## 3 Results and Discussion
### CV and Trend of NPP  
From 2000 to 2015, the average annual variation in NPP was 20.57 g C/m\({}^{2}\)-a (Figure 5a), indicating that the NPP as a whole trended upward during the research period. The maximum rate of growth in the inter-annual variation in NPP was 475.18 g C/m\({}^{2}\)-a, which increased significantly in the eastern \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Land Use Type** & **Farmland** & **Forest** & **Grassland** & **Water** & **Desert** & **Built-Up Area** & **Bare Land** \\ \hline Farmland & 659,998 & 2360 & 4346 & 1405 & 8956 & 124 & 294 \\ Forest & 764 & 46,2587 & 890 & 167 & 468 & 27 & 46 \\ Grassland & 4994 & 2966 & 1,793,473 & 1758 & 2648 & 3418 & 322 \\ Water & 1803 & 138 & 899 & 16,4937 & 466 & 1036 & 244 \\ Desert & 181 & 27 & 57 & 82 & 69,398 & 14 & 1 \\ Built-up area & 747 & 302 & 2856 & 1513 & 744 & 250,059 & 155 \\ Bare land & 18 & 7 & 101 & 73 & 15 & 11 & 265,807 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Land use transfer matrix from 2000 to 2015 (km\({}^{2}\)). Figure 4: Mean annual NPP from 2000 to 2015 (**a**) and the spatial pattern of conversion of land use types from 2000 to 2015 (**b**). Gansu, North-central Shaanxi, and northeastern Inner Mongolia regions. The maximum annual rate of NPP decline was 306.39g C/m\({}^{2}\)-a. The NPP in Henan, Shanxi, Hebei, and central Inner Mongolia had a significant downward trend, while that in Qinghai, Tibet, Xinjiang, and western Inner Mongolia had a slight downward trend. The significance test results of the inter-annual changes in NPP (Figure 5**b**) show that NPP increased significantly. The regions in the northeastern part of the study area that passed the significance test (\(p<0.05\)) were mainly distributed in Inner Mongolia and the Jilin and Liaoning provinces; in the middle of the study area, north-central Shaanxi, central and western Shanxi, southern Ningxia, and eastern Gansu provinces passed the significance test (\(p<0.05\)). The main reason for this result is the construction of the Grain for Green Program and the Three Northern Shelter Forest Programs over the last 35 years [40], effectively restoring the vegetation in the above-mentioned areas. The global temperature is increasing, and China's climate has also changed significantly in the past 100 years, consistent with the general trend of global temperature change [41]. The vegetation activity in most parts of China is also increasing due to earlier or longer growing seasons caused by global warming [42]. In addition, an increase in people's awareness of environmental protection and rationalization and sterilization of agricultural planting both significantly affect the upward trend of NPP. The NPP in the southern part of Ningxia increased significantly, and the rate of increase dropped from the south to the north. This is consistent with the conclusion reached by [PERSON] et al. [18]. The significant increase in NPP in north-central Shaanxi in the central part of the study area is consistent with the conclusion reached by [PERSON] et al. [43]. In addition, NPP in the northeastern part of the study area had an upward trend because of the widespread forest in this area, a small human population, and minimal anthropogenic influence. The water in the area is supplied by precipitation, snow cover, and meltwater from permafrost [44]. The regions in which NPP decreased significantly and passed the significance test (\(p<0.05\)) were mainly distributed in the Tibetan Plateau (Qinghai, Tibet, and Xinjiang). The observed decreasing trend for NPP in the Tibetan Plateau is consistent with the conclusion reached by [PERSON] et al. [45]. The coefficient of variation of the NPP in the study area was between 0 and 1.4, with an average CV of 0.144, indicating a small overall variation in NPP during the study period. The regions with larger CV ranges were distributed in the northeastern part of Inner Mongolia and southeastern part of Tibet, with scattered distributions around Beijing, Shaanxi, Shanxi, and Henan (Figure 5**c**).