Biogeography of Stress: Graded and Threshold Phenological Responses in European Beech-Dominated Forests Under Disruptive Heatwaves.

Climate change is increasing the frequency and intensity of summer heatwaves, posing a major threat to temperate forests. European beech (Fagus sylvatica) forests are particularly sensitive to thermal extremes. Given their wide distribution, responses to heatwaves are not uniform, and it remains unclear how sensitivity varies at the continental scale. This study aims to quantify how summer disruptive heatwaves (dHWs) reshape phenological dynamics in European beech-dominated forests (BDFs) over 2003-2023 across four major biogeographical regions: Alpine, Atlantic, Continental and Mediterranean. By combining remotely sensed vegetation time series with machine-learning and anomaly detection approaches, we specifically (i) identified the occurrence of summer dHWs across European BDFs; (ii) quantified the impacts of dHWs both on summer canopy functioning (short-term effect) and on autumn senescence phenology (lagged effect), and (iii) evaluated these phenological responses under different dHW intensity levels. Results revealed that dHWs in BDFs consistently reduced summer canopy greenness and advanced autumn senescence, with distinct regional patterns. The most pronounced dHW impacts were observed in Continental and Atlantic regions. Graded, intensity-dependent responses emerged in Atlantic, Continental and Alpine regions, with the latter showing a comparatively buffered response, likely due to a cooler climate baseline and shorter vegetative phase. By contrast, the Mediterranean region exhibited a threshold-type response, reflecting adaptation mechanisms of southern-edge populations under already limiting summer conditions. Late-summer and early-autumn emerged as the most sensitive time windows for dHW-induced phenological shifts, with temporal patterns varying along the European BDF climatic gradient. Together, these results demonstrate that dHWs drive both immediate and legacy impacts on forest vegetative dynamics, reducing seasonal productivity and advancing the end of the growing season, with intensity effects modulated by regional climate and baseline phenology. Our findings underscore the need for region-specific strategies to safeguard forest ecosystems as disruptive heatwaves continue to intensify.