Simultaneous assessment, through sap flow and stable isotopes, of water use efficiency (WUE) in thinned pines shows improvement in growth, tree-climate sensitivity and WUE, but not in WUEi

In water-limited regions, adaptive management of forest and water relationships has been put forward, to implement hydrology-oriented silviculture to reduce stand evapotranspiration and, at the tree level, to improve growth and water use efficiency (WUE). The main goal of this study was to evaluate the effect of thinning in the short and medium term on tree growth, climate (drought) sensitivity, WUE performed using growth and sap flow measurements and WUEi performed using δ13C and δ18O isotopes, in a typical semiarid forest. This approach also evaluated the reliability of isotopes as indicators of the effects of adaptive forest management. A stagnated Aleppo pine plantation was experimentally thinned at high intensity (H98) in 1998 and at High (H), Medium (M) and Low (L) intensities in 2008, along with a control (C). Substantial limitation of tree growth was observed in C. Thinning not only increased growth, but also changed the tree growth-precipitation relationships, with C trees depending more on precipitation than thinned trees did. WUEi after thinning was significantly affected only in the medium term, with C trees being more efficient (94.4 μmolCO2/molH2O) than H98 trees (88.7), especially in dry spells (100.7). WUEi was found to increase when precipitation decreased, regardless of the treatment. However, WUE increased sharply from C (1.26 g biomass/L H2O) to H (3.20 g/L), showing a clear difference with WUEi observed in the same years. Thinning caused an increase in δ18O in the short term, but no relationship was found between δ18O and tree water use. It can be concluded that forest management improved WUE in spite of higher tree transpiration, but WUEi remained unchanged, probably due to an underestimate of photosynthetic capacity. The dual isotope (δ13C and δ18O) conceptual model was not consistent with our experimental data. Thus, the question of whether stable isotopes can be used as a tool for addressing the ecophysiological impacts of thinning remains open.