Acclimation of leaf hydraulic conductance and stomatal conductance of Pinus taeda (loblolly pine) to long-term growth in elevated CO2 (free-air CO2 enrichment) and N-fertilization

We investigated how leaf hydraulic conductance (K-leaf) of loblolly pine trees is influenced by soil nitrogen amendment (N) in stands subjected to ambient or elevated CO2 concentrations (CO(2)a and CO(2)e, respectively). We also examined how K-leaf varies with changes in reference leaf water potential (Psi(leaf-ref)) and stomatal conductance (g(s-ref)) calculated at vapour pressure deficit, D of 1 kPa. We detected significant reductions in K-leaf caused by N and CO(2)e, but neither treatment affected pre-dawn or midday Psi(leaf). We also detected a significant CO(2)e-induced reduction in g(s-ref) and Psi(leaf-ref). Among treatments, the sensitivity of K-leaf to Psi(leaf) was directly related to a reference K-leaf (Kleaf-ref computed at Psi(leaf-ref)). This liquid-phase response was reflected in a similar gas-phase response, with g(s) sensitivity to D proportional to g(s-ref). Because leaves represented a substantial component of the whole-tree conductance, reduction in K-leaf under CO(2)e affected whole-tree water use by inducing a decline in g(s-ref). The consequences of the acclimation of leaves to the treatments were: (1) trees growing under CO(2)e controlled morning leaf water status less than CO(2)a trees resulting in a higher diurnal loss of K-leaf; (2) the effect of CO(2)e on g(s-ref) was manifested only during times of high soil moisture.