Change Of Grassland Aboveground Biomass In The Three-River Headwater Region And Analysis Of Its Response To Climate Factors

Grassland is the world's largest terrestrial ecosystem and plays a very important role in regulating climate change and maintaining ecosystem balance.The Three-River headwater region is one of the most vulnerable terrestrial ecosystems in China and the most sensitive regions to climate change.The remote sensing estimation of grassland aboveground biomass in this area,the study of its dynamic change and the analysis of climate factor impact are of great significance for the sustainable use of grassland and the maintenance of ecosystem balance.This paper firstly uses the CASA model to estimate the net primary productivity(NPP)of grassland in the Three-River headwater region by using MODIS remote sensing data,meteorological data and grassland type data of grassland vegetation in a long time sequence(2000-2017),uses NPP data to estimate the aboveground biomass(AGB),to explain the temporal and spatial variation characteristics of grassland aboveground biomass in the Three-River headwater region from 2000 to 2017,to explore the correlation between the aboveground biomass and the temperature and precipitation factors in each phenological period,and to clarify the main climatic factors that driving grassland aboveground biomass in different grassland types in the study area.This paper mainly achieved the following research results:(1)This paper establishes a remote sensing model to estimate the annual grassland aboveground biomass in the Three-River headwater region from 2000 to 2017,and compares it with the measured data.The estimation accuracy is about 80%,and the precision is superior,which proves that the estimation method is effective and feasible.(2)Based on the model estimation results,this paper analyzes the temporal and spatial variation characteristics of grassland AGB in the study area.In terms of time,the interannual variation of grassland biomass in the study area showed an increasing trend in slow and volatility over the past 18 years.The interannual variations of AGB values of different grassland types were different,but the overall trends were increasing in slow and volatility,and the peak value was in 2010 or 2011;the study area has shown a pattern of overall stability,the local area of great improvement,and the small area of sporadic degradation in the spatial change in the past 18 years.(3)This paper explores the impact of climatic factors on grassland AGB.It is found that the accumulation of aboveground biomass in the study area is mainly limited by the temperature of the regreening period,the precipitation of the pre-regreening period and the precipitation of the yellowing and withering period in the previous year.Different types of grassland vegetation are affected differently by climatic factors in each phenological period.The precipitation in the regreening period and the precipitation of the yellowing and withering period in the previous year play important roles in the growth of the three main grassland vegetations(alpine meadow,alpine steppe and temperate steppe)in the study area.The precipitation in the greening period is higher,the yield of vegetation this year is higher.The temperature of the greening period and the temperature of the re-greening period play important roles in alpine grassland.As the temperature increases,the accumulated biomass increases.The temperate steppe is mainly affected by the temperature of the dormancy period in the previous year,and the low temperature helps the grassland to accumulate more aboveground biomass in the coming year.