CARBON-DIOXIDE EFFECTS ON CROP ENERGY-BALANCE - TESTING ECOSYS WITH A FREE-AIR CO2 ENRICHMENT (FACE) EXPERIMENT

Elevated CO2 concentrations (C-e) have been observed to decrease short-term plant water use under controlled conditions by increasing stomatal resistance. The extent to which this decrease occurs over a growing season in the held is uncertain, however, because stomatal resistance is only one of many mechanisms that control water use. In this study, we tested the ecosystem simulation model ecosys, which reproduces an hourly energy balance through soil-vegetation systems under defined atmospheric boundary renditions, using energy exchange data measured as part of the Free-Air CO2 Enrichment (FACE) experiment at C-e = 550 vs. 370 mu mol mol(-1). The model reproduced reductions in measured upward latent heat fluxes that varied from -10 to +40 W m(-2), depending on atmospheric conditions. In the model, the primary effect of elevated C-e on latent heat fluxes was through canopy stomatal conductance. This effect was largely offset by secondary effects through canopy temperature that enabled the model to reproduce measured changes in sensible heat fluxes. The total effect simulated by ecosys of C-e = 550 vs. 370 mu mol mol(-1) on evapotranspiration during the entire PACE experiment was a reduction of 7%. This reduction compares with one of 11% estimated from accumulated daily measurements of latent heat flux. In the model, the different effects of C-e on plant water use depend on atmosphere and soil boundary conditions, and are highly dynamic. Consequently the simulated C-e-water use relationship is likely to be site-specific. The use of models such as ecosys allows site-specific boundary conditions to be considered in the study of C-e effects on plant growth and water use.