Acclimation of leaf hydraulic conductance and stomatal conductance of Pinus taeda (loblolly pine) to long‐term growth in elevated CO 2 (free‐air CO 2 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 CO 2 concentrations (CO 2 a and CO 2 e , respectively). We also examined how K leaf varies with changes in reference leaf water potential ( Ψ 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 Ψ leaf . We also detected a significant CO 2 e ‐induced reduction in g s‐ref and Ψ leaf‐ref . Among treatments, the sensitivity of K leaf to Ψ leaf was directly related to a reference K leaf ( K leaf‐ref computed at Ψ 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.