How seasonal temperature or water inputs affect the relative response of C₃ crops to elevated [CO₂]: A global analysis of open top chamber and free air CO₂ enrichment studies

Rising atmospheric carbon dioxide concentration ([CO₂]) has the potential to positively impact C₃ food crop production by directly stimulating photosynthetic carbon gain (A), which leads to increased crop biomass and yield. Further stimulation of A and yield can result from an indirect mechanism in which elevated [CO₂] decreases stomatal conductance and canopy water use, ameliorating drought stress. Experiments in open top chambers (OTC) and free air CO₂ enrichment (FACE) facilities have enabled investigation of crop responses to elevated [CO₂] in near natural, field conditions. Mechanistic understanding of physiological responses to elevated [CO₂] has led to predictions that the stimulation of A, biomass production, and economic yield will vary with the temperature and water supply experienced by the crop. This study tested current assumptions about the relationships between relative responses of yield and biomass to elevated [CO₂] and variation in growing season temperature and water inputs (precipitation plus irrigation). Growing season average temperature was not a good predictor of the magnitude of biomass and yield responses to elevated [CO₂], contradicting the prediction that responses to elevated [CO₂] would increase with increasing temperature due to the greater benefit from decreasing photorespiration. However, the prediction that the relative stimulation of yield by elevated [CO₂] would be greatest in dry conditions was generally supported. Thus, a simple CO₂ fertilization value is not appropriate for modeling future crop productivity under varying environmental conditions. Further studies are necessary across a broader range of environmental conditions in order to accurately predict how rising [CO₂] will interact with temperature and drought stress and alter future crop production.