Response Of Grassland Cover Change To Drought In Steppe Region Of Mongolian Plateau

As an important part of the surface ecosystem,grassland plays an extremely important role in the global material cycle and energy flow.Grassland coverage is an important indicator reflecting the health of grassland ecosystems,which can quantitatively describe the growth status of regional grasslands.In the context of global climate change,drought disasters occur frequently.The Mongolian Plateau Steppe is located in an arid and semi-arid region,and the relatively single vegetation type makes the ecosystem in this region extremely fragile.Therefore,it is of great significance to explore the response law of grassland coverage to drought in the Mongolian Plateau steppe region for protecting regional ecological security and responding to extreme climate change.This study used MOD09A1,MOD13A1,MOD11A2 and the measured data of land cover to analyze the temporal and spatial variation characteristics of grassland coverage and drought in the Mongolian Plateau steppe from 2001 to 2020,and further explore the response of grassland coverage changes to drought.The results of the study are as follows:(1)From 2001 to 2020,the grassland area in the Mongolian plateau grassland area showed an overall increasing trend.From 2001 to 2017,grasslands with coverage of 40%-60% and 20%-40% were dominant,and after 2017,the dominance changed to grasslands with coverage of 60%-80% and 40%-60%.Spatially,the coverage level gradually decreased from northeast to southwest.(2)There is a strong correlation between the size of the grassland at each level of coverage in the study area and the amount of its transfer in and out.In time,the grassland area with coverage above 60% increased,and the grassland area with coverage less than 40% decreased,but the direction and area of conversion between different coverages in each year were different.In terms of regional distribution,the areas with increased grassland coverage are mainly concentrated in the grassland areas where the coverage of typical grasslands is 20%-60%;the areas with reduced coverage are mainly concentrated in the grasslands with coverage of 40%-80% in typical grassland areas;The mutual transformation between grassland and other types is mainly distributed in the interlaced area of typical steppe and desert steppe in the southwest and the alpine grass area in the west.(3)From 2001 to 2020,the accumulated drought in the Mongolian plateau grassland area was mainly mild drought,followed by moderate drought,and the area with extreme drought was the least.The inter-annual fluctuation of the cumulative drought area of each grade is large.In the past 20 years,the occurrence frequency of cumulative drought events in the southwest of the study area was high,and the cumulative drought intensity was high,while the frequency and intensity in the northeast were low.(4)Both the change in grassland coverage compared with the previous year,and the range normalized coverage had significant spatial correlations with the cumulative temperature condition index.The grassland coverage in the regions with different intensities of drought also decreased significantly according to different degrees.The proportion of the area where the range standardized coverage and the cumulative temperature condition index are positively correlated is 95.98%.There are also negatively correlated areas,but they only account for 3.43% of the statistical area,and are mainly distributed in the central and northern high latitudes.(5)The order of response degree of grassland coverage changes to drought in each vegetation type area is: typical steppe > desert steppe > alpine grassland >meadow steppe.Among the four climate types,the change of grassland coverage in the semi-arid area was the most sensitive to drought intensity,followed by the humid and semi-arid areas,and the smallest in the semi-humid area.By administrative region,grassland coverage changes in Inner Mongolia,China,were more responsive to drought than Mongolia.Among the areas with different landform types,the change of grassland coverage in the platform was the most responsive to drought,followed by the hilly area,and the change in the grassland coverage in the sandy land and low mountain area was less sensitive to the stress effect of drought.