Drought and Heat Jointly Drive Forest Canopy Injury During Compound Climate Extremes

Forests worldwide are increasingly exposed to compound climate extremes, yet the physiological and ecological pathways through which concurrent heat and drought damage vegetation remain poorly understood. These compound stresses pose significant risks to ecosystem resilience, but their interactive effects have rarely been quantified across large landscapes. The unprecedented June 2021 Pacific Northwest “Heat Dome” provided a unique natural experiment to address this gap. Using high‑resolution satellite imagery and spectral–temporal diagnostics, we mapped leaf scorch across 93,420 ha with 87% accuracy and quantified the relative contributions of abiotic drivers and species identity. Unexpectedly, water-stress variables, particularly rapid atmospheric drought captured by vapor pressure deficit anomaly, dominated spatial variation in canopy injury (35.5%), slightly exceeding the contribution from heat‑associated stress (33.1%). The synergistic effect of hydraulic stress and heat stress further amplified canopy injury. Species identity accounted for 19.1%, with divergent sensitivities: Thuja plicata was disproportionately vulnerable to water deficit, whereas Abies amabilis was most sensitive to elevated heat. Trait‑based analysis linked these vulnerabilities to distinct functional syndromes, enabling predictive insight into species‑specific responses. By disentangling damage drivers at the landscape scale, our findings advance understanding of forest responses to compound climate extremes, trait based predictive frameworks and provides actional insights for adaptive management under accelerating climate change.