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-fertilizationpce
We investigated how leaf hydraulic conductance (Kleaf) of loblolly pine trees is influenced by soil nitrogen amendment (N) in stands subjected to ambient or elevated CO2 concentrations CO2 a and CO2 e, respectively). We also examined how Kleaf varies with changes in reference leaf water potential (Yleaf-ref) and stomatal conductance (gs-ref) calculated at vapour pressure deficit, D of 1 kPa.We detected significant reductions in Kleaf caused by N and CO2e, but neither treatment affected pre-dawn or midday Yleaf.We also detected a significant CO2 e-induced reduction in gs-ref and Yleaf-ref. Among treatments, the sensitivity ofKleaf toYleaf was directly related to a reference Kleaf (Kleaf-ref computed at Yleaf-ref). This liquid-phase response was reflected in a similar gas-phase response, with gs sensitivity to D proportional to gs-ref. Because leaves represented a substantial component of the whole-tree conductance, reduction in Kleaf under CO2 e affected whole-tree water use by inducing a decline in gs-ref. The consequences of the acclimation of leaves to the treatments were: (1) trees growing under CO2 e controlled morning leaf water status less than CO2 a trees resulting in a higher diurnal loss of Kleaf; (2) the effect of CO2 e on gs-ref was manifested only during times of high soil moisture.