Long-term forest monitoring reveals constant mortality rise in European forests

European forests are an important source for timber production, human welfare, income, protection and biodiversity. During the last two decades, Europe has experienced a number of droughts which were exceptionally within the last 500 years both in terms of duration and intensity. These droughts seem to left remarkable imprints on the mortality dynamics of European forests. However, systematic observations on tree decline with emphasis on single species has been scarce so far so that our understanding of mortality dynamics and drought occurrence is still limited at continental scale. Here we make use of the ICP Forest crown defoliation dataset, permitting us to retrospectively monitor tree mortality for all major conifers, major broadleaves as well as a pooled dataset of minor tree species in Europe. In total, we analysed more than 3 million observations gathered during the last 25 years and employed a high-resolution drought index which is able to assess soil moisture anomaly based on a hydrological water-balance and runoff model. We found overall and species-specific increasing trends in mortality rates accompanied by decreasing soil moisture. A generalized linear mixed model identified previous-year soil moisture anomaly as the most important driver of mortality patterns in conifers, but the response was not uniform across the numerous analysed plots. We conclude that mortality patterns in European forests are currently reaching a concerning upward trend which could be further accelerated by global change-type droughts in the near future.