| Grazing has been the main utilization process on the alpine steppe in the Qinghai-Tibetan Plateau.As the most important domestic livestock,Yak and Tibetan sheep exert different influences on the grassland ecosystem due to their distinct grazing behaviors.Therefore,the responses of plant functional trait and plant stoichiometry(C:N:P)of different hierarchical levels(from individual to functional groups)to livestock assembly(yak,Tibetan sheep,or both in different mixing ratios)will provide useful information to implementing rational grazing management on the alpine steppe.This study was conducted on an alpine steppe in the Qinghai Lake basin.Six treatments were set: Only yak grazing(YG),Only Tibetan sheep grazing(SG),Yak and Tibetan sheep grazing mixed as 1:6(MG1:6),Yak and Tibetan sheep grazing mixed as 1:4(MG1:4),Yak and Tibetan sheep grazing mixed as 1:2(MG1:2),No grazing(NG).We had measured 16 individual traits,including whole plant traits,leaf traits,leaf chemical stoichiometric and crude fiber of nine major species,Stipa purpurea,Stipa sareptana var.krylovii,Leymus secalinus,Kobresia humilis,Carex aridula,Astragalus laxmannii,Potentilla acaulis,Dracocephalum heterophyllum,Stellera chamaejasme,as well as biomass,coverage,stochiomestry and crude fiber content of four functional groups.The main results are summarized as follows:(1)Effects of different grazing modes on major speciesThe responses of whole plant traits(plant height,Individual weight and mass-height ratio)to grazing were greater than those of leaf traits(specific leaf area,specific leaf weight,leaf thickness and leaf dry matter content).Under grazing,the specific leaf weight of many species decreased,the leaf nitrogen content and leaf chlorophyll content increased,and the photosynthetic rate increased,indicating that the species responded to environmental change mainly through the rapid reaction mechanism of accelerating material absorption and utilization under grazing,the mechanism of overcompensation growth under moderate grazing stress is explained.The sensitivity of the same traits of different species to different grazing methods was different,and the sensitivity of the same traits of different species to grazing modes was different,asymmetric response is the main response mechanism of alpine steppe vegetation to grazing modes.(2)Species sensitivity of functional groups under different grazing modesThe results of the sensitivity traits of grass species under different grazing modes show that: The disturbance degree of grazing on grass species was SG>MG1:6 and MG1:4> MG1:2>YG,the sensitivity traits of sedge species was compared,the disturbance degree of grazing mode to sedge species was MG1:2> MG1:4> MG1:6> YG and SG,respectively.The results showed that the disturbance degree of grazing mode to legume species was MG1:4> MG1:6,MG1:2 and YG>SG;The results showed that the degree of disturbance to the forb species was YG and MG1:6>MG1:4>MG1:2>YG,respectively,the results of the comparison of the total sensitivity traits of the Functional Group showed that the disturbance degree of grazing mode to grassland was MG1:4> MG1:6and MG1:2> SG>YG,respectively.(3)Effects of different grazing modes on functional groupsThe results of functional groups also showed that the effect of grazing modes on grass biomass was much greater than that of other functional groups,the response of nitrogen and phosphorus to grazing was greater than that of plant carbon,and the response of coarse fiber of functional groups to grazing was worse,overall,there has been no significant change.To sum up,grazing with different livestock on alpine steppe has different effects on plant functional traits.Comparatively speaking,grazing with Tibetan sheep and mixed grazing have more effects on main species of alpine steppe than grazing with yaks.In addition,the response of alpine steppe plants to grazing was more sensitive at species level than at functional group level.The rapid response mechanism of leaf traits under grazing mode reveals the compensatory growth mechanism of grassland to some extent,which provides basic data for rational utilization and scientific management of alpine steppe. |
