Canopy vitality drives rainfall redistribution in an old-growth temperate beech forest

Forest disturbance increasingly manifests not only through abrupt events such as fire or windthrow, but also through progressive canopy vitality decline driven by pathogens, stress, and mortality—processes that fundamentally reorganize forest structure and function. The ecohydrological consequences of such vitality-driven disturbance remain poorly understood in old-growth temperate forests. This study examined how progressive canopy deterioration—from healthy crowns to branchless snags—affects rainfall partitioning and canopy hydrological parameters in an old-growth Hyrcanian oriental beech (Fagus orientalis Lipsky) forest of northern Iran. Over one full hydrological year, fifteen trees were randomly selected to represent five vitality stages and were instrumented to measure throughfall, stemflow, and interception under both leaf-on and leafless conditions. A reformulated Gash analytical model (RGAM) was applied to simulate interception dynamics. Results revealed that throughfall increased as both interception and stemflow declined systematically with decreasing canopy vitality, indicating a transition from hydrologically buffered to more transmissive canopy conditions. Nonetheless, snag trees exhibited measurable rainfall interception—7.9 % for branched and 2.8 % for branchless snags—challenging the assumption that snags contribute negligibly to canopy evaporation. Stemflow generation decreased sharply as crown connectivity deteriorated and was consistently lower in the leafless period. RGAM accurately reproduced interception for healthy and moderately degraded trees but overestimated losses in severely deteriorated canopies, suggesting that model parameters must account for canopy heterogeneity and vitality-dependent storage dynamics. These findings provide the first quantitative assessment of rainfall redistribution across a five-stage canopy vitality gradient, explicitly including both branched and branchless snags, demonstrating that canopy degradation substantially alters rainfall storage, channeling, and evaporation processes. Incorporating tree vitality and deadwood structure into interception modeling will improve predictions of rainfall redistribution, soil moisture, and water yield in old-growth and uneven-aged temperate forests worldwide.