The Impact Of Grazing Intensity And Habitat Resources On The Compensatory Growth Of Alpine Meadow Communities

Compensatory growth commonly defined as a positive response of plants to defoliation damage, which was affected by grazing intensity,habitat resources availability.The compensatory ability of grassland community is related with the species diversity of community and primary productivity,it was concerned in grazing ecology.There were more studies about compensatory growth which fastened on the level of individual plants and populations,but a few studies on the level of whole community.Most studies were reported that the grazing defoliation affected the character of grassland vegetation,which emphasized particularly that the changes in the components of community species,the character of soil,and the primary productivity of grassland resulting from grazing defoliation.A field experiment was conducted to study the compensatory growth,the species diversity of community,and its relationship to different factors in an alpine Kobresia humilis meadow through setting grazed/ungrazed plots under different habitats.Three habitats were assessed, i.e.1:pen pasture,with heavy defoliation ratio×highest nutrient availability.2:pass pasture,with moderate defoliation ratio×moderate nutrient availability.3:ungrazed pasture,with null defoliation ratio×lowest nutrient availability.The main results as follows:1 The changes of defoliation ratio,soil nutrients,water content in soil and the community characteristics among three habitatsThe order of the defoliation ratio and the community coverage of different habitats was pen＞pass＞control pasture.A greater water content of soil was found at pen,but there was no significant difference between the pass and control pasture.The pH values of soil did not differ among habitat types.The content of organic matter,nitrate nitrogen,total phosphorus,phosphorus pentoxide in soil was higher at the pen,there was no significant differences between the pass and control pasture in June.The content of ammoniacal nitrogen in soil did not differ between the pen and control pasture,but there was a reverse trend at the pass.The total nitrogen content was highest at control pasture,then at pen pasture,lowest at pass pasture.The soil content of organic matter, total phosphorus,phosphorus pentoxide was higher at the pen pasture,there was no significant difference between the pass and control pasture in August.Soil total nitrogen content was highest at pen,then at pass,lowest at control pasture.The ammoniacal nitrogen of soil was greatest at pen, then at control pasture,and lwoest at pass.The soil nitrate nitrogen content has no significant difference among habitats.No significant differences in the soil nutrient was found between grazed and ungrazed plots.2 Species diversityThe species diversity index H' and the species richness index R were 1.54,2.71,2.43,and 12, 26.29 at pen,pass and control pasture in June,respectively.In August,H' and R were 1.60,2.62, 2.45 and 4,35,28 at pen.pass and control pasture,respectively.The total number of species occurring at pen,pass and control pasture were 18,37,38 in June,and were 23,43,35 in August, respectively.This results showed that the number of species was higher at the pass and lower at the pen comparing with the control pasture,and supported that the prediction by the intermediate disturbance hypothesis.Furthermore,the dominant plant species at the three habitats was different, which were Elymus nutans and Potentilla anserina L at pen,Kobresia humilis and Elymus nutans at pass,and Stipa aliena at control pasture.3 The compensatory response of community to different factorsThe aboveground biomass of the community was overcompensation at the both pass pasture and pen pasture.Belowground biomass was exactcompensation at the pen pasture and overcompensation at the pass pasture.The aboveground biomass of the community was positively related to the defoliation ratio,total nitrogen,and phosphorus pentoxide (Y=0.718+0.308X₁+0.027X₂+0.018X₃:X₁=defoliation ratio of communities,X₂=total nitrogen, X₃=phosphorus pentoxide;R²=0.554,n=27,p＜0.05).The defoliation ratio,total nitrogen content and the phosphorus pentoxide have mainly influence on the compensatory growth of the community aboveground biomass after grazing and can explain 55.4%of the variance in the compensatory growth;The belowground biomass of community has no correlationship with the defoliation ratio, water content of soil,the nutrient content and the relative growth rate.4 The compensation response of the main species to different factorsAnalysis for the compensatory index G/C shown that the compensatory pattern of main species of different habitats was different.The aboveground biomass of the Elymus nutans occured overcompensation at the both pass and pen.Belowground biomass of the species occurred exactcompensation at the pen and overcompensation at the pass.The aboveground biomass of the Stipa aliena occurred undercompensation at the both pass and pen.Belowground biomass of the S. aliena occurred undercompensation at the pen and exactcompensation at the pass.The aboveground biomass of the Kobresia humnilis occurred exactcompensation at the pen and overcompensation at the pass.Belowground biomass of the species occured exactcompensation at the pen and overcompensation at the pass.The aboveground biomass of the Carex scabrirostris occurred overcompensation at the pen and exactcompensation at the pass.Belowground biomass of the C. scabrirostris occured overcompensation at the both pass and pen.The aboveground biomass of the Poa annua and Scirpus distigmaticus occured exactcompensation at the both pass and pen.The belowground biomass of the two species occured exactcompensation at the both pass and pen.The aboveground biomass of the Potentilla anserina occurred overcompensation at the pen and exactcompensation at the pass,respectively.The belowground biomass of the P.anserina occurred overcompensation at the pen and exactcompensation at the pass.The total aboveground biomass of the other species occurred exactcompensation at the pen and overcompensation at the pass.The belowground biomass of them occurred undercompensation at the pen and overcompensation at the pass. 5 The relationships of compensation response between the community and the main speciesAt pen pasture,the aboveground compensatory biomass of the community has a significant positively correlation with the that of Elymus nutans,Potentilla anserina,and others species.The belowground compensatory biomass was mainly determined by the that of Kobresia humilis,Carex scabrirostris,Potentilla anserina and other species.However,at pass pasture,the aboveground compensatory biomass of the community was mainly determined by the that of Kobresia humilis, Elymus nutans and other species.And the belowground compensatory biomass was greatly influenced by the that of the Kobresia humilis,Carex scabrirostris,and other species.This indicated that the specifical compensatory model of main species at different habitats determined the compensatory model of the community.Overcompensation occurred for aboveground parts at pen pasture mainly as a result of overcompensation in Elymus nutans,whereas overcompensatory of Kobresia humilis has a larger contribution to overcompensation occurred for belowground parts at the habitat.At pass habitat,overcompensatory of Kobresia humilis determined overcompensatory occurred for both above- and below-ground parts.In a word,the above results showed that the compensatory patterns of the different habitats in alpine meadow was directly correlated with the compensation variety of the community species in habitats,and the pattern was affected by the grazing intensity and the resources availability, especially the nitrogen nutrition,whereas the impact of soil moisture and then relative growth rate after grazing were relatively small.The species richness index R and the species diversity index H' at pass habitat was higher than another two habitats.Considering the varieties of R,H',G/C and the effects of water and nutrient content on the compensatory growth in different habitats,we belive the species diversity is very lower at pen habitat in despite of overcompensation occurred at the habitat. Potentilla anserina,which is a good predictor of grassland degradation,become dominant species at pen habitat by heavy grazing and has a great competive pressure to other species due to its clonal growth ability,and will finally results in reduction of value of grassland use.So long-term overgrazing can led to mosaic extinction of some native species and degradation of grassland. Ungrazed pasture have not compensatory potential when losing tissues by herbivore though there was higher species diversity at the habitat.From view of the ecosystem and/or landscape, maintaining intermediate grazing intensity is very importane to maintain higher speciesw diversity and primary productivity of grassland.The intermediate disturbance hypothesis is the fundamental rule of grazing ecology.