Effects of exclosure duration on the community structure and species diversity of an alpine meadow in the Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau is one of the richest areas in terms of biodiversity in China. The alpine meadow is the most widespread vegetation type on this plateau and plays an important role in the carbon cycle, conservation of biodiversity,and livestock husbandry. However,climate change and human activities have greatly affected this already fragile ecosystem. Although exclosure,one of the effective measures,has been widely applied in grassland restoration,the impact of exclosure duration on community structure and species diversity is of particular interest. Therefore,determining the influence of exclosure duration on the alpine meadow is important for the development of sustainable livestock husbandry. We examined the community structure and species diversity in patches of Kobresia humilis alpine meadow,which had been enclosed for 2,7,or 17 years. We have discussed possible reasons for the changes in community structure and species diversity. The results indicated the following: 1) With increase in exclosure duration,the percentage of grass species in the upper community increased significantly,especially of Stipa aliena. The percent change was 13. 87% for2-year,21. 76% for 7-year,and 23. 95% for 17-year exclosure. The percent change of leguminous species significantly decreased,whereas sedge and forbs species exhibited no obvious change in exclosure duration. 2) The [alpha]-biodiversity,Patrick index,and Margalef index of the plant communities indicated no significant changes in exclosure duration. In contrast,the Simpson index,Shannon-Wiener index,probability of interspecific encounter index,Alatalo index,and Pielou index decreased in exclosure duration( 2-year 7-year 17-year enclosures); there were significant differences between the 2-year and 7-year exclosures and between the 2-year and 17-year exclosures( P 0. 05). 3) The [beta]-biodiversity,Cody index,and Morisita-Horn similarity index indicated that differences inspecies were greater between 2-year and 7-year exclosures,as well as 2-year and 17-year exclosures,than that between 7-year and 17-year exclosures. 4)The amount of plant litter decreased in exclosure duration,but differences were not significant. Litter coverage time appeared to be one of the reasons for the decrease in species diversity because the litter layer could prevent the seeds from falling to the ground. 5) Soil organic carbon( SOC),soil total nitrogen( STN),and soil total phosphorus( STP) increased in exclosure duration due to