2.1. Site description
The study was conducted in the Dahuang Mountain Forest Reserve (100°22′ E, 38°43′ N, Fig. 1) in the Qilian Mountains. The study area is located approximately 45 km southeast of Shandan County, Gansu Province, China, in the northern part of the Qilian Mountains in a typical arid area, with P. crassifolia as a dominant tree species. Several afforestation projects have been implemented in this area, providing a perfect opportunity to elucidate spatial patterns of SWC following a vegetation change. The climate is semiarid and cold temperate, with a mean annual temperature of 2.5 ℃, and mean annual precipitation of 385 mm (for years 1994 to 2014); approximately 80% of the precipitation falls during the warmer months of June through September (He et al., 2019). Soils are gray cinnamon on shaded and semi-shaded slopes, and chestnut (Chinese soil taxonomy) on sunny slopes (Chen et al., 2016).
Forests are dominated by P. crassifolia , and are distributed on shaded and semi-shaded slopes; grasslands are primarily found on sunny and semi-sunny slopes, and shrubland is distributed in the transition between forest and grassland. Since the 1970s, most of the forest at low elevation had been cut down to satisfy an increasing demand for timber and crop production in the region. Dahuang Mountain has been included in Natural Forest Protection Projection (in response to deforestation-caused flooding) since 2001, and most of the grasslands on east- and west-facing slopes have been successively converted toP. crassfolia plantation forests, with an increase in the proportion of forest-grass cover of an average of 6.2 % from 2000 to 2010 (Sun, 2020).