Vegetation Carbon Use Efficiency Deduction Based On MODIS Data Products In Three-River Headwaters Region

Vegetation carbon use efficiency?CUE?is a percentage index representing the capacity of plants to transfer carbon from the atmosphere to terrestrial biomass,i.e.the ratio of net primary production?NPP?to gross primary production?GPP?,which reflects the allocation of photosynthesized products by plants.As a basic parameter of the mechanism of carbon cycle in ecosystem,CUE is paramount for regional carbon budget evaluation.But how this parameter changes with climates is still unknown.Faced by an aggravated global warming,there is a heightened necessity in unravelling the dependence of the NPP/GPP ratio on climates.Based on MODIS GPP,NPP data and vegetation type data,this thesis obtains the GPP,NPP and CUE values of different vegetation types in Three-River Headwaters region.The spatial distribution characteristics of vegetation primary productivity and carbon use efficiency in Three-River Headwaters region from 2001 to 2017 are discussed by using linear regression with elevation data.The response of vegetation carbon use efficiency to the changes of temperature,precipitation and evapotranspiration in the source area was analyzed by correlation coefficient method with meteorological data and MODIS ET data.The results not only reveal the vegetation productivity in Three-River Headwaters region under the climate and environmental changes,but also have great practical significance and scientific value for the response to climate change and the formulation of protection measures.The main conclusions are as follows:?1?The productivity characterisitics of all types of vegetation in Three-River Headwaters region is forest>shrub>alpine grassland>other alpine vegetation.NPP and GPP of all types of vegetation showed a slow and positive growth trend.From2001 to 2017,the mean GPP and NPP of vegetation in Three-River Headwaters region were 186.75 g C m^-2 a^-1 and 156.47 g C m^-2 a^-1 respectively.The spatial distribution of vegetation productivity increased gradually from northwest to southeast,which was significantly correlated with elevation.The GPP and NPP values of vegetation at altitude below 4500 m?Xinghai,Tongde,Zeku,Henan Mongolian Autonomous County,Gande,Jiuzhi and Banma in the east?were higher than those at above 4500 m?Zhiduo,Golmud,Qumali and Zaduo in the west and Maduo in the middle?.?2?The CUE of different vegetation types in Three-River Headwaters region was relatively high,and more than 80%of the CO₂ absorbed from the air by photosynthesis was preserved in the vegetation during 2001-2017.Carbon use efficiency of alpine grassland,shrub and other alpine vegetation in Three-River Headwaters region showed different growth rates,while forest CUE decreased slowly.?3?From 2001 to 2017,the CUE of different vegetation types and the whole vegetation system in Three-River Headwaters region increased first and then decreased in March-October of the year,and the CUE of vegetation in June was the highest.The annual carbon use efficiency of the vegetation system in Three-River Headwaters region was between 0.73 and 1,with an average level of 0.85.The spatial distribution of vegetation CUE was high in the North and low in the South and high in the West and low in the East.In the source area,Maduo had the highest use rate of vegetation carbon,while Henan Mongolian Autonomous County had the lowest use rate.The CUE values of vegetation at altitude below 4500 m were relatively low?0.79⁰.83?,while at above 4500 m areas,the CUE were relatively high,and some areas were above 0.85.?4?CUE of vegetation in Three-River Headwaters region had a significant lag effect on climate factors.From April to May,the vegetation CUE was most affected by the temperature,precipitation and evapotranspiration in the same period,showing a significant positive correlation.There was the highest sensitivity between vegetation CUE and climate?temperature,precipitation and evapotranspiration?in June-August,which were in the corresponding period,showing a significant negative correlation.From September to October,the vegetation CUE was positively correlated with temperature and negatively correlated with precipitation and evapotranspiration,which were affected by temperature lag for two months and precipitation and evapotranspiration lag for one month.?5?There was a negative correlation between vegetation carbon use efficiency and temperature,precipitation and evapotranspiration in April-October.The influence mechanism of precipitation and evapotranspiration on vegetation CUE in Xinghai,Tongde and Zeku in the low elevation region of the source area is consistent,showing negative correlation and positive correlation with temperature.The relationship between CUE and air temperature and precipitation in the southern part of Qumalai is negative in high altitude areas.