Mapping climate-related forest risks: An index-based approach

The environmental challenges faced by many tree species as a result of climate change show substantial spatial and temporal variation. One method to compile these variations is to construct a risk map combining various indexes representing hazards, exposures and vulnerabilities. In this study we combine climate envelopes with topographic and soil hydrological data to create a risk map over the territory of Germany using an index-based approach, for two different future climate scenarios (SSP245 and SSP585) and the four tree species Scots pine ( Pinus Sylvestris ), Norway Spruce ( Picea abies ), European beech ( Fagus sylvatica ) and Pedunculate oak ( Quecus robur ). By taking soil hydrological aspects into account, as well as extreme events via climate envelopes, we expand the application of climate envelopes thereby creating a new basis for risk analyses. Sensitivity and elasticity analyses of our risk model indicate that the upper climate envelope bounds for climate factors representing extreme heat events, exert the strongest influence on risk patterns across all species. We then identified pronounced spatial and species-specific variation in climate-related forest risk. Our results show a significant shift towards higher risk across all tree species with the SSP585 scenario assigning over 80% of locations for spruce and pine to a high risk category, with 75% and 65% for beech and oak respectively. The spatial analysis identifies low risk areas in specific river lowlands with high soil moisture capacity soils, but highlights that soil water availability has the potential to partially mitigate the effects of drought and rising temperatures. These findings, validated against historical NDVI data in the Harz region, suggest that current diversification strategies may be insufficient, as even deciduous species face substantial climate-envelope exceedance. Our findings therefore underscore the urgent need to re-assess tree species selection for diversification, to diversify forest locations, and expand nature restoration efforts beyond current targets.