Fig. 1. Location of sampling points (Fig. A). Each grassland type includes a paired no-grazing and grazing site. Fig. B shows a temperate desert sample site; Fig. C shows a temperate grassland sample site; Fig. D shows a montane meadow sample site. Fig. E shows the sample layout.
2.2. Experimental design and field sampling
In July 2021, three areas (Fukang city, Mubi County and Qitai County) on the eastern section of the northern slope of Tianshan were selected. Each area corresponded to one grassland type: temperate desert, temperate steppe and mountain meadow. The test sites were all located within the national fixed monitoring sites and have been fenced since 2012. The fenced areas are surrounded by spring and autumn grazing areas for sheep, and the grazing intensity is moderate (0.6-1.0 sheep/ha). At each site, paired plots (a long-term grazing plot and a grazing exclusion plot) were sampled. All paired plots shared soil type and geographical conditions, including slope, elevation, and topography. Three sample lines were set up at 50-m intervals in each of the grazing exclusion and grazing areas, and three 1 m × 1 m herb samples were laid out at approximately 50-m intervals on each sample line (total of 54 small samples). If there were shrubs in the plot, 5 additional shrub plots (10 m×10 m) were measured. The plant species present in each square were recorded. For each plant species, its cover, average height, density and biomass were documented. Plant coverage was measured by the projection method; natural heights were measured using a ruler, and individual quantities (densities) of each species were recorded with the help of statistical methods. The aboveground biomass of each species was estimated in each sampling plot by clipping the plant from the soil surface using scissors. The collected plant material was brought back to the laboratory for treatment (dried at 80 °C for 24 h to a constant weight). All the plant data are presented in Supplementary Table 1. Soil samples were collected at two depths: 0-5 cm and 5-10 cm. The samples from the same sample line and depth were mixed and placed in sealed bags. The samples were taken back to the laboratory in a vehicle refrigerator (-20 °C). Half of the samples were stored at 4 °C, while the rest were air-dried at room temperature. Plant roots, gravel, and other debris were removed from the air-dried samples. The remaining soil was ground, mixed, and sieved through 1-mm and 0.25-mm sieves for analysis.
2.3. Plant community diversity measurement
According to Whittaker’s scientific and comprehensive discussion on an evaluation index of plant diversity, the α-diversity index is often used to study the species diversity within a community caused by the differentiation of interspecific niches within a community and is a commonly used evaluation index in ecological research (Whittaker, 1977). This study used the method proposed by Whittaker. The community structure attributes were characterized by the importance value (IV), Patrick richness index (R), Simpson dominance index (D), Shannon‒Wiener diversity index and Pielou index (E)(Wu et al., 2009), and they were calculated as follows:
Importance value (IV): (1)
Patrick richness index (R): (2)
Simpson dominance index (D): (3)