| As one of the most widely distributed vegetation types,grassland not only reflects the productivity of the ecological environment,but also represents the ecological quality of the ecosystem.To reveal the spatial and temporal dynamics of grassland productivity and quantitatively analyze the impact of human activities on grassland ecosystem is conducive to making rational decisions on the protection and development of grassland ecosystem services.As an important ecological barrier and animal husbandry production base on the Qinghai-Tibet Plateau,grazing activities are the most important human disturbance activities.The balance state of grass and livestock directly affects the degradation and restoration of grassland,and then affects the service ability of grassland ecosystem.Based on remote sensing and model simulation,this study analyzed the spatiotemporal variations of grassland ecosystem NPP and grassland yield over the Tibetan Plateau from 2000 to 2018,and analyzed the effects of grazing on grassland productivity through stocking pressure.The main research results are as follows:(1)2000-2018,the qinghai-tibet plateau Tibetan areas the most significant number of sichuan province showed a trend of decrease,the grassland livestock density per unit area in high space on the east,southeast,north and west,the northern region,general Mongolia autonomous prefecture in Xinjiang,Gansu jiuquan city livestock density is the lowest,in aba autonomous prefecture,Sichuan province,liangshan autonomous prefecture,yunnan province Di Qing Zhou and xining of qinghai province,eastern city livestock density is the largest,seen from the change trend for many years,Tibet,sichuan livestock density showed a trend of decrease,qinghai,gansu province,xinjiang region,eastern yunnan province is a growing trend,Due to the ban on grazing and ecological migration,the livestock density in the Three-River Headwaters area of Qinghai Province also showed a decreasing trend.(2)The interannual variation trend of NPP in the past 19 years was generally high in the north and low in the south,high in the east and low in the west.Among them,the northern Qiangtang Plateau in northern Tibet,the northern Ngari region and the Qilian Qingdong alpine basin have the highest increasing trend,and the most obvious decreasing region is concentrated in the southern Tibet valley.The spatial distribution of grassland yield based on NPP and field sampling data was consistent with that of NPP.The grassland yield per unit area in the eastern region was higher than 600 kg/hm~2,and the grassland yield gradually decreased from east to west,which was highly correlated with the temperature,precipitation and vegetation types in the Qinghai-Tibet Plateau.According to the interannual variation trend,the annual grass yield in 65.32%of the Qinghai-Tibet Plateau showed an increasing trend,44.68%showed a decreasing trend,and only 35.56%showed a significant change.The average annual grass yield on the Qinghai-Tibet Plateau is 482.85kg/hm~2,the average annual change rate is 3.86%,and the change amount is 18.65kg/hm~2.On the whole,the fluctuation of perennial grass yield on the Qinghai-Tibet Plateau is small,and the grassland ecosystem is relatively stable.The average annual dry weight and fresh weight of the main grasslands on the Qinghai-Tibet Plateau were 5976693 million t and 193.6344 million t respectively.(3)The spatial distribution of theoretical stocking capacity was consistent with the distribution pattern of grass yield over the Qinghai-Tibet Plateau from 2000 to 2018,which showed a decreasing trend from southeast to northwest.The theoretical stocking capacity increased in a fluctuating manner over the years,but the overall increase range was small,and the average theoretical stocking capacity was 0.4-0.5 Shu/hm~2.From 2000 to 2018,the overall stocking pressure was in the state of grassland overload.In terms of the trend distribution pattern,the Qiangtang Plateau in northern Tibet showed a decreasing trend,while the southern Tibet Valley,Qilian Qingdong Alpine Basin and Qingnan Plateau wide valley showed a significant increasing trend(>0.1),and the most significant change area was mainly in the southern Xigaze of Tibet(>0.15).(4)From 2000 to 2018,the overall overloading on the Qinghai-Tibet Plateau was serious.64 counties were slightly overloaded,accounting for 31%of the total number of counties.Moderate and severe overloading accounted for 17%and 9%,respectively.In terms of the proportion of the number of overloaded counties in each province,the order of overloading from large to small is:Tibet>Xinjiang>Qinghai>Gansu>Yunnan>Sichuan,the pressure of grassland grazing is the highest in Tibet,especially in southern Tibet,and the pressure of grassland grazing is the lowest in Sichuan. |
