## 3 Results
### Spatial and Temporal Distribution of Grassland NPP in Inner Mongolia
#### 3.2.3 The Change Trend Distribution of Grassland NPP  
The relationships between the interannual fluctuations of grassland NPP and meteorological factors in Inner Mongolia from 2000 to 2020 are shown in Figure 8. The NPP ranges from 246.15 g C/m\({}^{2}\) in 2001 to 323.96 g C/m\({}^{2}\) in 2013. The change trend obtained by the OLS method shows a slight increasing trend of 2.43 g C/(m\({}^{2}\cdot\)yr) (\(p<0.01\)), and that obtained by the Theil-Sen method shows a slight increasing trend of 2.16 g C/(m\({}^{2}\cdot\)yr) (\(p<0.05\)). In general, the NPP of the Inner Mongolia grassland gradually improved from 2000 to 2020. Figure 5: Spatial distribution of the average NPP in the Inner Mongolia grassland during 2000–2020. Figure 6: Area proportion of the different NPP distribution intervals. From an overall perspective, as shown in Figure 8, the highest grassland NPP appeared in 2013, when there was adequate precipitation, suitably low temperatures, and abundant sunshine. The other periods of higher NPP were in 2012, 2018, 2019, and 2020, when the climatic conditions were better than average. The lowest grassland NPP was in 2001, when there was a lack of precipitation. The other years of lower NPP were 2000, 2007, and 2016. Figure 8: Interannual change trends of grassland NPP and meteorological factors obtained by the OLS method for the Inner Mongolia grassland. Figure 7: Coefficient of variation of NPP in the Inner Mongolia grassland during 2000–2020. Extreme weather conditions occurred in these years, i.e., drought, high temperature, and lack of sunlight, respectively. Although the growth trend of grassland NPP from 2000 to 2020 passes the significance test, the growth of NPP was not continuous and showed temporal heterogeneity (Table 5). From 2000 to 2010, the growth rate of grassland NPP was slower and more unstable, with opposite change trends obtained by the two methods, while the growth rate during 2010-2020 was faster. In order to better clarify the spatial and temporal variation of grassland NPP in Inner Mongolia, the different time series of grassland NPP were analyzed by the use of the Theil-Sen method in this study. As shown in Figure 9e,f, there was a positive annual NPP growth tendency in most areas in the Inner Mongolia grassland from 2000 to 2020. About 83.22% of the total grassland showed an increasing trend for NPP, of which 32.28% showed significant growth at the \(p\)-value of 0.05. The grassland areas that showed significant growth are mainly distributed in east Ordos, south Chifeng, south Tongliao, and Hinggan. At the same time, about 16.78% of the total area presented a decreasing trend for NPP, which took place mainly in west Xilingol and north Chifeng, but most of this change (93.24%) was not significant. In order to show more details of the short-term changes, the spatial and temporal variation of grassland NPP in the first 11 years and the second 11 years was also compared in this study. Figure 9a,b shows the results for 2000-2010. The changes of most of the grassland do not pass the significance test at a \(p\)-value of 0.05. About 44.85% of the grassland showed an insignificant increasing trend for NPP during this period, which took place mainly in west Ordos, middle Xilingol, and west Hulun Buir. Another 43.91% of the grassland showed an insignificant decreasing trend for NPP, which took place mainly in Baotou, Ulanqab, south-west Xilingol, and along both sides of the Greater Khingan Range. Only 8.08% of the grassland showed a significant growth trend for NPP, which took place mostly in east Ordos, while 3.16% showed a significant decrease, which took place mostly along the Greater Khingan Range. Figure 9c,d shows the results for 2010-2020. Compared with the first 11 years, the spatial distribution of the change trend of NPP changed to a certain extent, and more grassland (72.34%) showed an increasing trend for NPP. About 13.36% of the total area passed the significance test, which occurred mainly in east Ordos, south Chifeng, and south Tongliao.