Recent radial growth decline in response to increased drought conditions in the northernmost Nothofagus populations from South America

An emerging phenomenon of forest decline in Mediterranean-type ecosystems has been detected in response to climate change during the last century. It is expected that the Mediterranean regions will likely experience drought events during this century with consequences for biodiversity maintenance. Although the Chilean Mediterranean-type forests are among the most threatened forest ecosystems in South America, their responses to recent increased drought events due to global warming are poorly documented. In the same region, the endangered and endemic forests of Nothofagus macrocarpa (Nothofagaceae) are found on mountain peaks. It is unclear how N. macrocarpa forests are responding to increased drought conditions occurring in the area over the last few decades. Here, we analyzed how recent climatic variability has affected the growth of N. macrocarpa. We selected five sites along the whole geographic distribution of N. macrocarpa forests in central Chile (32.5–34.5°S) to develop tree-ring chronologies. Climate-growth relationships were analyzed through correlations with local (precipitation, temperature and drought index) and large-scale climate data (ENSO index and Antarctic Oscillation). N. macrocarpa growth was positively influenced by May to November precipitation (austral winter-spring seasons) and negatively influenced by temperature from October to December (austral spring/early-summer seasons). Using a piecewise regression analysis, we identified a significant decrease in growth from 1980 onwards that resembled a precipitation decline and temperature increase in central Chile during the same time period. Tree-ring chronologies were positively correlated to the ENSO index and negatively correlated to the Antarctic Oscillation index during the current growing season, and more strongly from 1980 onwards. Based on our results, we conclude that increased drought conditions have produced a decline in radial growth of N. macrocarpa forests in the last decades. We propose that increased drought conditions predicted for this century in this region will exacerbate this declining N. macrocarpa growth trend with unknown consequences for the survival of these endemic and endangered forest ecosystems.