Effects of Different Grazing Intensities on the Community Phylogenetic Structure of Grassland in Arid and Semiarid Region

Grazing is the main way of utilizing natural grasslands in Inner Mongolia. However, over the past several decades, grasslands on the Mongolian Plateau have experienced severe degradation due to unreasonable grazing. The changes in grassland biodiversity and productivity caused by grazing have been extensively studied, but studies at the level of community phylogenetic structure are still lacjing. In this study, four different grazing intensities, including no grazing, light grazing, moderate grazing, and heavy grazing were established, and the net relatedness index (NRI) was used to characterize the community phylogenetic structure. We studied the relationship between the biomass of three dominant grasses and the community phylogenetic structure, aiming to explore how different grazing intensities affected community phylogenetic structure in grassland ecosystems and its underlying mechanisms. The results showed that under different grazing intensities, the phylogenetic structure of typical grassland communities showed a clustering trend, and the NRIs under different grazing intensities were significantly higher than those of the non-grazed plots, indicating that habitat filtering played a dominant role in community construction, while grazing increase the degree of the community clustering. However, with the increased in grazing intensity, the proportions of the NRI and dominant species both first increased and then decreased. Linear regression results showed that the NRI was related to the relative quality of dominant species, indicating that changes in community phylogenetic clustering caused by grazing were related to the competitive advantages of several dominant grasses in the community over distant species caused by exclusion. This study shows that grazing as a habitat filter itself increases the degree of clustering of the community phylogenetic structure, and the selective feeding of livestock amplifies this advantage and leads to a further increase in the degree of phylogenetic clustering.