Forest dieback in Europe: Climate drivers, symptoms, and physiological processes

In recent years climate-induced forest stress and dieback apparently have been increasing in Europe, from dry areas such as Mediterranean mountains to mesic locations such as the Swiss Alps. While precise statistics are lacking, the area of European forests recently affected and currently threatened by dieback is on the order of several million hectares. Two key drivers of climate-induced forest dieback are increased aridity (e.g., drought intensity and duration) and warmer temperatures, resulting in physiological stress that can exceed mortality thresholds for particular tree species. Climate change models predict increased aridity and temperatures across large areas of Europe in coming decades, which will likely aggravate forest stress and associated mortality. Once climate-triggered dieback starts, other factors such as insects or pathogens can amplify mortality, as seen with Pinus sylvestris forests in the Alps and Quercus forests on the Iberian Peninsula. Current research is described on physical and remote sensing indicators of tree physiological status and forest stand characteristics with respect to mortality risk. It includes efforts to determine stand structural thresholds of resistance whereby mortality processes could be minimized with appropriate management (e.g. thinning) and to develop early indicators of dieback processes. Monoculture reforestations using conifers that currently cover millions of hectares in several countries around the Mediterranean basin may be at particular risk of extensive dieback and provide opportunities to evaluate factors driving forest mortality risk. These plantations are being evaluated in terms of species, stand structures (e.g., stem densities), crown health, local site and climatic conditions, and management histories, to determine the abiotic and biotic conditions that affect forest dieback processes.