Effects Of Grazing Exclusion By Fencing On Ecological Restoration Of Degraded Alpine Grasslands On The Northern Tibetan Plateau

Located in the hinterland of the Qinghai-Tibet Plateau,the Northern Tibetan Plateau is the most traditional pastoral area and the most important livestock base of Tibet.Alpine grassland is the most widely distributed natural ecosystem in the Northern Tibetan Plateau,serving as the basic production material for the development of local livestock industry.Driven by both climate warming and overgrazing,alpine grassland ecosystems in the Northern Tibetan Plateau have become severely degraded over the past years,with consequences of declined plant productivity and diversity.Grazing exclusion by fencing is one of the common measures for ecological restoration of degraded grassland on the Qinghai-Tibet Plateau,but its effectiveness is controversial.To scientifically evaluate the effects of short-term(2012 sampling)and long-term(2021 sampling)grazing exclusion by fencing on soil and plant community characteristics of alpine grassland,alpine steppe and desert steppe in Northern Tibet enclosed since 2006 were selected as the research object.Soil quality,root biomass,above-ground biomass,plant species diversity and nutrient content inside and outside the fences were measured and compared.We obtained the following main results:1)There was no significant effect of long-term grazing exclusion by fencing on soil quality and plant productivity of alpine steppe and desert steppe in Northern Tibet.The15-year grazing exclusion by fencing did not significantly change total carbon,total nitrogen,total phosphorus,organic carbon,trace elements,microbial biomass carbon and microbial biomass nitrogen contents and enzyme activities in topsoil,nor did it significantly change the plant aboveground and belowground biomass.However,the aboveground biomass in fenced plots of alpine steppe and desert steppe in 2021 increased significantly by about 52% compared with 2012.2)Long-term grazing exclusion by fencing reduced plant species richness in desert steppe,but had no significant effect on alpine steppe.No significant differences in plant Shannon-Wiener index and Pielou evenness index of alpine steppe and desert steppe and species richness of alpine steppe were found between fenced and grazed sites,while the plant species richness of desert steppe inside the fences is significantly lower than outside the fences.The plant species richness of alpine steppe in the enclosure increased in 2021 compared with 2012,while the Shannon-Wiener index and Pielou evenness of desert steppe in the enclosure decreased in 2021.3)Long-term grazing exclusion by fencing had no significant effect on plant nutritional quality of alpine steppe and desert steppe in Northern Tibet.15-year grazing exclusion by fencing did not significantly alter contents of plant crude protein,crude fat,neutral detergent fiber,acid detergent fiber,lignin,cellulose,crude ash and crude fiber.Soil quality and climatic conditions were the key factors affecting plant productivity,diversity and nutritional quality.Growing season precipitation was the most important environmental factor regulating plant productivity in alpine grassland,while growing season temperature was the most important environmental factor affecting plant nutritional quality.In conclusion,long-term grazing exclusion by fencing had limited effects on soil quality,plant productivity and plant nutritional quality of alpine steppe and desert steppe.Climatic condition was the main environmental factor affecting soil quality,plant community structure,productivity and nutritional quality.The results of this study are helpful to better understand the restoration process of degraded alpine grassland,and provide data support and theoretical basis for formulating more reasonable layout and management policies of fencing projects.