Temporal And Spatial Variation Of Carbon Source Carbon Sinks In Tibet Grassland Ecosystem And Its Relationship With Climate Factors

The impact of climate change on terrestrial ecosystems is the focus of current research on global change.The carbon cycle and greenhouse effect of terrestrial ecosystems can be used to quantitatively assess the carbon budget of ecosystems through carbon source/sink indicators to clarify climate change.The Tibetan grassland ecosystem in the core area of the Qinghai-Tibet Plateau is a sensitive area for climate change response.Studying the carbon source/sink spatial pattern of grassland ecosystems in Tibet and its relationship with climatic factors has practical significance for ecosystem security and socio-economic development in the region,and is also important for understanding the response of the Qinghai-Tibet Plateau to global climate change.Based on remote sensing and GIS,MODIS and meteorological data were used to simulate the net primary productivity（NPP）of grassland ecosystems in Tibet from 2000 to 2014.The soil respiration model was used to calculate the grassland from 2000 to 2014.After verifying the simulation results,the NPP and NEP in Tibet will be analyzed in time and space,and then the spatial and temporal trends of climate in Tibet from 2000 to 2014 will be combined to provide carbon source/sink and climate factors for Tibetan grassland ecosystems.The correlation is analyzed and the results show that:（1）The average annual value of NPP is 37.81 gC·m^-2yr^-1 on the grassland ecosystem of Tibet from2000 to 2014,and the average annual NEP of grassland net ecosystem productivity is 20.22 gC·m^-2yr^-1.After comparison with the model simulation results in other studies,it can be concluded that the NPP and NEP are estimated based on the VPM model of light energy utilization and the soil microbial respiration model in the Tibetan grassland ecosystem,and the estimation results are accurate.The accuracy of the results is also relatively high.（2）During the study period,the grassland ecosystem in Tibet showed a carbon sink status.The area of grassland carbon sink（NEP>0）was 487,400 km²,accounting for 58.12%of the total grassland area in Tibet.The area of the carbon source area（NEP<0）of the grassland ecosystem in Tibet is 351,200 km²,accounting for 41.88%of the total grassland area in Tibet.The average annual carbon release in Tibet is 0.6TgC·yr^-1,and the average carbon sequestration is 17.57 TgC·yr^-1.There is still a large carbon sequestration potential.（3）From 2000 to 2014,the spatial distribution of NPP and NEP in Tibet’s grassland ecosystem based on natural zoning was about the same,and the mean values were:East Evergreen Broadleaf Forest（OA1）in the southern Himalayas>Sichuan Dongshan Mountain needles Forest Zone（IIAB1）>Fruity Alpine Shrub Meadow Zone（IB1）>Southern Tibetan Mountain Shrubland Zone（IIC1）>Qiangtang Alpine Grassland Zone（IC2）>Ali Mountain Semi-Desert and Desert Zone（IID1）>The Kunlun alpine and desert zone（ID1）shows a trend of decreasing the southeastern part of the Tibet to the northwest.（4）From the trend of annual average NPP and NEP in the different natural zones of Tibet grassland ecosystem in 2000-2014,it can be seen that the temperature zone and the dry and wet climate zone of the same species trend zone are also roughly the same.The main trends of NPP can be divided into three types:“rise-down”,“down-slow-down”and“down-stable-slow”.The trend of NEP is more than that of NPP.There are four types of“rise-down”,“down-slow-slow-down”,“down-stable-slow”and“down-stable”.（5）From the significant distribution of carbon source/sink and climatic factors in Tibet,the area where NEP is significantly correlated with temperature and precipitation is significantly negatively correlated with precipitation,accounting for 34.30%,which is significantly positively correlated with temperature.The area is about 20.28%,both of which are distributed in the Kunlun alpine desert zone（ID1）in the northwest of Tibet and the northern part of the Qiangtang alpine steppe zone（IC2）.In most of the rest,NEP and climatic factors were not significantly correlated,and only a small part of NEP was positively correlated with precipitation and negatively correlated with temperature.