Management alters drought-induced mortality patterns in European beech (Fagus sylvatica L.) forests

The high tree mortality during the dry and hot years of 2018-2019 in Europe has triggered concerns on the future of European beech (Fagus sylvatica L.) forests under climate change and raised questions as to whether forest management may increase tree mortality. We compared long-term mortality rates of beech between managed and unmanaged stands including the years 2018-2019 at 11 sites in Hesse, Germany. We hypothesized that mortality would increase with climate water deficits during the growing season, initial stand density, decreasing dominance of trees, and decreasing intensity of tree removals. Initial stand density, tree removals, the climate water balance and the competitive status of trees were used as predictor variables. Mean annual natural mortality rates ranged between 0.5% and 2.1%. Even in the drought years, we observed no signs of striking canopy disintegration. The significantly higher mortality (1.6-2.1%) in unmanaged stands during the drought years 2018 and 2019 was largely confined to suppressed trees. There was no significant increase of mortality in managed stands during the drought years, but a shift in mortality towards larger canopy trees. Our study did not confirm a general influence of management, in the form of tree removals, on mortality rates. Yet, we found that during drought years, management changed the distribution of mortality within the tree community. To analyse the effects of management on mortality rates more comprehensively, a wider gradient in site moisture conditions, including sites drier than in this study, and longer post-drought periods should be employed.