Fine-scale analysis of the cumulative and time-lagged effects of drought on vegetation in the Ili River Basin, Central Asia.

Under intensifying climate change, drought impacts on vegetation have become increasingly complex. Although the Ili River Basin is highly sensitive to drought, the underlying mechanisms of drought-vegetation interactions remain insufficiently understood. This study quantifies the cumulative, time-lagged, and dominant effects of drought on vegetation gross primary productivity (GPP) at the daily scale, and investigates their variations across land use types, wetness gradients, and seasons. Fine-scale ecosystem management strategies are further proposed, tailored to both land use and seasonal conditions. The findings are: (1) Over the past 40 years, growing season GPP increased (k = 1.302 C/m/a), while drought conditions intensified (k = -0.002/a). (2) Cumulative drought effects suppressed GPP in over 90 % of the basin, with accumulation durations concentrated around 180 and 90 days. Cumulative effects intensified and extended with increasing drought severity. (3) Time-lagged effects exhibited spatial heterogeneity, with 58 % of the basin showing post-drought GPP promotion after 280 days and 42 % showing suppression after 117 days. Time-lagged effects also strengthened with drought intensity. (4) Positive cumulative effects dominated 77 % of the basin, with their spatial extent increasing under more severe drought. (5) Seasonally, spring exhibited strong lagged suppression but low sensitivity, summer showed the strongest and most sensitive cumulative suppression, while autumn displayed signs of compensatory recovery. Among land use types, grassland exhibited the highest sensitivity to drought. Differentiated management strategies should be adopted by season and land use, with emphasis on enhancing drought monitoring and water regulation in spring and summer, and prioritizing drought risk prevention and adaptive management in grassland.