Materials and methods
Study site
Populations of S. breviflora and C. songorica were identified in Siziwang, Inner Mongolia (41°46′43.6″N, 111°53′41.7″E, elevation 1456m). The region has a temperate continental arid and semi-arid climate, and is characterized by significant inter and inter-annual variability of hydrothermal conditions (Wang et al., 2011). The average annual precipitation is 223 mm, and 80% of the annual precipitation is concentrated in May to September. The main soil type is light chestnut soil (Chinese classification) or calcic Kastanozems (FAO soil classification). The soil texture is sandy loam, and the desert steppe in this region is dominated by S. breviflora ,Artemisia frigida and C. songorica . The composition of plant species is relatively simple, with average vegetation height of 8 cm and a canopy cover ranging from 17% to 20%.
Experimental design
To quantitatively test the effects of grazing intensity on the desert steppe ecosystem, twelve adjacent plots were established at a grazing experiment site in 2004. These treatments were arranged in a randomized complete block design, which included four grazing intensity treatments with three replicates (n = 3) of each treatment (see Fig. 1). The grazing intensities were identified as no grazing CK (0 sheep·ha-1·half year-1), light grazing LG (0.93 sheep·ha-1·half year-1), moderate grazing MG (1.82 sheep·ha-1·half year-1) and heavy grazing HG (2.71 sheep·ha-1·half year-1). Since 2004, the annual seasonal grazing period is from the beginning of June to the end of November, and is predominantly 2-year-old Mongolian sheep grazing. The daily grazing schedule was from 6:00 am to 6:00 pm. Water and salt were provided freely.