## 5 Discussion
### Spatial Variation of Tree Cover Change in the Loess Plateau  
From Region I to V, tree cover and slope decreased, and had a spatial pattern from northeast to southwest (Figure 5). The five types of tree cover regions were based on the 15 year tree cover value, providing the spatial distribution and limitation of trees in the Loess Plateau. The spatial pattern of the five regions had a close relationship with the vegetation zone distribution in the Loess Plateau. From the southeast to northwest of the Loess Plateau are the forest zone, forest steppe zone, typical steppe zone and desert zone. Trees could survive in the forest zone, forest steppe  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Variation Types** & **Area Percentage (\%)** & **Slope** & **Z Value** & **H** \\ \hline Sustained Significant increase & 16.31 & \(>\)0.1 & \(>\)1.96 & \(>\)0.5 \\ Sustained Slight increase & 10.05 & 0.0005–0.1 & –1.96–1.96 & \(>\)0.5 \\ Sustained Stability & 8.52 & –0.0005–0.0005 & –1.96–1.96 & \(>\)0.5 \\ Sustained Slight decrease & 2.00 & –0.1–0.0005 & –1.96–1.96 & \(>\)0.5 \\ Sustained Significant decrease & 0.39 & \(\leq\)–0.1 & \(<\)\(-\)1.96 & \(>\)0.5 \\ Undetermined future trend & 38.08 & - & - & \(\leq\)0.5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Trend in the tree cover changes in the Loess Plateau.  
zone and to a limited extent in the typical steppe zone, but could not live in the desert steppe zone or desert zone [49]. The ranges of Region I, II and III were very close to the forest zone and forest steppe zone and the scope of Region IV and V were comparable with typical steppe zone, desert steppe zone and desert vegetation zone, which matched well with the classification of the five tree cover regions. This showed the accuracy of the ecological meaning of the five tree cover regions.  
In the five tree cover regions, the proportions of tree cover change showed big tending differences (Figure 6). Tree cover increased occurred mainly in Region I to IV, but were due to different processes. In Region I, II and III, the increase trend represented the tree cover value of a tree covered pixel increase. In Region IV, the former process existed, but it was more likely to be a non-tree covered pixel change to tree covered. Overall, under the GGP, restoration and aforestation both promote the increase of tree coverage and tree covered area increase. Even though most of the regions were experiencing tree cover increase, tree cover decrease also existed. The proportion was much higher in Region III, where the slope was flatter and trees could persist but not forest. Human activities were more detrimental due to the urbanization in this region during the past 15 years [42], which were the dominating reasons causing tree cover decrease.  
An increasing trend of tree cover distributed in Northern of the Shaanxi Province, the east and west of the Shanxi Province, the north of Ordos Plateau, Northern of Pingliang in the Gansu Province and Northern of Ningxia. This was consistent with shifts observed in vegetation cover in most regions except in the Ningxia plain, Inner Mongolia and Liupan Mountains, where grasses and shrubs were the dominant vegetation [13; 50]. A sustained increase in tree cover were found mainly in Region III and IV, including all the potential revegetation regions (Huangfuchuan, Kuyehe, Wudinghe, and Yanhe watersheds) discussed in previous research [13]. The result regarding sustained and increasing of tree cover was based on the current situation. However, some studies had showed that afforestation in unsuitable regions leads to deterioration of soil ecosystems, decrease of land surface temperature and water shortage as a result of increased transpiration [16; 51; 52]. In order to predict tree cover change in the Loess Plateau more accurately in the future, further studies need to take the influence of environment change into account.