Similar patterns of background mortality across Europe are mostly driven by drought in European beech and a combination of drought and competition in Scots pine

Background tree mortality is a complex demographic process that affects structure and long-term forest dynamics. Here we investigated how climatic drought intensity interacts with interspecific and intraspecific competition (or facilitation) in shaping mortality patterns across tree species ranges. To this aim, we used data from five European national forest inventories to perform logistic regression models based on individual tree mortality in Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.). We computed the relative importance of climatic drought intensity, basal area of conspecific and heterospecific trees (proxy of indirect intra- and interspecific competition or facilitation) and the effects of their interactions on mortality along the entire European latitudinal gradient of both species range. Increase in climatic drought intensity over the study period was associated with higher mortality rates in both species. Climatic drought intensity was the most important driver of beech mortality at almost all latitudes while Scots pine mortality was mainly driven by basal area. High conspecific basal area was associated with high mortality rates in both species while high heterospecific basal area was correlated with mortality rates that were high in Scots pine but low in beech. Overall, beech mortality was directly affected by climatic drought intensity while Scots pine mortality was indirectly affected by climatic drought intensity through interactions with basal area. Despite their different sensitivity to drought and basal area, the highest predicted mortality rates for both species were at the ecotone between the cool temperate and Mediterranean biomes, which can be explained by the combined effect of climatic drought intensity and competition. In the context of global warming, which is expected to be particularly strong in the Mediterranean biome, our results suggest that populations at the southern limit of species ranges may experience increased mortality rates in the near future.