Spatiotemporal Dynamics of Vegetation Net Primary Productivity in Hilly and Gully Regions on the Loess Plateau

Since the implementation of forestry ecological engineering projects such as the Three-North Shelter Forest Program and the Grain for Green Program, the net primary productivity (NPP) of vegetation on the Loess Plateau has increased. Analyzing the spatiotemporal dynamics of vegetation NPP and clarifying the relationship between vegetation carbon sequestration capacity and land use change can provide scientific references for the carbon budget of terrestrial ecosystems and regional ecological security. This study focuses on the Ansai District of the Loess Plateau, utilizing Landsat TM images from 2000 and GF-2 images from 2017 as data sources for land use type identification. Based on an improved CASA model, vegetation NPP was estimated, and subsequently, the changes in land use, vegetation NPP, and the impact of land use on vegetation NPP changes were analyzed. The results showed following: The implementation of key forestry ecological engineering projects has increased the proportion of forest area in Ansai District from 9.76% in 2000 to 41.18% in 2017, with the main land use changes being the conversion of cultivated land and other land into forest land. The mean values of vegetation NPP in Ansai District in 2000 and 2017 were 170.28 g C·m-2·a-1 and 465.16 g C·m-2·a-1, respectively, showing a significant increase in vegetation NPP, though the distribution was uneven, with a decreasing trend from south to north. Compared with 2000, the forest area in Ansai District increased significantly in 2017, along with an increase in vegetation NPP. The conversion of cultivated land and other land into forest land made the greatest contribution to the increase in vegetation NPP. The implementation of forestry ecological projects promoted the conversion of cultivated land and other land into forest land in Ansai District, thereby enhancing the carbon sequestration capacity of vegetation.