Linking the single-leaf CO(2) effect on stomatal conductance to regional scale evapotranspiration

Research in plant physiology has demonstrated that in many species the stomatal aperture is reduced when exposed to elevated CO{dollar}sb2.{dollar} In these small scale measurements there is also a large corresponding decrease in transpiration which suggests that water use may decrease as atmospheric CO{dollar}sb2{dollar} concentrations continue to rise. However, single leaf measurements performed in gas exchange systems are not easily extrapolated to larger scales. Plant communities contain many additional complexities and interactions that are not evident at the measurement scale. Therefore, there is a need to examine how the additional complexity of natural and agricultural ecosystems interacts with increasing CO{dollar}sb2{dollar} and evapotranspiration.; Two crop species (soybeans and corn) were grown and measured outdoors at three CO{dollar}sb2{dollar} concentrations to evaluate the effect of CO{dollar}sb2{dollar} on stomatal conductance under a range of environmental conditions. Stomatal conductance decreased in response to CO{dollar}sb2{dollar} in both soybeans and corn. Several approaches are considered to represent these stomatal responses in a model.; The additional feedbacks involved with scaling single leaf stomatal conductance to larger scales are shown theoretically with numerical simulations, primarily based on the big-leaf energy balance equations. A number of theoretical feedbacks between the plant canopy and atmosphere are discussed, many of which have not been reported, which alter the simulated sensitivity of evapotranspiration to changes in stomatal conductance (or CO{dollar}sb2).{dollar} Surface layer and mixed layer processes, stomatal response to environmental variables other than CO{dollar}sb2{dollar} and soil evaporation are all shown to influence this sensitivity. Generally these feedbacks reduce the sensitivity of evapotranspiration to changes in stomatal conductance.; More realistic simulations are performed over a growing season to estimate more directly the integrative effect of CO{dollar}sb2.{dollar} Plant growth, soil hydrology, and surface fluxes are all represented over a growing season using the soybean and corn gas exchange data. The simulated reduction in evapotranspiration over the entire growing season for a doubling of CO{dollar}sb2{dollar} was 6.3% (soybean) and 8.9% (corn), much less than the direct change in stomatal conductance with CO{dollar}sb2{dollar} (52% and 59%). A system is developed to analyze the magnitude of individual feedbacks in these estimates.