Comparing absolute and standardized drought indices for modelling tree mortality of spruce, beech, pine, and oak based on the Crown Condition Survey in Germany

Increasing drought frequency and intensity driven by climate change have severely impacted tree vitality and contributed to rising mortality across Europe. To quantify and compare the intensity and regional extent of droughts, a variety of indices have been developed. Consequently, different methodological approaches are currently used in research to explain drought-induced tree mortality. The aim of this study was to evaluate the extent to which aridity (Climatic Water Balance, CWB; Site Water Balance, SWB) and drought indices (Standardized Precipitation Evapotranspiration Index , SPEI; CWB anomalies) were associated with tree mortality. The study used data from the German Crown Condition Survey (1990–2022) to derive mortality rates for four major tree species (Norway spruce, European beech, Scots pine, and oak). Using correlation analyses and generalized additive models (GAMs), we retrospectively quantified how aridity and drought indices, particularly the SPEI, explained tree mortality by comparing modelled mortality probabilities with observed rates. The results revealed distinct species-specific responses. For spruce, beech, and pine, SPEI and CWB anomalies showed the strongest links to mortality, with effects increasing over longer aggregation periods of up to five years. For oak, which exhibited lower mortality rates, the SPEI-based models did not adequately explain mortality, indicating the need for additional predictors. Overall, the results indicated that in Germany the mortality of the most common tree species was better described by relative deviations from long-term averages than by absolute aridity measures. The study shows that the SPEI is a robust indicator for predicting drought-related mortality across dominant tree species in Germany and contributes to improved tree species selection under climate change. • Tree mortality in Germany is better predicted by relative deviations from climatic long-term averages than by absolute aridity measures. • Standardized drought indices (SPEI) outperform aridity indices (CWB, SWB) in capturing mortality events. • SPEI robustly predicts drought-related mortality for Norway spruce and European beech. • Longer aggregations (up to 5 years) improve model performance. • Species-specific drought responses cause varying model performance across indices.