Influence of multiple biotic and abiotic factors on the crown die-back of European beech trees at their drought limit

•Increasing soil water storage capacity and light reduce crown dieback in beech trees.•Crown dieback is reduced by tree species diversity and neighbourhood interactions.•The upper part of tree crown is more vulnerable to soil drought.•Beech trees die permanently after crossing a threshold of 58% crown dieback.

Owing to the growing sensitivity of forests to drought under the warming climate, more attention should be paid to the role of soil drought, plant-plant interactions, tree species and structural diversity, and other abiotic factors on the crown die-back of trees. We studied how permanent soil water stress had impacted on crown die-back of beech trees (Fagus sylvatica L.) at their drought limit in near-natural temperate forests of Germany and Switzerland. Crown die-back was quantified by the proportion of dead above-ground biomass to total biomass for the individual beech tree. We quantified the available soil water storage capacity as a measure of soil drought stress in combination with other biotic (e.g., plant-plant interactions, tree species diversity, stand structural diversity, plant height and proportion of oak trees) and abiotic (e.g., light availability, soil pH, soil bulk density, potential evapotranspiration) factors to determine the influence of those stressors on crown die-back. We found increases in soil water storage capacity, neighbourhood interactions, plant height and light decreased crown die-back of the beech trees. These stressors differently influenced the die-back among different parts of tree crown. Soil water storage capacity, light availability, intra- and interspecific interactions, plant height and tree species diversity had the strongest influence on die-back in the upper part of the crown. Permanent death of beech trees occurred when the amount of dead above-ground biomass exceeded a 58% mortality threshold. We conclude that vitality of beech trees in drought stressed forests is related to soil water storage capacity, light availability, neighbourhood interactions and tree species diversity.