Abstract: Fertilization and enclosure are two important factors affecting plant biomass and diversity of alpine mea-dow. It is of great scientific significance to elucidate the processes and mechanisms underlying how both factors affect plant community for the sustainable development of alpine meadow. In this study, we analyzed the responses of plant community and species richness to fertilization (nitrogen, phosphorus, and potassium) and enclosure (grazing exclusion) in an alpine meadow on the eastern Qinghai-Tibet Plateau, and examined the underlying mechanisms of species loss. The treatments of fertilization and fertilization + enclosure significantly increased community aboveground biomass, while enclosure alone had no such effect. Community light transmittance and species richness significantly decreased in all the three treatments. At the functional group level, changes of aboveground biomass under fertilization and fertilization + enclosure mainly resulted from a significant increase in grass biomass and a significant decrease in sedge biomass. There was no significant difference in miscellaneous forb biomass among different treatments. At the species level, species composition changed significantly under fertilization and fertilization + enclosure treatments, with Elymus nutans and Poa poophagorum gradually replacing Kobresia setschwanensis as the dominant species. There was a significant negative linear relationship between light transmittance and total community biomass and a significant positive linear relationship between community light transmittance and species richness, indicating that the increase of total biomass intensified the light limitation in the lower layer, and then led to the decrease of species richness through light competition. In summary, our results demonstrated that increased light competition caused by the increases of total biomass was an important driver for species loss under fertilization and enclosure conditions, providing a scientific basis for future biodiversity maintenance and management of alpine meadow.

Key words: species richness, light competition, aboveground biomass, functional group.