[Spatial and temporal variations of vegetation coverage in the middle section of Yellow River Basin based on terrain gradient:Taking Yan'an City as an example.].

Vegetation restoration is an important requirement for ecological protection and high-quality development in the Yellow River Basin. In-depth understanding the temporal and spatial differentiation of vegetation has important practical significance. With the four Landsat TM/OLI remote sensing images as the data source, the pixel binary model was used to estimate vegetation coverage. The transfer matrix, geological information map, and the center of gravity migration model were used to analyze the temporal and spatial variations of vegetation coverage in Yan'an City in the middle of the Yellow River Basin from 1988 to 2018. The spatial variation of vegetation cover to elevation and slope was explored using terrain distribution index combined with terrain data. The results showed that vegetation coverage in Yan'an was low in the north region and high in the south region. The vegetation coverage increased significantly due to policy changes. From 1988 to 2018, the vegetation change pattern in Yan'an was dominated by continuous improvement and stability. Vegetation coverage increased in about 50% of the area. 83% of the study area with high vegetation coverage remained stable. At each elevation and slope level, the advantage of high vegetation cove-rage increased with time. At each slope level, vegetation cover increased with slope. The transition direction of vegetation cover of different levels in Yan'an was basically consistent with the overall spatial change trend of vegetation cover, with the overall shift being from north to west. The vegetation construction in Yan'an has achieved remarkable results, though the vegetation coverage in the north still needs to be improved. Optimizing vegetation types and structures is an important direction for future vegetation construction.