Remote Sensing Dynamic Monitoring And Driving Force Analysis Of Grassland In Tibet

Grassland,as the main ecosystem type in Tibet,plays an important role in developing grassland animal husbandry,ensuring food safety and protecting regional ecological environment,which is an important foundation for the development of animal husbandry and regional economic development in Tibet.However,due to the impacts of climate change,irrational use of land resources and population growth,grassland ecosystem in Tibet has deteriorated to varying degrees,which not only leads to the continuous loss of grassland ecological service function,but also leads to many serious environmental problems.Therefore,the quantitative assessment of the temporal and spatial responses of grassland cover and grassland productivity to climate change and human activities in Tibet has scientific theoretical support value for the restoration of degraded grasslands and the formulation of grassland protection policies by grassland resource management departments.In this study,the grassland ecosystem in Tibet was taken as the research object,and based on the land use data of Global Land Cover2000,the grassland types and distribution areas in Tibet were obtained.Using meteorological data,vegetation remote sensing data,human factors data and elevation data,supplemented by model simulation,the dynamic changes of grassland NDVI and NPP in Tibet and their responses to climate change and human activities were explored.Firstly,based on Theil-Sen trend analysis and Mann-Kendall mutation test,the spatiotemporal dynamic pattern of grassland NDVI in Tibet from 2000 to 2018 was systematically analyzed at the annual scale.The residual analysis and correlation analysis were used to explore the dual response mechanism of Tibetan grassland to climate change and human activities,and the correlation between NDVI of Tibetan grassland and temperature and precipitation was discussed from the annual scale.On this basis,the improved CASA model was used to simulate and estimate grassland productivity.The temporal and spatial dynamic pattern of grassland NPP in Tibet was systematically analyzed by trend analysis,stability analysis and Hurst index,and the correlation between grassland NPP and temperature and precipitation in Tibet was explored from the annual scale.Finally,the potential NPP of grassland was estimated,and the NPP of grassland occupied by human activities was calculated according to the actual NPP.By calculating the change rates of the above three NPPs,eight evaluation scenarios of dynamic change driving mechanism of grassland NPP were constructed to quantitatively evaluate the relative impact and contribution of climate change and human activities on grassland NPP change in Tibet.The main conclusions are as follows:(1)Annual NDVI of grassland in Tibet in recent 20 years was 0.21,the largest annual average NDVI appeared in alpine subalpine meadow(0.36),and the smallest annual average NDVI appeared in desert grassland(0.10).In addition,the annual average NDVI of grassland showed a decreasing trend from southeast to northwest in space.The area with increasing NDVI(61.01%)was larger than that with decreasing NDVI(38.99%),and the area with increasing NDVI in alpine subalpine meadow was the largest(57.54%).In terms of time,the NDVI of grassland generally showed a slight but insignificant upward trend,with an annual average growth rate of 0.0005.The NDVI of alpine subalpine meadow showed the most obvious upward trend,with an average growth rate of 0.0008.From the stability point of view,grassland NDVI is relatively stable.Specifically,the area with low volatility(0.05v<0.1)accounted for 49.41%,while the area with high volatility(C_v>0.2)accounted for only 4.65%.Among them,the alpine subalpine meadow is in a medium fluctuation state,and other grasslands are in a low fluctuation state.(2)The change of grassland NDVI in Tibet is affected by climate change,topographic factors and human activities.The areas where grassland NDVI was significantly positively correlated with temperature were mainly concentrated in Ngari and northwest Nagqu,and were scattered in Qamdo and other places,while the areas where grassland NDVI was significantly positively correlated with precipitation were distributed All over Tibet.Precipitation in this region is the main driving factor of NDVI change and plays a key role in promoting the accumulation of NDVI change.With the increase of altitude,grassland NDVI decreased first and then increased,and the growth rate of grassland NDVI on DEM>6000 m gradient was higher.The NDVI growth rate of grassland with slope of 2.5?10°was higher.The average NDVI growth rates on the west and southwest slopes were high.To some extent,the improvement of social development has promoted artificial grass planting,grassland protection and other activities,thus promoting the growth of grassland.(3)The annual mean NPP of grassland in Tibet during the study period was 136.46 g C·m^-2·a^-1,and increased significantly at a rate of 1.57 g C·m^-2·a^-1;NPP of grassland decreased from southeast to northwest.In the future,grassland NPP in Nagqu and other places will continue to increase,while grassland NPP at the junction of Shigatse,Ngari and Nagqu will have a reverse trend in the future.NPP of different types of grasslands was different,with the highest in alpine subalpine meadow and the lowest in plain grassland.Grassland NPP was positively correlated with air temperature,but not significantly correlated with precipitation,indicating that air temperature was the limiting climatic factor for grassland NPP in the region.The implementation of human activities such as rotational grazing and enclosure policies has an important role in promoting the increase of grassland NPP in Tibet in recent years.(4)The ANPP results of grassland generally showed a slight increase trend(37.85%vs36.13%).For the increase of ANPP in grassland,the contribution of climate change accounted for 17.60%,mainly in Ngari and the western Nagqu.The contribution of human activities accounted for 14.66%,mainly distributed in Shigatse,Nagqu and eastern Tibet.The decrease of ANPP in grassland was mainly caused by climate change and human activities(15.71%).For different grassland types,climate-dominated ANPP increase and human-dominated ANPP decrease mainly occurred in alpine subalpine grassland.In addition,although the climate-dominated alpine subalpine meadow vegetation ANPP decreased,but the anthropogenic ANPP increased.In different climatic regions,the contribution of human activities to the increase of ANPP in grassland is greater than that of climate change,which mainly occurs in semi-arid,semi-humid and humid regions.