The Response Of Community Structure And Function To Fertilization And Grazing In The Eastern Alpine Meadow Of Tibetan Plateau

The degradation of plant communities, especially in an ecologically fragile area, such as in an alpine meadow plant community on the Tibetan plateau, has caused great concern by ecologists. We carried out a series of original observations and control experiments in the Qinghai-Tibetan alpine meadow, trying to test community function and theirs potential mechanisms by way of the response of species characteristics and community structure to environmental changes. Then explore the mechanisms of plant community degradation, and in order to provide a theoretical basis for the recovery of the alpine meadow plant community. Our main objectives are as follows:(1) To test the response of species richness to environmental changes and the reasons that caused species richness decline,(2) To explore the response of species characteristics to environmental changes and potential patterns operated in this process.(3) To explore the common patterns of the response of reproductive allocation to environmental changes.(4) To test the effect of environmental changes on the temporal variability of community cover and cover of component species and to determine the importance of underlying mechanisms proposed for the diversity-stability relationships.Our main results are as follows:1 The effects of fertilization and grazing on species richness in alpine meadow plant community-potential patterns and processesOur results found that the competition of above and below ground is asymmetrical among species, with a change from belowground competition to aboveground competition following fertilization when grazers were excluded, which lead to species diversity drops. In contrast, there was no significant difference in aboveground competition ability and weak increase in belowground competition ability from grazing to grazing following fertilization. An alternative potential explanation for the biodiversity loss in grazing following fertilization is that belowground physical space and soil nutrients together determine the species richness variation. Our study also suggested that grazing did not change the fertilization effects, but delayed the effect of fertilization on species richness. This finding provided a strong experimental evidence for the intermediate disturbance hypothesis.2 The shifts of trait distributions across soil nutrient gradient in alpine meadow plant communityUsing a functional trait approach, we have found that fertilization caused plant communities to remain convergence in species traits within plots. Our studies also suggested that at least two distinct niche-based processes, environmental filters and limit similarity, contributed to species coexistence in grazing and enclosure following fertilization. In contrast, co-occurring species are often more ecologically similar in grazing and enclosure without fertilization. A result indicated either a balance between the two mechanisms or neutral processes control structure and function of community. Our results indicated that soil nutrients availability played an important role in the assembly of alpine meadow plant community at local scales.3 The effect of grazing and fertilization on allocation strategy in the Tibetan alpine meadowOur results suggested that both treatments and species identity significantly affected the allometric coefficient and the allometric exponent of the relationship between log (reproduce biomass) and log (vegetative biomass), although the effect of treatments on the allometric exponents is marginally significant. This suggested that treatments and species significant changed the efficiency of conversion of total plant biomass into reproductive biomass. The primary effects of the genetic marker on the reproductive output occurred via plant size (species identity), but the effect of treatments on species'allometric trajectory was relatively small compared with species identity. In other words, the reproductive output of species may chiefly be decided by maternal effects of species (plant size); but in the special climates in the alpine meadow of Qinghai-Tibetan, environment conditions as a phylogenic cause, which formed the severe selection pressures on local native plant species growth, also play a more important role in the process of the species evolution.4 The effects of long-term fertilization on the temporal stability of alpine meadow communitiesAlthough fertilization strongly reduced species richness, the temporal variability in community cover decreased with fertilization. Most species showed an increase of temporal variability in population cover with fertilization, but two grass species, which dominated fertilized communities after ten years, showed an increase of stability. Detailed analysis revealed that the increased dominance of these two highly stable grass species was associated with increased community stability at high levels of fertilization. In contrast, we found little support for other mechanisms that had been proposed to contribute to community stability, such as changes in asynchrony and portfolio effects. We conclude that the effect of dominant species on community stability outweighed the effect of species richness after fertilization.