2.4 Data analysis
In order to analyze changes in the above-ground standing crop of shrubby plants over time, we used monitoring data on standing crop by species in August from 2004 to 2017 to calculate the annual cumulative above-ground standing crop of shrubby plants and herbaceous plants, and plotted the dynamic curve of the cumulative above-ground standing crop. One-way ANNOVA was used to test the effects of stocking rate on cover (%), height (cm), basal diameter (cm), branch number (individuals) and the maximum branch diameter (mm) of four shrubs (Artemisia frigida ,Kochia prostrata , Caragana microphylla andCeratoides latens ) in the plots. Correspondence analysis was used to test the relationship between shrubs and stocking rate, which was analyzed using the importance value of shrubs and the stocking rate gradient. Importance value was calculated as (Iv )=(Cr +Hr +Dr +BNr +BDr )/5, where Cr (relative cover)=C (cover of a certain species)/∑C(total cover of all species), with similar calculations for Hr (relative height), Dr (relative basal diameter), BNr (relative branch number) and BDr (relative maximum branch diameter). Variance analysis and correspondence analysis were undertaken using SAS 9.4 (SAS Institute Inc., Cary, NC, USA) at the P <0.05 level of significance. To analyze change in the cover of shrubby plants, the plant community was divided into shrubby plants, perennial grasses and other herbaceous plants. Triangular diagrams of the cover of shrubby plants, perennial grasses and other herbaceous plants were plotted using the data on the cover by species monitored in August from 2017 to 2019. All figures were plotted using SigmaPlot Version 12.5 (Systat Software Inc., San Jose, CA, USA).