Effects of Extreme Drought Years on Radial Growth of Oak and Beech on the Swabian Alb, Germany

Climate change is increasing drought frequency and severity in Central Europe, with consequences for the productivity and stability of mixed broadleaf forests. We analyzed annual radial growth of European beech (Fagus sylvatica L.) and sessile oak (Quercus petraea (Matt.) Liebl.) along a water-availability gradient on the Swabian Alb (southwestern Germany). At three nearby sites, 45 trees per species were sampled. Tree-ring chronologies were related to meteorological variables and integrated drought indices (PDSI/scPDSI, SMI). Extreme dry and wet years were identified from detrended growing-season indices, and event responses were quantified using resistance, recovery, and resilience metrics and their humid-year analogues. Annual growth variation was primarily site-driven rather than species-driven, highlighting strong control by local soil water availability. Correlation analyses showed that integrated drought and soil-moisture indices explained radial growth more consistently than single climate variables. Oak showed higher drought resistance than beech in several events, although the magnitude and significance of this difference depended on site and drought year. Resistance generally declined toward recent events, particularly at Site 2, while recovery tended to increase. Recovery did not consistently favor either species, and resilience remained broadly stable, except for a decline in beech during the most recent drought events. Overall, sensitivity to drought is jointly controlled by species identity and fine-scale site conditions: oak has a comparatively robust resistance advantage, whereas post-drought recovery and resilience are strongly context-dependent. These findings support site-specific species mixtures and silvicultural strategies to sustain forest function under increasing drought risk.