Fig. 3 Population density difference diagrams of Cleistogenes songorica at different grazing intensities and spatial scales. Mean values (±SD ; n = 24) of density. Different lowercase letters indicate that grand means differ significantly between the grazing treatments (P < 0.05). (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.
Inter-specific association between Stipa breviflora andCleistogenes songorica populations
It can be seen from the Jaccard index (Table 3) of S. brevifloraand C. songorica populations that the value of Jaccard index under grazing treatment was greater than that under non-grazing treatment at different scales, indicating that grazing does increase the inter-specific affinity of S. breviflora and C. songorica . When comparing two sites under the same grazing intensity, we observed that the value of the Jaccard index increased as our sampling sized increased, implying an increase in the inter-specific affinity. At the spatial scale of 5 cm×5 cm, the Jaccard index showed a strong inter-specific competition between S. breviflora and C. songorica . At the spatial scale of 100 cm×100 cm, we observed the highest compatibility between S. breviflora and C. songorica under grazing. At this scale, it appears that grazing intensity has little effect the relationship between these grass species. With the increase of grazing intensity, the inter-specific affinity between the two species increased. Notably, we observed that the influence of grazing on inter-specific affinity was greatest at the smallest sampling resolution, and generally decreased as sampling resolution was increasingly coarse. In summary, grazing increased the inter-specific relationship affinity (competitive ability decreased) between S. breviflora and C. songorica in order to resist grazing disturbance, and this affinity was particularly prominent in large scale.
Table 3 Jaccard index of Stipa breviflora and Cleistogenes songorica populations