (r² ranged from 0.88 to 0.97) indicating the model is a robust predictor of canopy assimilation and biomass production.  Consistent with theory and empirical observation, the stimulatory effect of CO₂ increased as a function of maximum daytime temperature supporting the notion of the synergistic effects of elevated CO₂ and temperature on C₃ canopy assimilation.  Indirect temperature elevation caused a small (0.5 to 2%) but in some years significant decrease in canopy assimilation. Moreover, on average in years modeled here, we found that incremental increases in air temperature (+ 1, 2 or 3C) would reduce canopy assimilation by between 3% to 20% in ambient and 2% to 13 % in elevated CO₂, also consistent with the notion that elevating CO₂ enhances the photosynthetic optimum.  These results bring into question the frequently held assumption that modest increases in temperature at [CO₂] expected by 2050 will benefit plant productivity.