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.