Effects Of Grazing Gradient On Community Characteristics And Spatial Heterogeneity Of The Elm Sparse Forest Grassland In Horqin

In order to study the effect of grazing on the characteristics and spatial heterogeneity of the elm sparse forest grassland community,in 2021,the elm sparse forest grassland in the Wulijitu branch of Zhurihe Ranch,Kezuozhong Banner,Tongliao City,Inner Mongolia Autonomous Region,taking the herdsman’s pasture as the unit.According to the stocking rate of each plot,it is divided into grazing gradientⅠ(1.05-1.08 sheep unit^-1·hm^-2·a^-1),grazing gradientⅡ(2.25-2.31 sheep unit^-1·hm^-2·a^-1)and Grazing GradientⅢ(3.15-3.75 sheep units^-1·hm^-2·a^-1)3 grazing intensity gradients.Using the transect method,the quantitative characteristics of plant communities and main plant species were investigated,and the influence of grazing gradients on the spatial distribution of vegetation was analyzed using the power function rule.The main findings are as follows:（1）The species diversity of grazing gradientⅢwas lower than that of grazing gradientⅠand grazing gradientⅡ.The Margalef richness index,Shannon-Wiener diversity index,Simpson dominance index and Pielou evenness index are all grazing gradientⅠ>grazing gradientⅡ>grazing gradientⅢ.And the Margalef richness index,Shannon-Wiener diversity index and Simpson dominance index of grazing gradient I plot were significantly higher than those of grazing gradientⅢplot（P<0.05）.（2）Grazing gradientⅢplot showed a prominent decline in the population of Ulmus pumila,the density is 0.83 plants/ha,the importance value is 0.03.The density and importance of elm in Grazing GradientⅠwere 52.69 plants/ha and0.16,respectively,indicating that the increase in grazing intensity was an important reason for the decline of the arbor layer of the elm sparse forest grassland.（3）Grazing intensity has a significant effect on the quantitative characteristics of herbaceous layers.The density of community in Grazing GradientⅢwas significantly higher than grazing gradientⅠand grazing gradientⅡ（P<0.05）.The important value of perennial grasses in the community decreased from 0.38 of grazing gradientⅠto 0.16 of grazing gradientⅢ,while the one-and biennial plants increased significantly in grazing gradientⅢthan in grazing gradientⅠ（P<0.05）.Among them,the height,density,coverage,biomass,clump diameter,number of tiller branches per plant and important values of the excellent perennial grass forage Cleistogenes squarrosa.were significantly higher than those of grazing gradientⅢ（P<0.05）,while the density and coverage of Artemisia annua in grazing gradientⅡwere significantly higher than those ingrazinggradientⅠplots（P<0.05）;Amongthem,the height,density,coverage,biomass,clump diameter,number of tillering branches per plant and important values of the excellent perennial gramineous forage Cleistogenes squarrosa were significantly higher in grazing gradientⅠthan grazing grazing gradientⅡ（P<0.05）.The cover,density and biomass of underground shoots and annual plants were significantly higher in grazing gradientⅢthan in grazing gradientⅠ（P<0.05）;The biomass of xerophytes increased significantly in grazing gradientⅢcompared with grazing gradientⅠand grazing gradientⅡ（P<0.05）.After reaching the grazing gradientⅢ,the excellent perennial grasses represented by Cleistogenes squarrosa declined significantly,while the underground buds and annual mesophytes increased significantly.（4）The vegetation of Horqin elm sparse forest grassland and grassland had strong spatial heterogeneity under the three grazing gradients,behave as grazing gradientⅢ>grazing gradientⅡ>grazing gradientⅠ.After the grazing reached grazing gradientⅢ,the spatial distribution of fine forages such as Cleistogenes squarrosa was scattered,which reduced the spatial heterogeneity of the community vegetation.（5）The occurrence frequency and spatial heterogeneity index of species were affected by grazing intensity,and the spatial heterogeneity of the same species under different grazing gradients was specific.Perennial grasses promoted community spatial heterogeneity in grazing gradientⅠand gradientⅡplots and suppressed community spatial heterogeneity in gradientⅢplots,whereas biennials did the opposite.The spatial heterogeneity index of Cleistogenes squarrosa in the grazing gradientⅠplots was greater than the community spatial heterogeneity index,which promoted the spatial heterogeneity of the community,while the grazing gradientⅡand gradientⅢplots inhibited the spatial heterogeneity of the community Artemisia scoparia promoted the spatial heterogeneity of the community in all grazing gradient plots;（6）Community biodiversity was negatively correlated with community spatial heterogeneity,and there was a certain correlation between the density,frequency and biomass of various groups of plants and their spatial heterogeneity index.The biomass and spatial heterogeneity index of Cleistogenes squarrosa,legumes and annual plants were all significantly positively correlated,the correlation coefficients are 0.672,0.691 and0.708,respectively,it shows that there is a large correlation between biomass and spatial heterogeneity.To sum up,with the increase of grazing intensity,the vegetation species diversity of Horqin elm sparse forest steppe decreased significantly,the arbor layer declined,the important value of the perennial grasses represented by Cleistogenes squarrosa decreased significantly,the annual plants increased significantly,and the spatial heterogeneity of the community increased.