Reduced stomatal conductance is arguably the most ubiquitous response of plants to growth at elevated CO2. Reduced stomatal conductance acts to lower whole-plant water use. But, greater canopy temperatures associated with lower evaporative cooling can counteract reduced stomatal conductance. In addition, greater photosynthetic carbon gain at elevated CO2 can lead to greater canopy leaf area, which acts to increase whole-plant water use. Twenty five years of data from Free-Air CO2 Enrichment (FACE) experiments are now available to explore how interactions between stomatal conductance, canopy leaf area and canopy energy balance operate to influence total water use across a range of growing environments and plant types. Growth at elevated CO2 can also modify root growth, nutrient dynamics, hormone signaling, plant hydraulics and osmolytes. These responses to elevated CO2 combine to have complex effects on whole-plant performance under drought. But, the availability of experimental data describing canopy-scale fluxes of water and carbon are limited compared to physiological data.
Results/Conclusions
Models provide a means to dissect mechanisms of response and extrapolate experimental findings to larger spatial and temporal scales. Synthesis of the FACE dataset and model simulations reveals conserved mechanisms of response and knowledge gaps to be addressed by future research.