Abbreviations: (1) 5 cm×5 cm; (2) 10 cm×10 cm; (3) 20 cm×20 cm; (4) 25 cm×25 cm; (5) 50 cm×50 cm; (6) 100 cm×100 cm.

Synergistic changes in the density of Stipa breviflora andCleistogenes songorica with scale at different grazing intensities

The co-variation of the densities of S. breviflora and C. songorica with scales under different grazing intensities was shown on Fig. 4. No matter how the grazing intensity changes, there is a significant positive correlation between the densities of the two plant populations (P < 0.05). Under different grazing intensities, there was a linear correlation between the density ofS. breviflora and C. songorica . In CK, the correlation is density of C. songorica = 0.2623×density of S. breviflora ,r ² = 1.0000. In LG, the correlation is density of C. songorica = 0.0223+0.2331×density of S. breviflora ,r ² = 0.9999. In MG, the correlation is density of C. songorica = 0.2213×density of S. breviflora ,r ² = 1.0000. In HG, the correlation is density of C. songorica = 0.0201+0.2492×density of S. breviflora ,r ² = 1.0000. The determination coefficients were all close to 1, indicating that under different grazing intensities, the fitting between the densities of S. brevifloraand C. songorica a was excellent.

As grazing intensity increased, we observed an increase of density ofS. breviflora and a compensatory decrease in the density ofC. songorica decreased. We interpreted these results as proof of the competitive advantage of S. breviflora . We observed, however, that the difference in competitive ability was reduced as grazing intensity increased, suggesting that the increasing top down grazing pressure alleviated competition between species, indicating that in the three treatments, the competitiveness of S. breviflora was higher than that of C. songorica , until HG treatment, compared with the first three treatments, the competitiveness of C. songoricaincreased, and the competitiveness of S. breviflora decreased. It can also be seen from Fig. 3 that compared with other three treatments, the increase of C. songorica density in HG treatment was the largest. According to the intercept of this linear relationship, the value of LG treatment was 0.0223, and the value of HG treatment was 0.0201, indicating that the linear relationship between S. breviflora and C. songorica will be affected by the initial density of S. breviflora in LG and HG treatments. However, in HG treatment, due to the competitiveness of C. songorica has improved, the impact of initial density of S. breviflora will be reduced.

Fig. 4 The relationship between the density of Stipa brevifloraand the density of Cleistogenes songorica under different grazing intensities. CK, no grazing; LG, light grazing; MG, moderate grazing; and HG, heavy grazing.