## 4 Results
### Variation in Trends of Tree Cover Change  
According to the real conditions of the slope of tree cover trend, we adopted [PERSON]'s definition method to class the area into five classes [25]. The tree cover increasing area (58.16%) was considerably larger than the decreasing area (5.63%) from 2000 to 2014, and approximately 36.21% of the area in the Loess Plateau remained stable (Table 1).  
Figure 5: Five levels of tree cover regions in the Loess Plateau. Tree cover distribution of the five regions (**a**). Area percentage of the five tree cover classes (**b**). Violin plot for tree cover of the five regions (**c**), all the tree cover was zero in Region V. Violin plot for slope of the five regions (**d**).  
In Region I, most of the area showed an increasing trend in tree cover (in total approximately 88.82% of this region showed an increase, of which 38.39% showed a significant increase and 50.23% showed a slight increase). Significant increases were most often observed in the northern region of the mountains in Shanxi Province (Luliang and Taiyue Mountains), north of the Guangdong Mountain and northwest of Qinling Mountains, whereas a decreasing trend was observed mainly in the east of the Qinling mountains (Figure 6a).  
In Region II, the increasing trend in tree cover occurred across the majority of the area (in total, approximately 87.73% of this area showed an increase, of which 50.65% was characterized by a significant increase and 37.08%, by a slight increase). Only a few areas in the south were characterized by a decreasing trend in tree cover (in total approximately 6.91% of this region showed decreased cover, 0.71% of which was a significant decrease and, 6.20% slight decrease) (Figure 6b).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Trend of Tree Cover** & **Area Percentage (\%)** & **Slope** & **Z Value** & **\(p\) Value** \\ \hline Significant increase & 34.53 & \(\geq\)0.1 & \(>\)1.96 & \(\leq\)0.05 \\ Slight increase & 23.63 & 0.0005–0.1 & -1.96–1.96 & \(>\)0.05 \\ Stability & 36.21 & -0.0005–0.0005 & -1.96–1.96 & \(>\)0.05 \\ Slight decrease & 4.78 & -0.1–0.0005 & -1.96–1.96 & \(>\)0.05 \\ Significant decrease & 0.85 & \(\leq\)-0.1 & \(<\)–1.96 & \(\leq\)0.05 \\ \hline \hline \end{tabular} Note: the pixels with slope in \(-\)0.0005 and 0.0005 and \(Z\) > 1.96 or \(Z\) \(<\) –1.96 are classified as the stable.  
\end{table}
Table 1: Trend in the tree cover changes in the Loess Plateau.  
Figure 6: Trends of tree cover changes in Region I through IV (**a–d**) in the Loess Plateau from 2000 to 2014.  
In Region III, approximately 59.28% of the area had an increasing trend, which mainly occurred in the north (31.16% of which was a significant increase and 28.12%, a slight increase). Tree cover maintained relative stability at 31.92% in this region, scattered mainly in the southeast. Around 9.42% of the region was characterized a decreasing trend in tree cover (of which 1.60% underwent a significant decrease and 7.82%, a slight decrease). This occurred mainly in the southeast of the Loess Plateau along the Wei River and Fen River Plains, as well as in the northwest where belonging to Ningxia Hui and Inner Mongolia Autonomous Regions (Figure 6c).  
In Region IV, the tree cover increased in approximately 41.26% over the region (34.51% of which was a significant increase and 6.75%, a slight increase) mainly located in the southeast of this region (Yulin in North Shaanxi, Guyuan in South Ningxia, and Pingliang in Northeast Gansu Province). More than half of this region remained stable, the majority (58.18%) of which was observed in the northeast of this region. Only a very small portion of this region was characterized by a decrease (0.56%) (Figure 6d).