Drivers of individual-tree mortality in Picea abies and Pinus sylvestris: A large-scale analysis across Austria

Understanding the drivers of tree mortality is essential for developing proactive management strategies to mitigate forest decline under climate change. However, empirical individual-tree mortality models parameterized with long-term inventory and climatic data remain scarce. We applied Generalized Additive Mixed Models (GAMMs) to Austrian Forest Inventory data spanning a 40-year period (1981–2021) to evaluate mortality dynamics for Norway spruce ( Picea abies ) and Scots pine ( Pinus sylvestris ). Results reveal distinct, species-specific vulnerabilities to stand structure and climatic stressors. Both species are strongly driven by diameter at breast height (DBH), basal area of larger trees (BAL), and warm spring mean temperatures. Spruce is additionally sensitive to high summer maximum temperatures and exceptionally wet, stagnant summer conditions. Vapour pressure deficits (VPD) interacted with soil groups show no effect on either species, except for spruce on planosols and stagnic gleysols on loess. Notably, spruce benefits from warmer long- baseline temperatures across montane and lowland elevation zones—indicating vulnerability to weather volatility rather than thermal baseline warming—whereas Scots pine crosses its physiological threshold in chronically warmer lowlands. Admixtures also drive patterns: spruce mortality probability increase when mixed with European beech ( Fagus sylvatica ) and decrease when mixed with light-demanding conifers, whereas pine vulnerability rises when mixed with broadleaves or spruce. This divergence underscores the superior resilience of Scots pine to episodic climatic shocks, contrasted against a precarious state for Norway spruce, where minor climatic deviations trigger substantial mortality.