Seasonal ecological quality in China: concurrent and lagged drought responses and dominant factors.

INTRODUCTION: Global warming is intensifying seasonal droughts and increasing their pressure on terrestrial ecosystems. However, most assessments of drought impacts on ecological quality remain focused on annual-scale or concurrent relationships, with insufficient attention to drought seasonality, lagged effects, and differences along aridity-humidity gradients. METHODS: This study constructed spring, summer, and autumn Remote Sensing Ecological Index (RSEI) for China from 2001 to 2022, and analyzed the spatiotemporal variation characteristics of ecological quality at the seasonal scale. Meanwhile, by integrating the Standardized Precipitation Evapotranspiration Index (SPEI), other climatic variables, and human activity data, we investigated the concurrent and lagged effects of seasonal drought on ecological quality, incorporated these effects into the subsequent driving-factor analysis of ecological quality changes, and further compared drought responses and dominant-factor patterns across different dry-wet regions. RESULTS AND DISCUSSION: The mean RSEI in spring, summer, and autumn all increased significantly, with rates of 0.0032, 0.0017, and 0.0012 yr-1, respectively (p < 0.01). The significant response area of seasonal RSEI to SPEI was largest in summer, accounting for 30.15% of the study area, followed by spring and autumn, accounting for 26.78% and 23.53%, respectively. Spring RSEI was mainly affected by concurrent spring SPEI, summer RSEI was mainly affected by antecedent spring drought, and autumn RSEI was mainly affected by antecedent winter and summer droughts. Drought responses differed markedly among dry-wet regions. In spring, the significant impact range of drought on ecological quality generally decreased with increasing aridity, whereas in summer it was highest in semi-arid regions. In autumn, RSEI and SPEI were mainly negatively correlated in humid and semi-humid regions, but mainly positively correlated in semi-arid and arid regions. After accounting for lagged drought effects, SPEI was the dominant factor with the widest significant influence range across all three seasons, while temperature, human footprint, and soil moisture were the main secondary factors in spring, summer, and autumn, respectively. Seasonal ecological quality monitoring, drought-risk identification, and regional ecological conservation should therefore consider the season of drought occurrence, antecedent water conditions, and regional dry-wet background. These findings can provide scientific support for seasonal ecological monitoring, regionalized drought-risk management, and differentiated ecological conservation strategies in China.