Leaf d 18 O of remaining trees is affected by thinning intensity in a semiarid pine forestpce

Silvicultural thinning usually improves the water status of remaining trees in water-limited forests. We evaluated the usefulness of a dual stable isotope approach (d 13C, d 18O) for comparing the physiological performance of remaining trees between forest stands subjected to two different thinning intensities (moderate versus heavy) in a 60-yearold Pinus halepensis Mill. plantation in semiarid southeastern Spain. We measured bulk leaf d 13C and d 18O, foliar elemental concentrations, stem water content, stem water d 18O (d Ostem water), tree ring widths and leaf gas exchange rates to assess the influence of forest stand density on tree performance. Remaining trees in low-density stands (heavily thinned) showed lower leaf d 18O, and higher stomatal conductance (gs), photosynthetic rate and radial growth than those in moderate-density stands (moderately thinned). By contrast, leaf d 13C, intrinsic water-use efficiency, foliar elemental concentrations and d Ostem water were unaffected by stand density. Lower foliar d 18O in heavily thinned stands reflected higher gs of remaining trees due to decreased inter-tree competition for water, whereas higher photosynthetic rate was largely attributable to reduced stomatal limitation to CO2 uptake. The dual isotope approach provided insight into the early (12 months) effects of stand density manipulation on the physiological performance of remaining trees. Key-words: Pinus halepensis; competition; drought stress; dual isotope approach; oxygen isotopic composition; stomatal conductance.