Temporal And Spatial Dynamic Analysis Of Grassland Productivity In Mongolian Plateau Based On Chlorophyll Fluorescence Remote Sensing Monitoring

In the context of global climate change,the inherent natural processes of the Mongolian plateau grassland ecosystem,such as the structure of the vegetation community,biomass,phenology,and precipitation patterns,have changed.Therefore,rapid and large-scale estimation of the productivity and analysis of the Mongolian plateau grassland ecosystem Its temporal and spatial pattern and exploring the impact of climate and social factors on it are of great significance to the development of animal husbandry and the protection of ecological functions.Therefore,this study uses the SIF data of the Orbital Carbon Satellite（OCO-2）from April to October in 2017 and the MOD17A2 product of the same period to establish the relationship between sunlight-induced chlorophyll fluorescence（SIF）and total primary productivity（GPP）in the grassland growing season on the Mongolian plateau,Combined with the measured data to verify it,and on this basis,analyze the temporal and spatial characteristics of the Mongolian plateau grassland monthly,and finally analyze the reasons for the differences with the meteorological data and socio-economic data,in order to provide theoretical support for future grassland protection and sustainable development.The results of this study were as follows:（1）During the study period,the Mongolian plateau grassland OCO-2 SIF and MODIS GPP had a significant correlation（P<0.05）.As plants grow,the fitting relationship between SIF and GPP gradually became higher and stabilizes（R²=0.08-0.78）,the fit degree between September and October was higher than that in April,and the verification accuracy between GPP estimates and actual measured values was higher(R²=0.75,P<0.05、REE=0.16、RMSE=3.96 g C·m^-2、Bias=3.76).（2）The growth start time of the entire plateau grassland was April,and the growth end time was September to October.The growth was mainly concentrated in May to August.The longest growing season was between 120-150 days and the shortest was between 90-105days.The total amount of GPP in the entire plateau grassland was 306.88 g C m^-2 yr^-1,and the spatial pattern showed a trend of decreasing from the northeast of the plateau to the southwest,and from the edge of the plateau to the middle.The grasslands in the northern Mongolian Plateau and the east of the Greater Khingan Mountains had the highest accumulation of GPP in the entire plateau,and the total GPP was greater than 450 C·m^-2·yr^-1.The monthly variation of GPP was between 5.83-104.59 g C m^-2 M^-1,and the average value of plateau grassland GPP from May to August was 78.86 g C m^-2·M^-1,and the accumulation of GPP was mainly concentrated in the Mongolian Plateau.In the north and east of Daxinganling,GPP was greater than 120 g C m^-2 M^-1.（3）Climate,especially precipitation was the main reason for the differences in the spatial pattern of grassland GPP in the Mongolian Plateau.In this study,northern Mongolia and northeastern Inner Mongolia,where GPP accumulation was relatively high,soil moisture is close to field water holding capacity during the growing season.GPP accumulation was closely related to temperature and was regulated by moisture;grassland productivity in central and western Inner Mongolia Autonomous Region was mainly affected by precipitation.As a result,high temperature had caused a drop in moisture,which was not conducive to the accumulation of grassland GPP,so it has a negative correlation with temperature.The climatic conditions in the Inner Mongolia Autonomous Region and Mongolia were similar,but the accumulated value of GPP was higher than that in Mongolia.This is due to the“returning farmland to forests and grasslands”in the Inner Mongolia Autonomous Region in recent years to protect the grassland,which has increased the grassland coverage in recent years.However,the mining of copper mines near the western part of the ancient country caused serious degradation of grassland and reduced grassland biomass.