Ecophysiological Responses of Nothofagus obliqua Forests to Recent Climate Drying Across the Mediterranean‐Temperate Biome Transition in South‐Central Chile.

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°−40.3°S) using carbon isotope discrimination (Δ13 C) and intrinsic water use efficiency (iWUE) in tree rings during 1967–2017. Moreover, tree ring δ18O 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 13C, probably due to reduced photosynthetic rates associated with increasing light competition. The strongest associations between climate and Δ13C were found in the northernmost stand, suggesting that warmer and drier conditions could have reduced 13C discrimination. Tree growth in this site has not decreased, and δ18O was negatively related to annual rainfall. However, a shift in this relationship was found since 2007, when both precipitation and δ18O decreased, while correlations between δ18O and growth increased. This implies that tree growth and δ18O 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°−40.3°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. Key Points: Despite a common climate trend, the response of N. obliqua to climate change is not uniform, but rather specific to the stand and siteTree growth and δ18O have been coupled during the megadrought in the northernmost site, but rainfall does not influence this patternN. obliqua northern forests are able to maintain radial growth rates and acclimate to persistent droughts by tapping deeper water sources [ABSTRACT FROM AUTHOR]