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_2014

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 and CO2, 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 CO2, but neither treatment affected pre-dawn or midday Yleaf. We also detected a significant CO2-induced reduction in gs-ref and Yleaf-ref. Among treatments, the sensitivity of Kleaf 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 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 controlled morning leaf water status less than CO2 trees resulting in a higher diurnal loss of Kleaf; (2) the effect of CO2 on gs-ref was manifested only during times of high soil moisture. Key-words: capacitance; embolism; reference conductances; reference leaf water potential; time constant; turgor loss point.