Wednesday, November 4, 2009: 2:15 PM
Convention Center, Room 325, Third Floor
Canopy stomatal conductance (GS) is perhaps the most important physiological property of a vegetated surface, because it regulates the rate of water consumption of plant communities and ultimately, land surface–atmosphere interactions. An energy balance method was devised to determine aerodynamic conductance, gA, of wheat in plant growth chambers. This method enabled the calculation of GS from gA and from canopy surface conductance (Gsurf) estimated from rates of canopy evapotranspiration. The effect of CO2 concentration on canopy transpiration (Tr) was studied by raising CO2 concentration of a vegetative wheat canopy in steps from 400 to 700 to 950 to 1200 mmol mol-1. Increasing CO2 concentration from 400 to 700 mmol mol-1 decreased mean daily GS by 23%, resulting in 23% lower transpiration rate, and a 25% reduction in latent heat. Increasing CO2 concentration from 400 to 1200 mmol mol-1 decreased mean daily GS by 62%, resulting in 48% lower transpiration rate, and a 40% reduction in latent heat (LE). The sensitivity of transpiration to changes in Gs, dTr/dGs, was reduced by 6%, 17%, and 33% when CO2 concentration increased from 400 to 700, 900, and 1200 mmol mol-1 CO2. Reductions in transpiration between 400 to 700 mmol mol-1 and between 400 to 900 mmol mol-1 correspond to decreases in latent heat of 115 W m-2 and 170 W m-2, respectively. The canopy becomes hotter as transpiration is reduced and the observed changes in sensible heat (H) as a fraction of the changes in LE (dH/dLE) increase from 0.60 at 700 mmol mol-1 to 0.86 at 900 mmol mol-1. These results suggest that CO2 enrichment reduces GS and reduces the degree of stomatal control of Tr.