Spatio-temporal Response Of Grassland Ecosystem To Climate And Grazing In Hulun Buir Grassland

As a widely distributed type of land use,grassland has important economic and ecological functions.Hulun Buir grassland is a high-latitude grassland ecosystem with good ecological protection and rich landscape types,and it is the most ideal area to study the multi-level ecological pattern and its driving factors of grassland.Taking Hulun Buir grassland as the research object,this paper discusses the characteristics of temporal and spatial variation of vegetation and its influencing factors from the three scales of remote sensing,sampling line and sampling point.The results can provide theoretical guidance for scientific and rational utilization and protection of grassland resources in this area.In this study,based on the MODIS NDVI data from 2001 to 2018,the change intensity and trend of vegetation coverage in Hulun Buir grassland were calculated by temporal information entropy and time series information entropy,and the relationships among precipitation,temperature,SPEI,grazing pressure and NDVI were studied by partial correlation analysis.In terms of sampling line scale,the data of 990 plots in 4 transects of Hulun Buir grassland were classified and ordinationed.At the same time,similarity analysis(ANOSIM)and similarity percentage analysis(SIMPER)were used to quantitatively describe the differences among vegetation types and communities,and the main factors influencing vegetation distribution pattern were analyzed and explored.Based on the sampling point scale,660 samples were investigated in different sampling distances and sampling directions,and the effects of grazing pressure on the differences of vegetation community characteristics and soil physical and chemical characteristics were analyzed.Based on the above research,the main conclusions are as follows:1.From the study of remote sensing scale,the increased area of the whole Hulun Buir grassland vegetation coverage is larger than the reduced area,and the vegetation coverage shows an upward trend as a whole.The temporal information entropy changes greatly in the main part of the typical steppe,and tends to decrease to the meadow steppe and desert steppe on both sides of the east and west,and the time series information entropy is higher in the central and northern part of the typical steppe.The desert steppe in the southwest is the lowest,showing a decreasing trend from northeast to southwest as a whole.The NDVI of 92.1% area of the study area is positively correlated with precipitation,indicating that the change of vegetation coverage in the study area is mainly affected by rainfall.On the banner(city)scale,it is mainly affected by grazing pressure,which has a positive feedback effect on vegetation coverage in the study area,indicating that the overall grazing level of Hulun Buir grassland does not exceed its carrying capacity,which is consistent with the intermediate disturbance hypothesis.The patchy distribution on the small scale of sum is mainly affected by microtopography.According to the community investigation records,it is found that the change of vegetation coverage of sums which located in mountain,sand,puddle,meadow is significantly different from that in adjacent areas.The results show that the water condition controlled by precipitation is the most important factor restricting the vegetation coverage of Hulun Buir grassland,and the effects of grazing pressure and micro-topography play a leading role in the banner(city)and sum scale,respectively.2.According to the study based on the transect scale,196 species of plants were investigated in all the sample plots of Hulun Buir grassland,belonging to 43 families and 122 genera.The water ecological types are mainly mesophytes and the life forms are mainly perennial herbaceous plants.It fully illustrates the superiority of water conditions of Hulun Buir grassland in the northern grasslands of China.According to the indicator species analysis method,330 sample plots were divided into 3 vegetation types and 17 community types.The difference between desert steppe and typical steppe was obvious,while the meadow steppe had a tendency to change to typical steppe.Hulun Buir grassland as a whole developed in the direction of xerophyte.Precipitation is the common dominant factor driving the vegetation distribution pattern of each transect.Elevation,temperature,SPEI,grazing pressure,soil organic matter content and soil clay content have an important impact on the vegetation distribution of each transect.3.From the scale of the fenced grazing site,the "Piosphere" phenomenon caused by the fencing grazing activity centered on the water point obviously changed the distribution pattern of the plant community and increased the spatial heterogeneity of the plant community.The community characteristics,plant diversity near the water point was the lowest,and increased at first and then decreased with the increase of sampling distance;The soil bulk density,p H and soil nutrient content near the water point increased significantly,that is,the "fertile island effect" was formed,and the distribution pattern of plant community in fenced pasture was determined by sampling distance and soil physical and chemical properties.With the increase of the distance from the water point,the community indicator species appeared from annual short-lived plants to perennial constructive species.Finally,the community characteristics,species diversity and soil physical and chemical properties in the center and periphery of "Piosphere" were most affected by grazing,while the middle ring area between them was less disturbed.This provides an important reference for the study of "Piosphere" in similar areas,corresponding fence grazing activities,scientific management and rational use of pastures.To sum up,precipitation is the most important factor affecting the spatial distribution and community characteristics of vegetation at different scales in Hulun Buir steppe,and the whole vegetation evolves to xerophytic direction,it is caused by climate change on a long time scale and some unreasonable grazing activities.The “Piosphere” phenomenon caused by fencing grazing in water points has a significant impact on community characteristics,biodiversity,soil physical and chemical properties,which is the main cause of grassland degradation in small scale.It is suggested that in the degraded grasslands in similar areas,proper regulation of grazing intensity and strict measures such as no grazing and grazing in strict accordance with national regulations should be taken to protect and restore the grasslands in order to ensure the sustainable development of the grasslands in northern China.