| The alpine grasslands of the Qinghai-Tibet Plateau are not only an important animal husbandry base,but also an important ecological security barrier for China and even for Asia.Overgrazing is considered to be a main factor leading to grassland degradation among many human activities.To a certain extent,grazing intensity directly determines the degree of grassland degradation.In this study,two grazing intensities（e.g.,moderate grazing（MG）and heavy grazing（HG））and an enclosure（control,CK）were designed to study the effects of grazing intensity and grazing years on the main species of alpine grassland plant communities in the Pengbo Valley.Firstly,we aimed to study the influence of different grazing intensities on characteristics main species,such as height,coverage and important values.Secondly,we analyzed the response feature of these characteristics to different grazing years,and we systematically analyzed the ecological effects of these two factors on the main plant species.Finally,we analyzed the changes in niche width and niche overlap of major species under different grazing intensity and grazing years.We inspected the variation characteristics of interspecific relationships of major species under different grazing intensities.The study is not only important for the stability and sustainable utilization of the alpine grassland ecosystem,but also has important practical guiding significance for the sustainable development and adaptive management of animal husbandry in the Qinghai-Tibet Plateau.The main results are:（1）Tripogon bromoides is dominant and constructive species at research site and it is also a favorite plant for grazing livestock.There was no significant difference in its height before grazing（2016）,after one year of grazing,there was no significant difference between the moderate grazing and the control,and in the third and fifth year of grazing,its height occurred significant changes,manifested as CK > MG > HG.As the indicator species of grassland degradation,Euphorbia fischeriana,Astragalus strictus and Ligularia rumicifolia showed a certain difference in their height under different treatments.Therefore,different grazing intensities have different effects on the heights of different species,and the heights of different species vary significantly between different grazing years.（2）The results of variance analysis showed that,there were no significant differences in the coverage of Tripogon bromoides,Kobresiapygmaea,Carex atrofusca,Astragalus strictus,Euphorbia fischeriana,Stipa capillacea and Ligularia rumicifolia（P > 0.05）in 2016.After three years of grazing,the cover of Tripogon bromoides and Carex atrofusca was significantly higher under MG than CK and HG（P < 0.05）in 2019.After five years of grazing,in 2021,the grazing treatments（MG and HG）had significantly lower coverage of Pennisetum centrasiaticum and Stipa capillacea than CK（P < 0.05）.Under HG treatment,the coverage of Tripogon bromoides,Kobresia pygmaea and Carex atrofusca was significantly lower than that under CK and MG（P< 0.05）.Therefore,different grazing intensities had different effects on different species cover,but the effects of grazing intensity and grazing years on species coverage were smaller than those on species height.（3）The results of variance analysis showed that,the important values of Tripogon bromoides,Kobresia pygmaea,Carex atrofusca,Astragalus strictus,Stipa capillacea,Ligularia rumicifolia and Ceratostigma minus were not significant differences under each treatment（P > 0.05）from 2016 to 2021.However,the important values of above species fluctuated among different grazing years.Therefore,different grazing intensities and grazing years had little effect on species importance values.（4）Different grazing intensity and grazing years had little effect on species niche width.The Shannon niche width of the constructive and dominant species in the community under each treatment was in absolute dominance,and the niche width was in the order of Tripogon bromoides > Pennisetum centrasiaticum > Carex atrofusca >Kobresia pygmaea.The niche overlap of between species differed under different grazing intensities,but had little difference in different grazing years.（5）Grazing has altered the associativity between species.Under the CK treatment,there was a significant negative association between the Tripogon bromoides and Pennisetum centrasiaticum except for 2019,indicating the two species had a strong competition on resource.Under the MG treatment,the Tripogon bromoides and Carex atrofusca was significantly associated negatively at the beginning of grazing,and then（2017,2019,and 2021）moderate grazing changed the association between species（a significant positive association,or not related at all）.It is worthing that under HG treatment,there was a significant negative association between Pennisetum centrasiaticum and Tripogon bromoides,Pennisetum centrasiaticum and Carex atrofusca,Tripogon bromoides and Carex atrofusca at the beginning of grazing（2016）.Heavy grazing treatments altered the associativity between these species in 2017,2019,and 2021,either positively or not.Therefore,grazing prohibition makes the competition between Pennisetum centrasiaticum and Tripogon bromoides stronger for resources,while grazing weakens the competition intensity.It shows that grazing changes the connection between dominant species and other species by feeding on dominant species.（6）The relationship between dominant species is close,while the relationship between dominant species and other species is loose.Grazing alters the probability of species appearing together in a community.Under the CK treatment,the OI values between the Pennisetum centrasiaticum and Tripogon bromoides,Kobresia pygmaea and Carex atrofusca were all larger in the different the years,indicating that they have a higher probability of appearing together,and the relationship in the community is relatively close.Other species in the community are less likely to appear together,and their relationship in the community is looser.In the grazing areas（MG and HG）at the beginning of grazing（2016 and 2017）,the OI values between Pennisetum centrasiaticum and Tripogon bromoides,Kobresia pygmaea and Carex atrofusca,Stipa capillacea and Astragalus strictus were larger,In the third and fifth years of grazing（2019 and 2021）,only the OI values between Pennisetum centrasiaticum and Tripogon bromoides,Kobresia pygmaea and Carex atrofusca were larger.Therefore,the dominant species and subdominant species are more likely to appear together,while other species are less likely to appear together in the community,indicating that the dominant species are closely related but loosely related to other species. |
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