Spatial decoupling of drought exposure and growth loss in German forests during the 2018–2022 drought sequence

Abstract. The 2018–2022 drought sequence coincided with widespread forest stress in Central Europe, but tree-growth responses differed markedly among species and hydroclimatic settings. Here, we analyzed radial-growth responses along a west-east transect across Germany using a multi-site tree-ring network spanning five common forest species: European beech (Fagus sylvatica), pedunculate oak (Quercus robur), sessile oak (Quercus petraea), Scots pine (Pinus sylvestris), and Douglas fir (Pseudotsuga menziesii). We assessed drought severity and duration using the Standardized Precipitation–Evapotranspiration Index (SPEI, 1951–2024) and estimated growth anomalies relative to the baseline of expected growth under neutral moisture conditions derived from hierarchical generalized additive mixed models.

The 2018–2022 period was the most severe multi-year drought event across the sampled regions at longer accumulation scales (SPEI-24 and SPEI-48), exhibiting exceptional persistence in severity and temporal extent. Growth losses peaked in 2019 for four of the five species, with annual anomalies in that year ranging from -21 to -42 %. Douglas fir responded already in 2018 and reached its strongest single-year deficit of the recent sequence in 2022 (≈-66 %). When integrated over five years, the recent sequence produced some of the strongest sustained growth deficits in the study period for most species within the sampled stands. Analysis across individual drought events suggests additional growth suppression at a given severity in beech and Douglas fir and less suppression in sessile oak during recent events.

Despite the strong regional synchrony of the climatic driver, growth responses remained highly heterogeneous along the transect. Across all species, growth anomalies were less spatially coherent than the climatic driver, and this decoupling emerged even between neighboring stands under near-identical forcing. These results show that a climatically unprecedented, coherent drought produced growth impacts that were neither uniform nor predictable from regional water-balance severity, so that translating regional severity into stand-level growth losses requires additional information on local water availability and site context.