Drought tolerance and mortality risk of European beech

Little is known about the drought-induced mortality risk for most tree species. This makes it difficult to predict their future distribution. Here, we quantified the drought tolerance and mortality risk of the economically important European beech ( Fagus sylvatica L.) by analysing its response to decreasing water potentials in the soil-plant-atmosphere continuum. From 2014 to 2023, we measured ecohydrological parameters at eight forest sites covering a large part of the species' ecological niche. The hourly time series at all sites included temperature, vapor pressure deficit (VPD), and soil water potential (Ψ soil ). At one site, the time series also included soil hydraulic conductivity (K), root water uptake (RWU), changes in stem radius, and dendrometer-derived leaf water potential (Ψ leaf ). Additionally, we determined the annual mortality rate of beech trees at all sites. At sites with a positive climatic water balance, beech mortality only occurred in soils with a low water storage capacity. However, at sites with a negative climatic water balance, beech trees also died in soils with high storage capacities. Mortality dynamics indicated that Ψ soil was more important for beech decline than VPD, heat, and excessive radiation. An average Ψ soil of −0.8 MPa in the root zone was identified as the mortality threshold for all sites, as beyond this threshold, RWU ceased due to extremely low K. Consequently, tree water reserves were largely depleted, and the percent loss of xylem conductivity exceeded 50% induced by midday Ψ leaf values as low as −3.5 MPa. Beyond the mortality threshold, annual mortality rates increased with the duration of drought. Furthermore, our study shows that the recent increase in beech mortality is induced by increased atmospheric drought, earlier leaf sprouting, and incomplete soil water replenishment during winter. These factors have resulted in more frequent soil drying and threshold exceedances in recent years.