Initial 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 from plant communities is more limited. A series of weighing lysimeters in which tallgrass prairie species had been planted were exposed to a subambient to elevated gradient in CO2 in a field chamber. Lysimeters with intact monoliths of three soil types were represented along the CO2 gradient. We used regression analysis to determine how CO2 effects on evapotranspiration per unit of soil surface area (ETsoil) and per unit of leaf area (ETla) depended on variation in leaf area index (LAI) and diurnal changes in environmental variables during the initial 6 weeks of CO2 treatment. CO2 enrichment reduced ETsoil and ETla, and together with air temperature and LAI accounted for most of the variance in daily values of evapotranspiration explained by multiple regression models. The CO2 effect on ETsoil did not depend on values of other variables, but CO2 enrichment reduced ETla most at relatively low air 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 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 evapotranspiration per unit of leaf area will vary with seasonal change in temperature and LAI, independent of seasonal shifts in leaf age and physiological activity.