## 5 Results
### Future Trend of Forests NPP in Northeast China  
#### 5.2.1 Analysis on Spatial Changing Trend of NPP, from 2001 to 2019, in Northeast China  
In order to explore the changing trend variation of NPP in Northeast China, this study estimated the Sen trend degree of NPP from 2001 to 2019 (z = 0.0990, slope = 0.2982). The overall NPP in the study area presented a not-significant increasing trend from 2001 to 2019, and the area with an increase and decrease trend accounted for 52.58% and 47.42%, respectively (Figure 9a). In terms of varying rates, the area with the NPP change rate between \(-\)10 and 0 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) accounted for 44.36% of the whole area and it was mainly distributed in the south of Zhangguangcai Mountain and the north of Changbai Mountain. To a certain degree, the ecological environment in many regions showed a weak trend of decline. Additionally, there was a large proportion of the area with a change rate between 0-5 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\), mainly located in the western part of Wuchang City, the north of Yushu City, and the eastern part of Ningan City in Zhangguangcai Mountain. On the contrary, the regions with the change range of NPP between 5-10 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) and over 10 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) accounted for 14.37% and 2.19% of the whole area, respectively, and they mainly distributed around the edges of the study area within the spatial pattern of belts. The regions with a change rate under \(-\)10 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) only made up 3.07% and they were mainly distributed in the central part of Wuchang City, the western part of Shanghai City, the eastern part of Jiahe City and the western part of Dunhua City in the central part of Zhangguangcai Mountain. Thus, only few regions showed very serious ecological environment degradation.  
In order to further determine whether the above regions showed a significant trend, we firstly performed the Mann-Kendall (M-K) test, and divided the significance test results into significant changes (Z \(>\) 1.96 or Z \(<\) \(-\)1.96) and insignificant changes (\(-\)1.96 \(\leq\) Z \(\leq\) 1.96) at 0.05 confidence level. Then, the NPP change trend at the pixel scale was calculated by superposing the changing trend results of Theil-Sen Median and significance results by M-K test, and by further dividing the results into four types (Figure 9b). The outcome demonstrated that the NPP change trend in most Northeast China was not significant from 2001 to 2019, and the areas with significant change only accounted for 3.99% of the whole study area. In terms of the upward trend, 50.78% of the areas showed a slight improvement, and only 1.79% showed a significant improvement. The areas with extremely significant improvement in ecological environment were mainly present in Bin County, the middle of Yanshou County, Acheng City, the west of Wuchang City, the east of Shuangcheng City, and distributed in the west of Shulan City in Zhangguangcai Mountain. The areas with slight improvements were mainly  
Figure 9: Changing trend of NPP (**a**) and its significance (**b**) in Northeast China, during 2001–2019.  
distributed in the periphery of the whole area. In terms of declining trend, 45.22% of the regions in Northeast China showed a slight degradation trend. They were mainly distributed in the south of Shulan City, east of Wuchang City, west of Dunhua City, Shangzhi City, and the intersections of Acheng City and Yanshou County in Zhangguangcai Mountain. The severely degraded areas accounted for only 2.20%, mainly distributed in the south of Longjing City, Helong City in the east of the Changbai Mountains, and the Jiahe City in the middle region.