## 3 Results  
### Vegetation Distribution over the Loess Plateau  
The distributions of the average values, linear trends, and F-test results of NDVI over the three selected periods are presented in Figure 2. Figure 2a,c shows that the vegetation coverage generally decreases from southeast to northwest with a relatively low NDVI of 0-0.4 in the northwestern region. The overall vegetation coverage rate improved during S compared with that during S2, which suggests that the major contributor to the vegetation growth in S is the vegetation growth of S1 (Figure 2d,f). The statistical results reveal that the average trends over Loess Plateau during S, S1, and S2 were 0.022/10a, 0.013/10a, and 0.059/10a, respectively. After 2000, the growth rate of vegetation over Loess Plateau was 4.5 times that before 2000, In addition, [35] shows that the vegetation change rate of the whole China during 1982-2012 is 0.002/10a, and the vegetation growth rate of Loess Plateau during S and S2 is 11 and 29.5 times than that of the whole China. From the spatial distribution of NDVI, vegetation decreased in the southern and northern portions of Loess Plateau, especially in area I. Figure 2g-i shows the F-test distributions vegetation change in the three periods of S, S1, and S2 in the Loess Plateau; Table 4 provides the statistical results of the F-test showing decreasing and increasing trends for different significance levels. Approximately 56.76% of the pixels revealed an increasing trend during S2, while 72.23% of the pixels showed an increasing trend during S (P \(<\) 0.01).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Range** & **-1—0.4** & **-0.4—0.2** & **-0.2–0.2** & **0.2–0.4** & **0.4–1** \\ \hline Correlation degree & Strong negative correlation & Moderate negative correlation & Weak correlation & Moderate positive correlation & Strong positive correlation \\ \hline \hline \end{tabular}
\end{table}
Table 3: Range of Pearson correlation coefficients and correlation degrees.  
Figure 2: The (**a–c**) are average values, (**d–f**) are linear trends (unit: year-1), and (**h–j**) as F-tests of the GIMMS NDVI for three periods, where S, S1, and S2 represent the periods of 1982–2015, 1982–1999, and 2000–2015, respectively.  
According to the spatial distribution of vegetation in the Loess Plateau (Figure 1), in the north and south (area I), the vegetation decreased significantly. The main reason is that there are cities of Zhong, Yinchuan, Shizuishan, Wuhai, Linhai, Baotou, Hohhot and others in the north of area I. These cities have sufficient sunshine, strong evaporation, short summer, and late spring, and are prone to extreme days such as cold waves and sandstorms. In recent years, with the acceleration of urbanization, the area of construction land has increased, and the change of land use has led to the obvious degradation of vegetation in this area in the past 30 years. For the south of zone I, there are 12 cities, such as Shuozhou, Linfen and Yuncheng. The open-pit coal mining in these areas all year-round has caused serious damage to the surface vegetation in this area and is not easy to repair. These reasons directly lead to the significant degradation of vegetation in zone I of the Loess Plateau.