Early leaf senescence reveals drought stress thresholds and mortality risk across temperate forests

Abstract. Drought-induced early leaf senescence is increasingly widespread in temperate forests, yet the thresholds triggering this response and whether it reflects adaptive drought avoidance or progressive stress remain poorly constrained. Combining six years (2018–2023) of Sentinel-2 observations with atmospheric and soil drought indicators and canopy mortality maps across German beech (Fagus sylvatica) and oak (Quercus robur, Q. petraea) forests, we show that early senescence emerges once cumulative drought stress exceeds species-specific hydraulic thresholds. Sustained atmospheric vapour pressure deficit (VPD) followed by late-summer soil water depletion sharply increases senescence probability, with critical thresholds occurring after six consecutive weeks of daily maximum VPD above 1.9 kPa in beech and 2.1 kPa in oak, and after two weeks of root-zone soil water potential (ψ_soil) below −0.8 MPa (beech) and −0.9 MPa (oak). Sensitivity differs between forest types: beech-dominated stands respond more strongly to soil drought, whereas oak stands are more sensitive to atmospheric drought. Elevated spring VPD further amplifies early senescence risk in both species, pointing to cross-seasonal legacy effects in canopy stress responses. Critically, early senescence – and particularly its recurrence across consecutive years – is associated with significantly elevated canopy mortality, indicating progressive hydraulic stress rather than protective drought avoidance. Clustering forests by senescence dynamics and mortality outcomes reveals distinct drought-response types ranging from resistant stands buffered by favourable soil and climate conditions to vulnerable lowland forests with elevated mortality risk. Together, these results establish early leaf senescence as a landscape-scale indicator of drought stress severity and emerging mortality risk, providing a new framework for identifying forest vulnerability under intensifying climate extremes.