Response of evapotranspiration from tallgrass prairie vegetation to CO2 at subambient to elevated concentrations

Effects of CO₂ enrichment on leaf transpiration are well-documented, but our understanding of how CO₂ interacts with other variables to regulate evapotranspiration is more limited.  We installed weighing lysimeters planted to species characteristic of tallgrass prairie into a field chamber used to regulate CO₂ along a subambient to elevated gradient.  Lysimeters with three soils types were studied to determine how CO₂ effects on evapotranspiration per unit of leaf area (ETla) vary with leaf area index (LAI), soil type, and environmental variables (soil water deficit from field capacity, photosynthetically-active radiation, air temperature, air vapor pressure deficit).  CO2 enrichment reduced ETla.  The CO2-caused decrease in ETla was greatest at relatively-low temperatures and low LAI for all soils combined.  Higher temperatures countered the CO₂ effect by increasing ETla more at elevated than subambient CO₂.  Higher LAI also countered the CO₂ effect by decreasing ETla more at subambient than elevated concentrations.  Plant (LAI) and environmental effects on ETla differed among soils, possibly because plant growth patterns and physiology differed among soils. Our results imply that the CO₂ effect on ETla will vary with seasonal changes in temperature and LAI, independently of seasonal shifts in leaf age and physiological activity.  The CO₂-caused decrease in ETla should be greatest early in the growing season when temperatures are mild and the plant canopy is re-establishing.