Interactive effects of elevated CO2 and temperature on water transport in ponderosa pine

Many studies report that water flux through trees declines in response to elevated CO2, bur this response may be modified by exposure to increased temperatures. To determine whether elevated CO2 and temperature interact to affect hydraulic conductivity, we grew ponderosa pine seedlings for 24 wk in growth chambers with one of four atmospheric CO2 concentrations (350, 550, 750, and 1100 ppm) and either a low (15 degrees C nights, 25 degrees C days) or high (20 degrees C nights, 30 degrees C days) temperature treatment. Vapor pressure deficits were also higher in the elevated temperature treatment. Seedling biomass increased with CO2 concentration but was not affected by temperature. Root shoot ratio was unaffected by CO2 and temperature. Leaf sapwood area ratio (A(L)/A(S)) declined in response to elevated temperature bur was not influenced by CO2. 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 CO2 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.