Event-stage evidence reveals weakened vegetation recovery stability under multi-scale drought accumulation in the Yellow River Basin

Regional drought-impact assessments often emphasize productivity loss, but less attention is given to whether vegetation stabilizes after drought and whether long-duration drought events are associated with distinct response pathways. This study developed an event-based drought–GPP response framework to evaluate how drought accumulation timescales reorganize vegetation response pathways in the Yellow River Basin during 2001–2022. Drought events were identified using the high-spatial-resolution Standardized Precipitation Evapotranspiration Index at 3-, 6-, and 12-month accumulation scales (HSPEI3, HSPEI6, and HSPEI12) and were matched with monthly standardized gross primary productivity anomalies. Three response components were quantified: main productivity loss, stable recovery among loss events, and duration-conditioned persistent-drought response. Drought-event frequency declined sharply from HSPEI3 to HSPEI12, while event duration, core drought duration, and drought intensity increased, showing a transition from frequent short-term deficits to fewer but more persistent accumulated drought events. Vegetation response changed more strongly after productivity loss than during the initial loss stage. Main productivity-loss occurrence varied only moderately across accumulation scales, whereas stable recovery declined from 89.7% under HSPEI3 to 76.5% under HSPEI12, and duration-conditioned persistent-drought response increased from 1.6% to 10.2%. Drought-event attributes showed weak statistical separability for initial productivity loss but much stronger separability for unstable recovery, especially under HSPEI12. Recurrent hotspots and persistence-prone regimes were concentrated mainly in the northern and north-central basin. Long drought accumulation was therefore associated more clearly with weaker post-loss recovery stability than with increased initial GPP loss, making recovery stability, duration-conditioned persistent-drought response, and spatial recurrence central indicators of vegetation drought vulnerability.