Understanding Responses of Canopy Transpiration to Elevated CO2.

Canopy stomatal conductance (G_S) 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, g_A, of wheat in plant growth chambers. This method enabled the calculation of G_S from g_A and from canopy surface conductance (G_surf) 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 G_S 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 G_S 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 G_S and reduces the degree of stomatal control of Tr.