Snowmelt Timing and Contrasting Vegetation Responses to Compound Dry–Hot Events in Northwest China’s Mountains

Seasonal snow regulates spring water supply and vegetation development, thereby shaping vegetation responses to summer compound dry–hot events. Whether snowmelt timing relative to event onset explains the contrast between peak damage and cumulative deficit remains unresolved. Regional observations from 2006 to 2025 were integrated with high-resolution satellite data across Northwest China to combine regional association analyses with local comparisons within the same events. Snowmelt completion timing, the snowmelt–event interval, antecedent root-zone moisture, and local snow–terrain conditions were evaluated. Earlier snowmelt was associated with lower peak damage but greater seasonal cumulative vegetation deficit. As the snowmelt–event interval increased from 56 to 148 days, cumulative deficit decreased by 9.30 z·observations. The mean standardized root-zone moisture residual declined from 0.12 during the initial post-snowmelt period to −0.43 in the last valid period. In local annual-baseline comparisons, late-snow candidates showed greater peak damage and cumulative deficit than matched controls. Under an event-preceding baseline, candidate pixels had 0.0489 lower cumulative EVI2 deficit than controls across 77 event-sensitive samples, and this direction persisted after sequentially excluding mountain systems. Future scenarios showed jointly high peak damage and cumulative deficit in the Altai under some climate combinations. These results provide evidence for incorporating snowmelt preconditioning into assessments of mountain vegetation risk that account for both event impacts and seasonal losses.