| Grassland is one of the terrestrial ecosystems that has been intensively influenced by human disturbances.In the past a few years,a large part of grasslands on the Qinghai-Tibetan Plateau have experienced dramatic decreases in vegetation coverage and productivity and increasing declines in ecosystem service function because of the long-term overgrazing.Fencing is one of the common measures for grassland recovery.A study was conducted in an alpine meadow on the Tibetan Plateau,in order to explore the underlying mechanisms of how grazing affected the persistence and development of plant community and nutrient cycle,and examine the changes of community structure,plant functional traits,functional diversity and plant nutrients features under fencing and grazing The main results are as follows:(1)The results of five years showed that: compared with grazing,fencing obviously increased total community cover and aboveground biomass,and interaction between groups and years was significant(P<0.05).For graminoids,the cover and aboveground biomass increased significantly in response to fencing(P<0.05),and showed an increasing trends with fencing time,which were mainly attribute to the increase in the cover and aboveground biomass of Elymus nutans.For sedges,the cover and aboveground biomass were unaffected by grazing,but the interaction between groups and years was significant for aboveground biomass(P<0.05).For forbs the cover and aboveground biomass did not change in response to grazing(P<0.05).The aboveground biomass of sedges in the fencing treatment increased first and then decreased,while the aboveground biomass of forbs showed an increasing trend,which was mainly attribute to the increased aboveground biomass of Anemone trullifolia and potentilla fragarioides.Grazing had no effect on underground biomass,but significantly reduced litter biomass(P<0.05).(2)Compared with fencing,grazing significantly increased the Richness index but decreased the Pielou index of vegetation in 2016(P<0.05).Shannon-Wiener index did not change between the treatments(P<0.05).The three indexes reflecting species diversity showed no obvious difference between the treatments in other years(P<0.05).The results indicated that grazing had limited effects on community diversity.(3)Grazing significantly decreased specific leaf area and plant height at the community level(P<0.05).Under grazing,specific leaf area of forbs tended to decrease at the species level,while there were no obvious changes for sedges and grasses in the two treatments(P<0.05).Leaf dry matter content was significantly and negatively correlated with specific leaf area in both fencing and grazing sites(P<0.05).When SLA is equal,grazing reduced LDMC compared to fencing.When LDMC is equal,the grazing sites had lower plant height.Grazing decreased the average dissimilarity of traits among species(P<0.05),thus resulted in a convergence for the functional traits on the whole,which could be reflected by the narrowed difference in leaf dry matter content and plant height among species.Compared with fencing sites,grazing obviously increased the functional evenness,but decreased the functional divergence(P<0.05).(4)Compared to fencing sites,leaf N content and N/P in leaves of most species under grazing tended to increase(P<0.05),while C/N tended to decrease as a result of a more pronounced effect in unpalatable than in other groups.However,leaf C,P content and C/P remained unchanged(P<0.05).For stem tissue,stem N content obviously increased under grazing at the community level,and stem C,P contents remained unchanged(P<0.05),which resulted in a decreased C/N in stem,while C/P and N/P in stem showed no difference between the two treatments(P<0.05).At the community level,no obvious changes for root C,N,P concentrations and stoichiometry were found between the treatments(P<0.05).However,grazing obviously increased root P content of graminoids,sedges and palatable species,but had no effects on C,N content in root(P<0.05). |
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