Drivers of drought-induced canopy mortality in conifer and broadleaf forests across Luxembourg

Climate change-induced weather extremes are increasing the intensity and frequency of disturbance events, posing a major threat to forests globally. In Central Europe, hotter and drier summers, such as those during the 2018–2020 drought period, have led to widespread forest damage. To adapt forests to a hotter and drier future it is important to identify sites more vulnerable to canopy mortality during drought, but high-resolution tree mortality data across continuous landscapes are still sparse.

This study addresses this research gap by utilising a high-resolution (single-tree-level), spatially continuous dataset covering the entire Central European country of Luxembourg. We used generalized additive models (GAMs) to explore the contribution of biotic and abiotic drivers to the observed canopy mortality of conifer and broadleaf forests during the 2018–2020 summer droughts. Specifically, we assessed drivers related to forest structure, mortality clustering and spread, and topographic conditions to understand how they relate to spatial variability in canopy mortality.

In total, the model explained 44.7 % of spatial variation in canopy mortality in conifers and 25.3 % in broadleaf forests. ... mehrEnvironmental drivers related to clustered mortality, showed the strongest influence on canopy mortality in conifer forests, indicating that bark beetle infestation shaped spatial mortality patterns under drought. Forest height also emerged as an important driver associated with increased canopy mortality in both forest types. In contrast, topography- and soil-related drivers showed only limited influence on canopy mortality. Our study highlights the potential of using high-resolution canopy mortality data across a national-scale landscape to unravel how abiotic and biotic processes shape spatial variation in forest mortality under severe drought conditions.