Spatiotemporal Changes in Vegetation Cover and Oasis Expansion in Yengisar County, Northwest China

This study used 30 m Landsat imagery from 1995 to 2020 to investigate vegetation dynamics and their driving factors in Yengisar County, an arid oasis region in northwestern China. The Normalized Difference Vegetation Index (NDVI) was calculated, and the Dimidiate Pixel Model was used to estimate fractional vegetation cover (FVC). Trend analysis and a center-of-gravity migration model were employed to examine the spatiotemporal evolution of vegetation and its responses to climatic and human factors. The results showed that: (1) FVC exhibited clear spatial heterogeneity, with high vegetation coverage mainly distributed along river systems in the southwest and concentric expansion occurring around urban areas in the northeast. (2) From 1995 to 2020, the total vegetated area increased by 409.38 km2, representing a 76.4% increase relative to 1995, while the average FVC increased by 15.4%. Except for the extremely low-coverage class, all vegetation classes expanded, with the medium-coverage class showing the largest increase (142.9%). (3) The vegetation center of gravity gradually shifted toward the southeast, and the period from 2015 to 2020 represented the most rapid stage of vegetation improvement. (4) Human activities, particularly agricultural reclamation and grassland expansion, exhibited a strong association with recent vegetation changes and played an important role in the expansion of the artificial oasis. The results indicate that recent oasis expansion in Yengisar County has been closely associated with land-use change and irrigation activities. These findings provide useful information for oasis management, land-use planning, and water resource allocation in arid regions.