Community Phylogenetic Structure Of Grassland And Desert Ecosystem In Arid And Semi-arid Region

The coexistence of species in the community and species'interaction with the abiotic environment has always been the focus of ecological research.At present,similarity limitation,habitat filtering and neutral theory are the three main mechanisms which are used to explain the maintenance of community construction.Researches on community phylogenetic structure emphasize the relationship between phylogenetic relationships and their phenotypic traits and niches among coexisting species within the community.Generally,closely related species tend to share similar niches,which will result in the more intensive competition for resources.Therefore,by analyzing the relative phylogenetic distance between co-occurring species within the community,the strength of the interaction between species and its relationship with the abiotic environment can be determined,thus revealing the community construction process and the coexistence mechanism among different species.However,there is still a lack of in-depth understanding of the mechanisms for community composition and species coexistence in arid and semi-arid regions.In present study,the grassland and shrub ecosystems in the Mongolian Plateau were chosen as typical cases,and the Net Relatedness Index?NRI?was used as the core index to systematically study the phylogenetic structure of the Mongolian Plateau.Firstly,the effects of scale and species pool on phylogenetic structure of grassland ecosystem were studied.Then,the effects of different grazing pressures on community phylogenetic structure under controlled experimental conditions were analyzed.Finally,the control experiments and regional-scale community surveys were combined to explore the community construction mechanism and its influencing factors of grassland and shrub ecosystems in arid and semi-arid regions.The main results and conclusions of this study are as follows:1)The community phylogenetic structure is greatly influenced by scale and species pool.The phylogenetic overdispersion increases with the increasing scale when considering only species presence or absence(i.e.,NRI_pr).However,when the species are weighted by abundance?represented by relative mass?,the scale has little effect on the community phylogenetic structure(i.e.,NRI_ab).The phylogenetic clustering increases when the species pool is expanded from the local scale to the regional scale,indicating that species migration across regions exerts influence on the construction of local communities.2)The NRI_pr is reflective of the background phylogenetic structure of the community and is less susceptible to interference;NRI_ab is more sensitive to the change of external environment.3)Different grazing pressures increase the phylogenetic clustering.The degree of phylogenetic clustering is the highest under the light grazing pressure,while the degree of clustering decreases with the increasing grazing pressure.This unimodal trend is related to the selective feeding of livestock that alters the relative competitive advantage of gramineous plants in the community.4)Strong phylogenetic signals are found in plant functional traits in grassland and shrub communities on the Mongolian Plateau.The leaf length of herb and leaf width of shrub?and subshrub?show significant phylogenetic conservation,that is,species with close relatedness show similar functions traits than expected.5)The phylogenetic structure of the grassland ecosystems in the Mongolian Plateau shows an overall NRI_pr,indicating that the driving factor for the construction of background community is similarity limit?i.e.niche differentiation?.When the abundance weight?relative quality?is taken into considered,the clustering of community phylogeny is reduced,although no obvious phylogenetic structure is exhibited.The relative mass of Gramineae and regional factors are important factors that having an effect on NRI_ab,indicating that human disturbance plays an important role in reducing the phylogenetically overdispersion;functional NRI(i.e.,NRI_fun)shows significantly clustering,which is affected by environmental filtration and interference.6)The shrub community in the western Mongolian Plateau also shows a trend of phylogenetic overdispersion,but there are differences in different habitats.Among them,the typical desert and sandy shrub communities show phylogenetic overdispersion,while the steppification desert communities has a tendency of phylogenetic clustering.The community phylogenetic structure of shrub communities on the Mongolian plateau tends to be clustering in steppification-desert transition zone due to the lower temperature and the competition of herbaceous plants.The increase in mean temperature of driest quarter?MTD??i.e.,low temperature in winter?since the Last Glacial Maximum?LGM?is conducive to the phylogenetic overdispersion of shrub community.7)Inter-regional species migration significantly affects the formation of community species assembly.The diffusion restriction isn't treated as the main limiting factor of community phylogenetic structure and species migration on the Mongolian plateau.The climatic factors restrict the phylogenetic?diversity of the shrub communities,indicating that on the regional scale,habitat filtering is an important factor affecting the community's phylogenetic dissimilarity,but at the local scale,the limiting similarity determines the construction of the shrub communities on Mongolian plateau.The results of this study highlight that,1)habitat filtering is not the main driving factor for the construction of grassland and shrub communities on the Mongolian Plateau;2)climatic factors have little effect on community phylogenetic structure;3)diffusion restriction has little effect on regional community structure differences,in contrast,migration plays an important role in community species assembly.To sum up,this study suggests that climate change,at least to a certain extent,will not severely impact the community construction of the main ecosystems on the Mongolian Plateau,while excessive human disturbance?e.g.long-term overgrazing?is responsible for the change of the community structure in arid and semi-arid regions.