Leaf Isohydricity as a Predictive Axis for Divergent Drought-Induced Mortality Pathways Across Subtropical Tree Species

Understanding the mechanisms of drought-induced tree mortality is essential for predicting forest vulnerability under climate extremes. Leaf isohydricity has been proposed as a key trait shaping drought survival, yet whether it consistently explains interspecific mortality differences and whether mortality pathways diverge between isohydric and anisohydric strategies remain unclear. Here we imposed an extreme-drought experiment on saplings of 25 subtropical tree species across the isohydric-anisohydric spectrum. We quantified key hydraulic traits, including xylem embolism resistance (P₅₀, P₈₈) and hydraulic safety margin (HSM₅₀, HSM₈₈), and leaf turgor traits (Ψ_tlp and C_tlp). Growth and photosynthetic traits were measured across multiple periods to diagnose carbon-related constraints. Across species, isohydric species showed lower mortality than anisohydric species. This survival advantage was associated with greater embolism resistance and wider HSM, indicating a more conservative hydraulic design under extreme drought. A divergence was uncovered in the dominant mortality pathway across the isohydric-anisohydric spectrum. Mortality was primarily linked to hydraulic failure via xylem embolism in isohydric species, whereas strongly associated with carbon limitation following photosynthetic suppression in anisohydric species. We identify isohydricity as an axis capturing both drought survival and the dominant mortality pathway, providing constraints to improve model predictions of forest vulnerability to future extreme drought.