2.1 Study area
From southwest to northeast on the MP, the pattern of vegetation cover grades from desert to grassland to forest, and the plateau shows dramatic spatial and temporal variations in land cover (Liu, 1993). According to the Global Aridity Index dataset, which is based on UNEP’s (1997) aridity degree classification, the MP can be divided into an arid area (including an extremely arid area), a semi-arid area, and a semi humid area (including a humid area). The desert, grassland, and forest are located mainly in the arid, semi-arid, and semi humid areas, respectively (Miao et al., 2014). Although climate change, especially precipitation changes, have affected the boundaries of these climate zones to some extent (Jeong et al., 2011; Sternberg et al., 2015), they have remained relatively stable on the whole. Here, we divided the MP into three subregions, namely desert, grassland, and forest, in reference to the UNEP (1997) aridity degree divisions. The distribution of land cover is shown in Figure 1.
The desert subregion (DSR) is located in the southwest of the MP and has an annual mean precipitation of <200 mm. The main land cover is bare land and sparse vegetation (65%) and grassland (32%). The grassland subregion (GSR) is located in the central and northern portions of the plateau, and receives annual precipitation of between 200 and 400 mm. This is a subregion of the Eurasian grassland, which is one of the largest grassland ecosystems in the world (Batima and Dagvadorj, 2000). The main land cover is grassland (90%) and farmland (4%). The forest subregion (FSR) receives annual precipitation of approximately 400 mm, and the forest landscape is fragmented across this region (Bohannon, 2008). The main land cover in this region is grassland (41%), forest (34%), and farmland (18%).