| The terrestrial ecosystem carbon cycle is an important part of the global carbon cycle,and the Gross Primary Productivity(GPP)of vegetation is deeply involved in the terrestrial ecosystem carbon cycle.GPP can not only reflect the productivity level of terrestrial ecosystem,but also represent the carbon exchange flux between biosphere and atmosphere.Grassland ecosystem productivity is an important ecological factor to assess global terrestrial carbon sink and regulate terrestrial land surface process,and plays an important role in the terrestrial carbon cycle in the context of global change.Grassland ecosystem plays an important role in the global carbon cycle,but it is also vulnerable to climate change,especially in recent years,the increasing frequency of drought and extreme heat wave has a significant impact on grassland ecosystem.Therefore,I estimated the productivity of grassland vegetation and explored the relationship between productivity and drought,in order to provide a reasonable scientific basis and data theoretical support for the protection and restoration of grassland ecosystem.Based on the corresponding parameters of Xilinguole vegetation photosynthesis model,this study used MOD09A1 remote sensing data,photosynyhetically active radiation data,meteorological data,vegetation type data and site data to simulate the gross primary productivity of vegetation in Xilinguole during the growing season from 2001 to 2018,and it is verified by using site data.This paper also used the Palmer drought severity index and the standardized precipitation evapotranspiration index,to characterize the drought conditions of the grassland in Xilinguole during the growing season.The spatial and temporal distribution characteristics and changing trends of vegetation GPP and drought situation in the study area were studied and analyzed by unitary linear regression and correlation analysis.Finally,the response of vegetation GPP to drought in the study area was preliminarily discussed.The main research results and conclusions were summarized as follows:(1)From 2001 to 2018,the average annual GPP of the grassland growing season in the study area was higher in the northeast and lower in the southwest.The annual mean values of the three grassland types were as follows:meadow steppe(510.39 g C·m-2·a-1),typical steppe(373.07 g C·m-2·a-1)and desert steppe(193.63 g C·m-2·a-1).The mean value of grassland GPP in the study area was the highest in July,and the lowest in September.The mean GPP of meadow steppe and typical steppe reached the maximum in July,which were 164.35 g C·m-2·a-1and 109.86 g C·m-2·a-1,respectively.The highest value in desert steppe was 53.3 g C·m-2·a-1in August.Meadow steppe showed the most obvious growth trend in June,and its k value was 1.432.However,the typical steppe and desert steppe showed the largest increasing trend in August,with k value of 0.63 and 0.61,respectively.(2)From 2001 to 2018,the frequency of drought in Xilinguole was relatively high,with the PDSI range of-4.6 to 1.28.The slight drought was dominant,followed by the moderate drought,and the extreme drought occurred in 2007.Overall results of SPEI were consistent with those of PDSI.According to the PDSI values of the three grassland types,the degree of drought in the meadow steppe increased from May to September,and the frequency of extreme drought was the highest among the three grassland types,concentrated at 9.In the typical grassland,slight and moderate drought occurred in May,June and July,while severe drought occurred in August and September.The frequency of moderate drought in desert steppe was the highest and concentrated in August.According to the SPEI values of the three grassland types,the shorter the time scale,the greater the fluctuation of the time series curve,and the occurrence frequency of moderate drought was the highest.(3)According to the results of Chapter 3 and Chapter 4,it was found that the GPP value of all grassland types in the study area also decreased to the lowest in 2007when extreme drought occurred.The GPP value difference between any two grassland types was larger in dry years than in wet years.The correlation analysis between GPP value and drought index in the study area shows that GPP and SPEI03 have the highest correlation among the five different drought indexes,which means that GPP in the study area has the most correlation with meteorological drought at the three-month scale.In terms of months,drought in July and August has a greater impact on GPP than that in September.From the perspective of grassland types,desert steppe has the highest correlation with drought,followed by typical steppe,and meadow steppe has the least correlation with drought. |
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