| It has been the most important issue for decades for ecologists to explore the mechanisms underlying the community assembly. However, there are still many uncertainties in it, and particularly for the alpine meadow community, it is still unclear. From the debates between super-organism concept and individualistic concept to the debates between niche theory and neutral theory, a problem always perplex ecologists: whether the community assembly is a deterministic process or a neutral process? The main purpose to determine the mechanisms of community assembly is to explain and predict the variation of ecosystem functioning, such as the productivity of ecosystem which plays an important role in ecosystem services and human live. The objects of our study is to answer the following questions: The primary productivity of alpine meadow community in Tibetan Plateau is limited by nitrogen or phosphorus? How the productivity of alpine meadow community will vary under the global climate change? And what are the mechanisms underlying the changes?To answer these questions, two fertilization experiments that with different nutrients and gradients were conducted in an alpine meadow in eastern Tibetan Plateau. We measured six functional traits for 24 common species in each treatment(height; specific leaf area, SLA; leaf dry mass content, LDMC; seed size; leaf nitrogen content and leaf phosphorus content). Community structure(species abundance, species cover and species biomass) and soil properties(soil available nitrogen, soil available phosphorus, soil pH, soil water content and soil temperature) were also measured. We systematically analyzed those data and found:1, The production of alpine meadow is co-limited by both nitrogen and phosphorus. Nitrogen is the primary limiting nutrient in our study site, after nitrogen limit alleviated by fertilization phosphorus becomes limiting nutrient. There is little effect of potassium addition on production.2, Nitrogen fertilization significantly influenced the relationships between species relative abundance and functional traits, but the effect of phosphorus on abundance-traits relationships was little. The trait-based competitive(e.g. light competition) process was ubiquitous in alpine meadow community, especially after fertilization. Our results support previous studies showing that functional traits have considerable potential for explaining and predicting species relative abundance in different habitats.3, The competition-dispersal trade-off exists in alpine meadow community and species are constrained to be either good competitors or dispersers, but not both. We also found that the effect of competition-dispersal trade-off on species abundance distribution depended on spatial scale.4, It is better to use combination of different aspects of diversity(e.g. functional diversity and phylogenetic diversity) rather than single species diversity for explaining variation of production after fertilization. In accordance with previous studies we found the two important hypothesis, mass ratio hypothesis and niche complementary hypothesis, simultaneously exist in alpine meadow community without mutually exclusive of each other.5, The deterministic traits-abundance relationships ubiquitously appeared both in control and fertilized community, those results documented that trait-based niche process play an important role in community assembly process.6, Species coexistence and community structure should be determined by the balance between two antagonistic processes associated with trait convergence and divergence: habitat selection and competitive exclusion. Habitat selection was the main force in low nitrogen addition plots, while the competitive exclusion played a more important role with increased nitrogen addition, and finally these two antagonistic forces reach balance in highest nitrogen fertilization treatment. However, we did not find there were significant effects of phosphorus fertilization on the assembly process of alpine meadow community. |
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