Trends in background mortality in unmanaged forests across Europe over the last century

Anthropogenic climate change has led to increasing background tree mortality rates worldwide. Tree species have different ways of dealing with changing environmental conditions due to their life-history strategies and location within their ecological niche. Trees growing further from the centre of their niche are likely to experience higher levels of climatic stress and potentially higher mortality, whereas trees growing closer to the niche centre may experience higher intraspecific competition. To study the complex interplay of abiotic and biotic factors leading to tree mortality, we used a comprehensive network of permanent plots with repeated censuses spanning from 1936 to 2020 in 299 unmanaged forest reserves across Europe. The database includes 1.5 million stem records covering a total sampled area of 853.7 ha. We (1) calculated background mortality rates, (2) analysed trends in mortality rates and (3) investigated how the trends relate to the location of each tree within its ecological niche. We used Species Distribution Models (SDMs) to locate trees within their niche and generalized linear mixed models (GLMMs) to model mortality. We observed an overall median annual background mortality rate of 1.1%. Spruce-dominated forests had the largest increase in annual mortality from 1.5% to 3%. Similarly, the models showed a significant increase in oak-dominated forests, whereas beech-dominated forests showed a significant decrease in annual mortality rates over time. We identified three distinct mortality responses with respect to the location of trees in their ecological niche. For oak, spruce and mountain pine, higher mortality probabilities occurred when they were located further from their niche centre. Hornbeam, ash and Scots pine showed the opposite pattern and beech and fir featured an almost negligible effect of distance from the niche centre. Synthesis. In contrast to previous studies, our results suggest that dominant tree species in European natural forests have not consistently experienced increasing background mortality under anthropogenic climate change. Conversely, forest reserves dominated by competitive species such as beech may have benefitted from warmer growing conditions. Most studied tree species show no clear effect of distance from the niche centre, indicating high resilience and adaptability to environmental stress.