Drivers of background mortality in European mixed mountain forests of Picea abies (L.) Karst., Abies alba Mill., and Fagus sylvatica L

Mixed mountain forests of Norway spruce (Picea abies (L.) Karst.), silver fir (Abies alba Mill.), and European beech (Fagus sylvatica L.) are ecologically important across Europe, providing ecosystem services, supporting biodiversity, and contributing to the bioeconomy as crucial timber sources. Understanding background mortality in these forests is essential to distinguish the effects of this relatively continuous endogenous process from those induced by exogenous disturbances, and to inform sustainable management under changing conditions. To assess how stand density, tree-size dominance, species competition and diversity, site geomorphology, and climate influence background mortality, expressed as the annualised basal area loss rate, we applied generalised linear mixed models. We used tree measurements from 78 plots located in Bulgaria, Bosnia and Herzegovina, Slovenia, Slovakia, Poland, Germany, and Switzerland, inventoried from 1912 to 2016, with most of them first surveyed between 1953 and 1964. At the stand level, mortality increased with stand basal area, greater species diversity, and the interaction between fir dominance and drought. Spruce mortality increased with stand density, species diversity, and fir dominance, indicating a higher probability of spruce mortality under fir dominance pressure. Fir mortality increased with species diversity and mean annual temperature, revealing fir as sensitive to rising temperatures, and with the interaction between fir dominance and drought, with moisture effects varying along the fir dominance gradient. Beech mortality increased with stand density and fir dominance, suggesting that beech suffers more when fir occupies a dominant canopy position. These findings suggest that sustainable management of mixed mountain forests requires targeted silvicultural interventions to regulate stand density, manage species diversity, and limit fir size dominance.