Community Assembly Of Sub-alpine Meadows In The Eastern Qinghai-Tibetan Plateau,China

Community assembly has become a more important issue in ecology under the background of global biodiversity loss. Currently most ecologists generally believe that community assembly is mainly up to the joint effects of regional and local process. Regional process (geological and historical accidents, speciation, migration and dispersion and distinct) could determine current numbers of local species pool. Moreover it limits species categories which can successfully colonize local community, so it is the key ecological process of exploring the differences of species diversity among different local communities. Local process which includes species interaction, habitat filtering and dispersal limitation and so on can choose and determine the numbers of coexisting species in local community ultimately, so it is the key process of investigating the species coexisting mechanism of single local community. Hence understanding how these processes work in communities at different successional stages is important for developing effective preservation and restoration strategies.We examine community assembly using a chronosequence of sub-alpine meadows in Qinghai-Tibetan Plateau that range from’natural’(never farmed), to those that have been protected from agricultural exploitation for1,3,7,9and10years. We use method of multivariate variance partitioning, spatial analysis, MEM analysis and model fitting to test the role of local and regional process in determining community assembly along the successional gradient from the combined aspect of species diversity and functional diversity.From the aspect of species diversity, we firstly examine the relationship between species richness (S) and evenness (Pielou’s J’evenness) and their assembly mechanism using the data of30sampling quadrats with size of0.5m×0.5m of each meadows being studied. We also test for shifts in species and traits among meadows and also for changes in environmental and spatial correlates of species distributions within meadows. In addition, we explore the relationship between species pool and beta diversity of each experimental sites. From the aspect of functional diversity, we investigate the relationship between functional trait diversity of each successional meadow and successional age. We compare the observed trait metrics and null ones which are generated by regional process too. After that we perform MEM and multivariate variation partitioning analysis to explore the role of environmental and spatial variable in explain the variation of functional composition of each successional meadow community. Meanwhile we calculate ecological distance of species and functional composition of each successional community. At last comprehensive and comparative analysis of these results were carried out. We found that:1. Degrading meadows have very fast recovering speed after establishing fence to forbid agricultural planting. Species richness and composition are all associated with successional age, but the differences of species pool among these meadows can’t give rise to the differences of species composition of each local community.2. Functional trait diversity also show a similar increased variation with the increasing successional age to species diversity. However there only exists a clear significant trait convergence along the successional gradient at trait level, neither convergence nor divergence at species level are observed.3. Although all meadows has their specific evenness and physical environment along the successional gradient, there exist a consistent negative correlation between S and J’in these successional sub-alpine communities along the successional gradient at the sampling scale of0.5m×0.5m.4. Niche-preemption dominated niche differentiation mechanism determines the assembling mechanism of each studying meadow.5. Like species richness and evenness, inter-specific traits variation of all meadows are also associated with successinal age. Abundant species had high seed mass and specific leaf area in late-successional meadows, whereas the opposite occurred in early-successional meadows.6. Local environmental and spatial variables accounting for less than50%of the variations in species and functional composition. Despite these shifts among meadows of different ages, spatial distributions of species within meadows did not change, when associated with abiotic variables. Although soil phosphorus demonstrate a significant change with successionalg age, spatial patterns of species and functional trait within meadows only were reflected by changes in soil pH and nitrogen. In light of all results above, we can draw the following conclusions:1. Although there exist distinctive local species pool and species composition along the successional gradient, all successional meadows have consistent community assembly mechanism which namely large scale regional process (successional age) instead of local process(habitat filtering, species interaction and dispersal limitation) dominates community assembly of each local successional meadows. Hence there is no need to take e different local assembly processes of each successional meadows into account.2. Regional process determines the differences of each local species pool along the successional gradient, local species pool causes the different inter-specific trait varaition of each successional meadows in turn. Meanwhile such trait variation affect species composition ultimately. Hence, seed addition or restoration of soil seed banks may be an effective tool to protect and recover seriously degrading meadows.3. Niche-preemption is the dominating community assembly mechanism of each local community. However biotic processes (weak competitor exclusion) instead of abiotic processes result in such niche differentiation (niche-preemption).