Abstract
We measured leaf‐level stomatal conductance, xylem pressure potential, and stomate number and size as well as whole plant sap flow and canopy‐level water vapour fluxes in a C4‐tallgrass prairie in Kansas exposed to ambient and elevated CO 2 . Stomatal conductance was reduced by as much as 50% under elevated CO 2 compared to ambient. In addition, there was a reduction in stomate number of the C4 grass, Andropogon gerardii Vitman, and the C3 dicot herb, Salvia pitcheri Torr., under elevated CO 2 compared to ambient. The result was an improved water status for plants exposed to elevated CO 2 which was reflected by a less negative xylem pressure potential compared to plants exposed to ambient CO 2 . Sap flow rates were 20 to 30% lower for plants exposed to elevated CO 2 than for those exposed to ambient CO 2 . At the canopy level, evapotranspiration was reduced by 22% under elevated CO 2 . The reduced water use by the plant canopy under elevated CO 2 extended the photosynthetically‐active period when water became limiting in the ecosystem. The result was an increased above‐ and belowground biomass production in years when water stress was frequent.