Interactive effects of elevated CO2 and temperature on water transport inponderosa pine.

Many studies report that water flux through trees declines in response to elevated CO(2), but this response may be modified by exposure to increased temperatures. To determine whether elevated CO(2) and temperature interact to affect hydraulic conductivity, we grew ponderosa pine seedlings for 24 wk in growth chambers with one of four atmospheric CO(2) concentrations (350, 550, 750, and 1100 ppm) and either a low (15°C nights, 25°C days) or high (20°C nights, 30°C days) temperature treatment. Vapor pressure deficits were also higher in the elevated temperature treatment. Seedling biomass increased with CO(2) concentration but was not affected by temperature. Root : shoot ratio was unaffected by CO(2) and temperature. Leaf : sapwood area ratio (A(L)/A(S)) declined in response to elevated temperature but was not influenced by CO(2). Larger tracheid diameters at elevated temperature caused an increase in xylem-specific hydraulic conductivity (K(S)). The increase in K(S) and decrease in A(L)/A(S) led to higher leaf-specific hydraulic conductivity (K(L)) at elevated temperature. Stomatal conductance (g(S)) was correlated with K(L) across all treatments. Neither K(S), K(L), nor g(S) were affected by elevated CO(2) concentrations. High K(L) in response to elevated temperature may support increased transpiration or reduce the incidence of xylem cavitation in ponderosa pine in future, warmer climates.