Spatiotemporal dynamics of vegetation cover and its responses to drought in arid northwestern China

Under the dual pressures of accelerating global warming and intensifying human activities, arid and semi-arid ecosystems are confronting unprecedented ecological challenges. Fractional Vegetation Cover (FVC), as a critical indicator of vegetation dynamics and ecosystem health, has attracted increasing research attention; however, the spatiotemporal evolution patterns of FVC and its underlying response mechanisms to drought stress remain insufficiently understood. Using a dimidiate pixel model, FVC was derived from Landsat imagery during 1990-2022 for the arid northwestern China. Its spatiotemporal dynamics and drought responses were analyzed by integrating trend analysis, spatial statistics, and multi-timescale Standardized Precipitation Evapotranspiration Index (SPEI) data. The results indicate that: (1) Over the past 33-year, FVC exhibited an overall increasing trend at a rate of $0.98\times {10}^{-4}{\text{yr}}^{-1}$ , albeit with considerable interannual variability. The sharp decline in FVC in 2001 was closely associated with an extreme drought event, while the pronounced increase after 2010 coincided temporally with the implementation of regional ecological restoration programs. (2) Spatially, FVC displayed a heterogeneous pattern, with relative stability in the west and dynamic changes in the east. Low-high clusters dominated the local spatial autocorrelation structure, reflecting a fragmented vegetation distribution under drought stress, while high-high clusters showed progressive spatial expansion in oasis areas and ecological restoration zones. (3) The response of FVC to drought was scale-dependent: at the growing season scale (SPEI-4), FVC exhibited a positive correlation with drought indices, whereas at the interannual scale (SPEI-12), this relationship shifted to a negative correlation in certain sub-basins. Vegetation persistence was predominantly associated with no-to-mild drought conditions, while severe and extreme drought events significantly suppressed FVC. These findings results important theoretical and practical implications for ecosystem restoration efforts globally.