*2.1. Study Area*

This study focused on the forest resources in the Helan Mountains National Nature Reserve (38◦19 –39◦22 N, 105◦49 –106◦41 E) in Ningxia Province (Figure 1). The Helan Mountains belongs to the temperate arid climate zone with typical continental monsoon climate characteristics. The lack of rain and snow all year round leads to a dry climate. Although the average annual temperature is −0.7 ◦C, there is a wide seasonal variation in precipitation. For instance, the average precipitation from June to September, which accounts for over 62% of the annual precipitation, reaches 260.2 mm. Due to the steep mountain and complex terrain, the Helan Mountains are an important dividing line between climate and vegetation in western China. To the east is the grassland climate and grassland vegetation, and to the west is the desert climate and desert vegetation. It is located at the junction of the Qinghai-Tibetan Plateau, the Mongolian Plateau, and the North China Plain. The special geographical environment has shaped the unique biological groups of the Helan Mountains, making it the only biodiversity hotspot in northern China. Furthermore, the Helan Mountains National Nature Reserve in Ningxia Province has played a major role in studies on the virtuous cycle of vegetation development, succession, and restoration of ecosystems in semi-arid areas.

**Figure 1.** The geographical location of the Helan Mountains.
