Effects Of Grazing Exclusion Duration On Plant Functional Traits,Diversity And Productivity In An Alpine Meadow

Alpine meadow is an important part of alpine ecosystem type on the QinghaiTibetan Plateau.However,in recent decades,it has suffered varying degrees of degradation due to the combined impact of climate change and human activities.This degradation has resulted in a range of problems,including reduced vegetation coverage,increased water and soil erosion,as well as a decline in biodiversity and productivity.Since 2003,grazing exclusion has been widely adopted in Qinghai-Tibetan Plateau and places to restore degraded grasslands.Nevertheless,the effectiveness of these measures remains a topic of debate due to the spatial and temporal variability among different studies.Consequently,there is a need for long-term monitoring of grazing exclusion grassland.The study was conducted at the Qinghai Haibei National Field Research Station of Alpine Grassland Ecosystem,focusing on the alpine meadow plant communities under different grazing exclusion durations(0,3,5,8,10,15 years).The firstly objective of the study was to investigate the functional traits of plant species pool in the alpine meadow.Next,the study analyzed plant functional traits and phylogenetic structure to explore the mechanisms of plant community assembly under different grazing exclusion durations.Finally,the study discussed the effects of grazing exclusion on plant community aboveground net primary productivity(ANPP),taking into account species diversity,functional diversity and phylogenetic diversity.The main finding as follows:(1)Among the 100 plants in the regional species pool,the Asteraceae family has the highest proportion of plant species,followed by the Ranunculaceae,Poaceae and Gentianaceae families.In terms of life form,both of leaf carbon content and C:N ratio of annual/biennial plants are significantly lower than that of perennial plants,but the leaf nitrogen content is significantly higher than that of perennial plants.From the view of functional types,the leaf dry matter content of grass and sedge species is significantly higher than that of forb and legume species,while the leaf nitrogen content of forb and legume species is significantly higher than that of grass and sedge species.(2)The community-weighted mean height,leaf dry matter content,leaf tissue density,leaf thickness and leaf carbon content were all significantly increased by grazing exclusion,while the community-weighted mean specific leaf area decreased.However,community-weighted mean leaf nitrogen content significantly decreased in 5and 8 years of grazing exclusion compared to other years.All seven plant functional traits showed phylogenetic signals and weak conservation.The net relatedness index(NRI)and nearest taxon index(NTI)were both greater than zero under different years of grazing exclusion.The community phylogenetic structure exhibited clustering,and environmental filtering was found to be the dominates process in the assembly of plant community under different grazing exclusion durations.(3)The community structure and heterogeneity in grazing exclusion plots differed from natural plots,and the difference became more pronounced with increasing grazing exclusion duration.Grazing exclusion significantly reduced species diversity,functional diversity and phylogenetic diversity in alpine meadow,with the lowest level observed in the 10 th year.Additionally,ANPP was significantly lower in the 5th and15 th years compared to other years of grazing exclusion.Regarding functional types,ANPP of grass species initially increased then decreased,while sedge species did not show a significant change.In contrast,forb and legume species showed a trend of decreased ANPP initially,followed by an increased.ANPP was negatively correlated with the Pielou evenness index,Simpson dominance index and mean nearest taxon distance(MNTD),but no significant correlation was observed with other diversity indices.In summary,environmental filtering plays a dominant role in the process of plant community assembly across different years of grazing exclusion.As grazing exclusion continues,the structure difference and heterogeneity of community increase.Grazing exclusion significantly reduced the plant community diversity of alpine meadow,intensified interspecific competition,and decreased the utilization efficiency of species to resources.Furthermore,grazing exclusion regulates community ANPP by altering the proportion of different functional types.These findings offer a theoretical foundation for the rational utilization and sustainable development of alpine meadow.Due to the influence of uncontrollable factors such as epidemics,the study on the plant community diversity and productivity of alpine meadow at different grazing exclusion durations was conducted only at Haibei Station,and failed to obtain a larger range of survey data.Therefore,the generalizability of the results needs to be further verified,which is the shortcoming of this study.