Discussion
When the external environment changes, dominant species can choose different strategies to maintain growth, survival and fecundity (Liu et al., 2016). New biomass, for example, is allocated to leaves and roots, or to organs such as stolons, rhizomes and seeds to improve plant access to resources (Drenovsky et al., 2008; Hermans et al., 2006). S. breviflora and C. songorica are the dominant species in desert steppe, and have obvious control effect on the formation of community composition and community environment in the desert steppe. Perhaps due to the strong grazing resistance of these two species, we did observe increases in density as grazing increased (Fig. 2-3). We also found that a broader sampling method (sampling a 100 cm×100 cm quadrat compared to a 5 cm×5 cm plot) resulted in increases in plant density. This phenomenon may be related to the random walking, trampling and foraging behaviors of grazing livestock (Limb et al., 2010; Lin et al., 2010). As the grazing resistant S. breviflora and C. songorica are gramineous plants that can quickly absorb nitrogen and regenerate from their grazed meristem, which may also be one of the reasons for their increased density even under heavy grazing (Stampfli et al., 2018).
It should also be noted that S. breviflora and C. songorica are perennial clumpy grass, but the difference is thatS. breviflora is dense clumpy grass, and the tillering node is located on the ground, which is easily affected by grazing behavior, resulting in the fragmentation of plant clusters. C. songorica is sparse clumpy grass, and the tillering node is located underground. Because of this, simple grazing behavior does not change the number of plant clusters, which may explain why the plant communities appeared to fragment early in S. breviflora (the LG treatment), but not in the C. songorica species until grazing intensity was sufficiently high (the HG treatment) (An and Li, 2015; Lv et al., 2019; Salt and Mayes, 1991). This physiological difference may also explain why we observed the competitive advantage of C. songorica in HG region (Fig. 4). In this experiment, with the increase of grazing intensity, the linear relationship between S. breviflora and C. songorica was also affected by the initial density of S. breviflora (intercept of regression line in Fig. 4). The shift of the regression line on the longitudinal axis and the increase of the regression slope will lead to the relationship between the two become more complex.
The formation of plant population relationships is closely related to the environmental conditions of the community (Wang et al., 2015). Grazing may alter the availability of environmental resources in communities and ultimately alter interactions between species (Liu et al., 2000). In this study, with the increase of grazing intensity and spatial scale, the inter-specific relationship affinity between S. breviflora and C. songorica increased, and the inter-specific competition ability decreased. This is consistent with the results of Soltani et al. , which suggest that long-term grazing generally reduces competition between dominant species (Soltani et al., 2020). The increasing affinity of the two may be interpreted as a tolerance or resistance to grazing disturbance (Levine and HilleRisLambers, 2009). It should also be noted that at the scales of 10 cm×10 cm and 50 cm×50 cm, the inter-specific affinity of S. breviflora and C. songorica in the LG treatment was higher than that of CK and MG, which implies the existence of a critical point where scale influence inter-specific relationship transformation. It also raises questions about the potential biases induced by different sampling regimes, something which should be considered for studies in the future.
The change in inter-specific association can also be explained by the ecological compensation effect caused by grazing. In the absence of grazing, the limiting factor of population growth is natural resources, and the ability of a given species to access resources induces/produces competition. In the S. breviflora desert steppe, grazing could lead to niche differentiation in the ecosystem, which could be directly reflected by the ecological compensation effect between species. This ecological compensation effect changed the resource utilization mode of species (Fock and Kraus, 2016; Liu, 2019). When grazed, the community is exposed to a new pressure the top down influence of livestock feeding and trampling (Levine and HilleRisLambers, 2009). The pressure gradient hypothesis also points out that competition plays a major role in the natural environment. When the interference exceeds a certain limit, the inter-specific interaction between plants will decrease (Bertness and Callaway, 1994; Wan et al., 2011). Therefore, in this experiment, heavy grazing led to the decrease of inter-specific competition ability of dominant grass populations in desert steppe. In summary, in order to better explore the changes in inter-specific relationships of plant populations under grazing disturbance, specific analysis should be carried out in combination with habitat conditions, plant physiological and biochemical characteristics, and the internal mechanism of plant populations should be explored.