Tree-ring stable isotopes reveal twentieth-century increases in water-use efficiency of Fagus sylvatica and Nothofagus spp. in Italian and Chilean mountains

Changes in intrinsic water use efficiency (iWUE) were investigated in Fagus sylvatica and Nothofagus spp. over the lastcentury. We combined dendrochronological methods with dual-isotope analysis to investigate whether atmosphericchanges enhanced iWUE of Fagus and Nothofagus and tree growth (basal area increment, BAI) along latitudinal gradients inItaly and Chile. Post-maturation phases of the trees presented different patterns in d13C, D13C, d18O, Ci (internal CO2concentration), iWUE, and BAI. A continuous enhancement in isotope-derived iWUE was observed throughout the twentiethcentury, which was common to all sites and related to changes in Ca (ambient CO2 concentration) and secondarily toincreases in temperature. In contrast to other studies, we observed a general increasing trend of BAI, with the exception ofF. sylvatica in Aspromonte. Both iWUE and BAI were uncoupled with the estimated drought index, which is in agreementwith the absence of enduring decline in tree growth. In general, d13C and d18O showed a weak relationship, suggesting themajor influence of photosynthetic rate on Ci and d13C, and the minor contribution of the regulation of stomatalconductance to iWUE. The substantial warming observed during the twentieth century did not result in a clear pattern ofincreased drought stress along these latitudinal transects, because of the variability in temporal trends of precipitation andin specific responses of populations.