Vegetation Phenology Shifts Driven by Cascade Reservoir Operations in the Lancang–Mekong River Basin

Large-scale infrastructure represents one of the most pervasive anthropogenic disturbances to fluvial systems, yet the cascading interactions between reservoir operations and alpine land-surface processes remain elusive. This study targets the Lancang–Mekong River Basin, a critical transboundary hotspot originating from the Qinghai-Tibet Plateau, to quantify how hydrological regulation mediates the coupling between local microclimates and vegetation phenology (including the start of the growing season (SOS) and the end of the growing season (EOS)). We developed an analytical framework integrating long-term multi-source remote sensing observations with structural equation modeling and interpretable machine learning to disentangle the cumulative, spatially heterogeneous responses to damming. Our results reveal a fundamental regime shift: over the past 24 years, the vegetation growing season in dam-concentrated reaches has extended by over 30 days, characterized by a 22-day advance in SOS and a 9-day delay in EOS. While natural climatic drivers typically dominate alpine phenology, reservoir-induced impoundment has perturbed the local hydrothermal equilibrium and alleviated water stress in dry-hot valleys. Attribution analysis reveals that reservoir-regulated soil moisture dynamics account for 42.7% of vegetation variability, representing a mechanistic transition from climatic dominance to a coupled human-environment regulation regime. This mechanistic shift provides essential geomorphic and eco-hydrological insights for the adaptive management and ecological restoration of disturbed river systems in high-altitude hotspots.