Hydraulic adjustments underlying drought resistance of Pinus halepensis

Drought-induced tree mortality has increased over the last decades in forests around the globe. Our objective was to investigate under controlled conditions the hydraulic adjustments underlying the observed ability of Pinus halepensis to sur-vive seasonal drought under semi-arid conditions. One hundred 18-month saplings were exposed in the greenhouse to 10 different drought treatments, simulating combinations of intensities (fraction of water supply relative to control) and dura-tions (period with no water supply) for 30 weeks. Stomata closed at a leaf water potential (Ψl) of −2.8 MPa, suggesting iso-hydric stomatal regulation. In trees under extreme drought treatments, stomatal closure reduced CO2 uptake to −1 µmol m−2 s−1, indicating the development of carbon starvation. A narrow hydraulic safety margin of 0.3 MPa (from stomatal closure to 50% loss of hydraulic conductivity) was observed, indicating a strategy of maximization of CO2 uptake in trees otherwise adapted to water stress. A differential effect of drought intensity and duration was observed, and was explained by a strong depen-dence of the water stress effect on the ratio of transpiration to evapotranspiration T/ET and the larger partitioning to tran-spiration associated with larger irrigation doses. Under intense or prolonged drought, the root system became the main target for biomass accumulation, taking up to 100 % of the added biomass, while the stem tissue biomass decreased, associ-ated with up to 60 % reduction in xylem volume