Abstract

The forests of south-central Chile are facing a drying climate and a megadrought that started in 2010. This study addressed the physiological responses of five Nothofagus obliqua stands across the Mediterranean-Temperate gradient (35.9 degrees-40.3 degrees S) using carbon isotope discrimination (Delta C-13) and intrinsic water use efficiency (iWUE) in tree rings during 1967-2017. Moreover, tree ring delta O-18 was evaluated in the northernmost site to better understand the effects of the megadrought in this drier location. These forests have become more efficient in their use of water. However, trees from the densest stand are discriminating more against C-13, probably due to reduced photosynthetic rates associated with increasing light competition. The strongest associations between climate and Delta C-13 were found in the northernmost stand, suggesting that warmer and drier conditions could have reduced C-13 discrimination. Tree growth in this site has not decreased, and delta O-18 was negatively related to annual rainfall. However, a shift in this relationship was found since 2007, when both precipitation and delta O-18 decreased, while correlations between delta O-18 and growth increased. This implies that tree growth and delta O-18 are coupled in recent years, but precipitation is not the cause, suggesting that trees probably changed their water source to deeper and more depleted pools. Our research demonstrates that forests are not reducing their growth in central Chile, mainly due to a shift toward the use of deeper water sources. Despite a common climate trend across the gradient, there is a non-uniform response of N. obliqua forests to climate drying, being their response site-specific. Plain Language Summary The forests of south-central Chile are facing a drying climate and a megadrought that started in 2010. This study addressed the physiological responses of five Nothofagus obliqua stands across the Mediterranean-Temperate gradient (35.9 degrees-40.3 degrees S) to climate drying using stable isotopes in tree rings during the period 1967-2017. These forests have become more efficient in their use of water. However, trees from the densest stand are showing a poor adaptability to climate change, probably due to a decrease in photosynthetic rates associated with the increase in competition. Our research demonstrates for the first time that forests are not reducing their growth in central Chile, mainly due to a shift toward the use of deeper water sources. However, it is not certain until which point this acclimation will persist as these pools get depleted in the future with continued drying. Despite a common climate trend across the latitudinal gradient, this research points to a non-uniform response of N. obliqua second-growth forests to a drying climate, being their response stand and site-specific.