NDVI Changes In Vegetation Of Daur-Hulunbuir Grassland And Its Response To Climate Change

The Daur-Hulunbeier Grassland is located in the semi-arid region,with continuous marsh wetlands and numerous lakes.It is an important channel for the migration of inland birds and the migration and breeding habitat of international waterfowl migratory birds.It is an important part of the ecological barrier area of the forest wetland grassland in northern China.Under the trend of continuous warming of the global climate,exploring the response mechanism of vegetation growth change to meteorological water and heat factors is one of the hot issues of global change research at present.This study is based on MODIS normalized difference vegetation index data,combined with meteorological data,uses trend analysis method to obtain the trend change of time series,and uses partial correlation analysis to detect the lag response and simultaneous response of NDVI change of Daur-Hulunbuir grassland vegetation to hydrothermal conditions since 2000.The results show that:(1)From 2000 to 2020,NDVI in the vegetation growth season show a fluctuating growth trend(R2=0.26),with a growth rate of 0.025/10 a.Further analysis of NDVI interannual changes in different countries in the study area shows that the NDVI rising trend rates of China,Russia and Mongolia are 0.022/10 a,0.032/10 a and 0.026/10 a respectively;The NDVI change trend is different in different vegetation types.The change trends of grassland,forest,wetland,cultivated vegetation and saline-alkali marsh are 0.031/10 a,0.021/10 a,0.026/10 a,0.032/10 a and 0.04/10 a respectively;The change trends of the climate zones of Dwc,Dwd and BSk are 0.03/10 a and 0.021/10 a respectively.The annual change of NDVI in the growth season has three stages: the fluctuating growth stage from 2000 to 2013,the decreasing stage from 2013 to 2016,and the rapid growth stage from 2016 to 2022.(2)The spatial distribution of NDVI in the average growth season of the DaurHulunbuir grassland has obvious spatial differentiation characteristics,showing a pattern of "low in the southwest and high in the northeast".The proportion of the total area of the vegetation cover of very low,low,medium,high and extremely high grades varies greatly.The area of very low and low vegetation covers 46.66% of the total area,the medium vegetation covers 21.98% of the total area,and the high and very high vegetation covers only 31.36% of the total area.The slope of NDVI change trend of vegetation in 80% of the area covered by vegetation is greater than 0,and large areas of vegetation show an improvement trend,mainly distributed in the north-central and northeast of the grassland.Areas with NDVI change trend slope less than 0 are scattered around Huihe River and Hulunbeier City,where human activities are frequent.The area with insignificant vegetation change accounts for a large proportion,mainly distributed in the central and southern part of the grassland.(3)The process of continuous warming affects climate change.The thermal factor in the Daur-Hulunbuir grassland area shows a decreasing trend from southwest to northeast in space,while the water factor is on the contrary,with the characteristics of high in the east and low in the west.Precipitation and relative humidity showed an increasing trend,while temperature and solar radiation showed a very slow trend,and2013 was an important year.The thermal factor reached the lowest value,and the water factor increased sharply.In terms of spatial distribution,the overall trend of heat factor is not significant,while the water factor changes with a significant increase trend.The woodland in the southeast and grassland in the west are significantly increased,and the overall regional precipitation increases and changes significantly.(4)The interannual variation of NDVI and climate change in vegetation growing season showed that,on the whole,the correlation between NDVI and temperature and solar radiation in Daur-Hulunbuir grassland was generally low,while the response to precipitation and relative humidity was violent,and the growing water factors were more conducive to vegetation growth.NDVI has a certain lagging response to climate change,and temperature has a promoting effect on vegetation entering the growing season.During the growing period,precipitation and relative humidity became the main control factors affecting the lag of vegetation growth,which played a significant role in promoting vegetation growth.Vegetation change has a lag of 1-3 months on temperature,and the response to moisture change in the current month is more obvious.Vegetation cover in the growing season shows great fluctuations,mainly due to the fluctuating effects of precipitation and relative humidity.(5)Since 2000,vegetation has been increasingly affected by human activities,with the greatest impact in areas with concentrated population distribution.The main way is to increase the cultivation area and convert typical grassland into cultivated plants.In the analysis of the contribution rate of regional impact factors of significant vegetation change,it is found that the contribution rate of climate change is higher in the regions with significant vegetation improvement,while the contribution rate of human activities is higher in large areas with significant vegetation degradation.