Simulating drought-driven mortality of European Beech and Norway Spruce in German forests:insights on predisposing, inciting and contributing factors

Drought is increasingly recognized as a critical driver of forest dynamics, altering tree species' dominance, growth, and survival. To better understand these dynamics, we tested a recently developed predisposing-inciting (PI) framework for drought-related tree mortality within the forest gap model ForClim v4.1, focusing on two key European species: European beech (Fagus sylvatica) and Norway spruce (Picea abies). To better capture contributing factors of drought-related stress, we also developed a bark beetle module for Norway spruce to better account for interactions between abiotic and biotic stress factors.

Our study addressed three hypotheses:

• The PI framework remains effective across broader ecological and climatic ranges beyond its initial application.
• Soil water holding capacity (AWC) exerts a significant influence on drought-induced mortality, complementing climatic drivers.
• Reduced soil heterogeneity amplifies mortality risks by limiting microsite variability, thereby exacerbating drought stress.

We conducted simulations across hundreds of ICP-Level II sites in Germany, spanning diverse climate and soil gradients.

Results indicate that ForClim can reproduce general patterns of drought-induced mortality, though mismatches in magnitude and trends highlight areas for improvement. Discrepancies were attributed to sparse mortality data, the drought sensitivity of the bark beetle submodule, and limited regional calibration. 

Soil water availability emerged as a critical driver of drought resilience. Sites with low AWC experienced significantly elevated mortality rates, while high AWC provided a buffering effect, bringing modeled outcomes closer to observed data. Furthermore, soil heterogeneity played a mitigating role: sites with uniform soils exhibited higher mortality risks, thus emphasizing the importance of spatial variability in dampening drought impacts.

This study underscores the value of process-based models like ForClim in disentangling the mechanisms underlying forest vulnerability and drought-induced mortality. However, improvements such as finer-resolution mortality and crown condition data, as well as regional model calibration, are essential to enhance predictive accuracy. By advancing our understanding of drought-induced mortality, these findings contribute to better forecasting and management of forest resilience under current and future climate scenarios.