The effects of atmospheric water demand, water availability, and exposure on the drought response of Swiss temperate forests

Projected increases in drought frequency and strength in Central Europe in the next two decades due to anthropogenic climate change pose challenges for European temperate forests. Understanding the correlation between drought stress, local conditions, and forest responses is crucial for effective forest management and climate mitigation measures. We examine how local water dynamics determine the response of Swiss forests during the European drought in 2018. We particularly investigate how increased atmospheric water demand, reduced soil water availability, and increased exposure of forests to potentially harsh abiotic conditions at the edge of the forest affect forest health. We used Sentinel-2 data to calculate the normalized difference water index (NDWI) as proxy for forest health. Weather data, data from a process based hydrological model, a digital elevation model, and airborne LiDAR data were used to assess hydrological drivers. Our analysis revealed that forest exposure and water availability were more important than atmospheric water demand in explaining forest drought resistance. Regions with more limited water availability (47 % of Switzerland) had systematically higher proportions of forest areas that exhibited weak drought resistance (R² = 0.56 for moderate NDWI decrease and 0.55 for severe NDWI decrease). Forest exposure (i.e. the degree to which a forest patch stands out from the surroundings) could best explain weak drought resistance, with strong statistical relationships (R² = 0.69, R² = 0.50). Finally, atmospheric water demand had only a moderate effect on weak drought resistance (R² = 0.45 and 0.27). Our findings highlight the complex interplay of local water dynamics and forest responses to drought, while providing insights on how forest structure and exposure conditions at local scales affect responses and need to be considered when examining forest health under changing climatic conditions.