## 3 Results and Discussion
### NPP Spatial Pattern and Land Use Change  
The spatial pattern of NPP in the study area for the 2000-2015 period was obtained by using pixel-level statistics and was divided into seven grades by the natural discontinuity method (Figure 4a). The variable spatial distribution of NPP is consistent with the distribution of precipitation, with a gradual decrease from the southeast to the northwest. This is because, except for some parts of the Tibetan Plateau, most regions of China have less precipitation, resulting in a lack of moisture for vegetation growth. Under the background of climate warming, soil moisture evaporation in arid and semiarid areas in northwest China intensifies the degree of drought. Thus, precipitation is an important limiting factor for NPP in areas with a low moisture. In addition, the surface elevation of the study area gradually increases from east to west, making it difficult for the warm and humid airflow in the southeast coast to reach the northwest inland region. On the other hand, the rise of elevation causes a decrease in temperature, further limiting the growth of crops. This phenomenon is most obvious on the Tibetan Plateau. The average NPP in the research area was 1640.19 g C/m\({}^{2}\) during the study period. Generally, the average NPP in the east and south were higher than that in the west and north, respectively. Areas in which the NPP was in the range of 800-2008 g C/m\({}^{2}\) were concentrated in Inner Mongolia and the southeastern edge of the Tibetan Plateau. This accounted for 22.28% of the total study area. The NPP in the southeastern part of the study area ranged from 3217 to 10330 g C/m\({}^{2}\), accounting for 15.19% of the study area. This area has relatively good hydrothermal conditions and includes southern Gansu and Shaanxi, most of Henan, Shandong, and central Hebei. The NPP in the Tibetan Plateau was significantly lower than that in other regions, and only the southeast region had an NPP that was slightly higher (800-3216 g C/m\({}^{2}\)), while the NPP in most other regions was lower than 800 g C/m\({}^{2}\). Precipitation in the southeastern region of the study area was higher as a result of the warm and humid airflow from the Indian Ocean that gets uplifted by the terrain, while the northwest of the Tibetan Plateau, located deep inland and at a higher altitude, had less precipitation and lower temperatures. The part of the study area in which the spatial distribution of NPP was 2008 g C/m\({}^{2}\)  
Figure 3: Local spatial agglomeration (**a,b**) and significance (**c,d**) of average and Max NPP values. correlated with an annual average precipitation of 400 mm. According to the spatial patterns of the land use transfer matrix for 2000-2015 (Figure 4b), farmland was mainly found in the Henan, Shandong, Hebei, Shaanxi, Shaanxi, Jilin, and Liaoning provinces, which have good hydrothermal and topographic conditions. Grassland and bare land were predominantly found in Qinghai, Xinjiang, and Inner Mongolia, and the distribution of NPP corresponded to the land use type. Table 1 shows the statistical results of land use transfers that occurred from 2000 to 2015. According to the retention rate of land use types, from 2000 to 2015, the study area was dominated by grassland (\(179.35\times 10^{4}\) km\({}^{2}\)) and farmland (\(66.0\times 10^{4}\) km\({}^{2}\)), accounting for 48.29% and 17.77% of the total study area, respectively. Forest was the third most dominant land use type (\(46.26\times 10^{4}\) km\({}^{2}\)) and accounted for 12.46% of the study area. The desert areas were the smallest (\(6.94\times 10^{4}\) km\({}^{2}\)). The area of farmland converted into desert was the largest of the land use transfer areas, reaching 8956 km\({}^{2}\). The area of grassland that was converted into desert was 2648 km\({}^{2}\). This indicates that the study area is ecologically sensitive and at risk of further deterioration. The area converted from grassland to farmland was 4994 km\({}^{2}\) and from farmland to grassland was 4346 km\({}^{2}\).