Research On The Mechanism Of Species Diversity Loss Due To Fertilization Of Alpine Meadow On The Tibetan Plateau

The earth is experiencing ecological and environmental changes because of human-driven land use change, eutrophication, and accelerating loss of biodiversity has become a primary concern.Ecologists have long concentrated on the species diversity maintenance mechanism of plant community. Community assembly theory has traditionally been divided into niche theory based on competition and neutral theory based on dispersal. However, some ecologists found that the progress of plant community ecology is not insufficient, and niche theory and neutral theory can not explain the community assembly. Recently, most plant ecologists tend to believe that niche theory and neutral theory are not opposite, and proposed that a concept of integrated community including stochastic processes and determined processes.Ecologists have long concentrated on explaining the relationship between productivity and diversity, but the oberserved patterns are different. Diversity usually decreases or shows a unimodal pattern along natural gradients of increasing productivity. However, artificial fertilization gradients, the pattern is more consistent:diversity always decreases with increasing productivity. Many hypotheses have been proposed to explain diversity loss, yet understanding the mechanisms responsible for the reduction in plant diversity after fertilization remains a great challenge. In addition, general theories that attempt to account for the species composition and dynamics of ecological communities have as their major axes variation in productivity and disturbance, but productivity and disturbance, however, are often correlated and each can have both direct and indirect effects upon the other.In the past ten years, traits-based approaches can open a path to faster progress in community ecology, the explanation and the prediction of the response of plant community and species relative abundance to nutrition enrichment by functional trait has recently attracted considerable interest. However, the linkage between functional trait response and species abundance distribution along environmental gradients often fail in field study. In this study, we selected three key plant functional traits (specific leaf area, height and seed size) to further understand the mechanism of species diversity loss due to fertilization. Our objectives of this study are as following:1) understanding the mechanism of species diversity maintenance by fertilization, clipping and shading; 2) Seeking the reasonable explanation leading to the diversity loss due to fertilization by measuring three key functional traits for 30 common component species in fertilized and unfertilized communities in a Kobresia capillifolia dominated alpine meadow on the Tibetan Plateau.We conducted 3 years experiments in perennial herbaceous communities in an alpine meadow on the Tibetan Plateau at the Research Station of Alpine Meadow and Wetland Ecosystems of Lanzhou University (N 33°58', E101°53'). We applied four levels of fertilization, three intensities of clipping and two levels of shading in a factorial design to study the effects of fertilization, clipping and shading on species diversity and productivity. We chose 30 component species at their flower phases in fertilizing (two levels) from 2008 to 2009 to measure three key functional traits (specific leaf area, height and seed size). Correspondingly,9 quadrants were sampled in each treatment at middle of September in each year for investigates the response in communities level. Light and soil resource of each treatment were measured at same time of vegetation monitoring. Univariate general linear models (GLM) was used to estimate the effects of fertilization, clipping and shade on above- and belowground vegetation characteristics. We performed a linear regression modeling to test the relationship between the response of species relative abundance and individual functional traits in the fertilized and control community. Data for each growing season were analyzed separately.The results are as following:1) There was a significantly negative correlation between productivity and diversity intergrated from fertilization and clipping treatments.2) Fertilization significantly decreased the species richness and diversity and the trend of declining increased with fertilization levels increasing; Clipping significantly increased species richness and diversity and the trend of increasing increased with clipping levels increasing.3) There was a significantly positive correlation between individual density and diversity, but we used the density as covariate, fertilization and clipping remain keep significant effects on species richness and diversity.4) Fertilization significantly increased above-ground biomass and these changes were mainly driven by the response of grasses in our results; Clipping significantly increased above-ground biomass and these responses were mainly driven by the compensatory growth of forbs.5) Photosynthetically active radiation was similar in the understory of shaded and fertilized plots, but only fertilization reduced species richness and diversity, suggesting that light competition alone could not explain diversity loss after fertilization; The root biomass and R:S ratio had a significant increase in shaded plots, but the richness and diversity did not change, suggesting that root competition alone also could not explain diversity loss after fertilization in this community.6) Specific leaf area and height of the component species significantly increased in response to fertilization, yet not obvious change among functional groups were observed (grasses, forbs, and legumes); The response of species relative abundance was significantly positively correlated with specific leaf area and height, but not with seed size.From this study, we have drawn the following conclusions:1) The interaction between fertilization and clipping has significant effects on species richness and diversity and clipping can maximize the richness and diversity with fertilization levels increasing.2) Density effect can not explain plant community change after fertilization.3) Neither root nor light competition alone could explain diversity loss after fertilization in this alpine meadow community, and the interactions between root and shoot competition, investigated from a functional groups perspective, should be themost reasonable explanation leading to diversity loss due to fertilization.4) Definite processes based on species functional traits can explain plant community structure after fertilization and specific leaf area and height can be used as a practical guide for predicting species diversity loss following fertilization.