Biomass Of Alpine Grassland In Saini District,Nagqu City Based On Radiative Transfer Model Spatial And Temporal Variation And Climate Impact Analysis

As an important part of the Qinghai-Tibet Plateau ecosystem,alpine grasslands have a significant impact on China’s climate,biodiversity,soil and water conservation,and other aspects.By monitoring the above-ground biomass of alpine grasslands,it is possible to analyse the patterns and trends of changes and provide a valuable scientific basis for the conservation and restoration of alpine grasslands.The Saini District(Nagqu County)is part of the Nagqu Region of the Tibet Autonomous Region,and basically covers the main types of alpine grasslands in the northern Tibetan Plateau,with the main vegetation type being alpine grassland.In this paper,the above-ground biomass of alpine grassland in Saini District was selected as the research object,based on PROSAIL model,MODIS Popper response function and BP neural network,and MOD13A3 data was used as an important data source to invert the above-ground biomass of alpine grassland in the study area.Then,the correlation between above-ground biomass and climatic factors(rainfall,temperature,sunshine duration and relative humidity)was investigated and the influence of climate on above-ground biomass of grassland in the study area was analysed.The main results and conclusions are as follows.(1)Based on the PROSAIL model,MODIS Popper response function and BP neural network,the inversion of above-ground biomass of grassland in the study area was carried out using MOD13A3 data,and the RMSE value of the inversion was 15.02g/m~2 and the R~2 value was 0.73,which were in good agreement with the measured results(the RMSE of 16.25 g/m~2 and R~2 of 0.7).The RMSE of 17.25 g/m~2 and R~2 of0.69 between the inversion results and the NPP conversion results are in good agreement with each other and the error between them is small.(2)A long time series grassland above-ground biomass dataset was established in the study area using BP neural network with month-by-month MOD13A3 data from2000 to 2019.The analysis revealed that the above-ground biomass biomass in the study area peaked in August;May to August was the rising period;and September to November was the falling period.(3)The above-ground biomass of grassland in the study area during the peak period was selected to represent the annual above-ground biomass of grassland in the study area,and spatial and temporal changes were analysed using the trend method,spatial autocorrelation analysis and optimized hotspot analysis.The results showed that the above-ground biomass of grassland in the study area declined somewhat between2000 and 2019,with an overall very slow rate of decline.Some areas in the eastern part of the study area showed a certain trend of improvement,but its rate of improvement was not obvious and its area share was relatively low;while some areas in the western and central parts of the study area showed a very obvious trend of decline and a higher area share.(4)Using the results of the correlation analysis between the above-ground biomass of grassland in the study area during the peak period and climatic factors in each period,it was shown that:the above-ground biomass of grassland in the study area was most significantly influenced by climatic factors during the rising period of above-ground biomass of grassland,which had the most prominent area share related to relative humidity,followed by temperature,then rainfall and sunshine duration in that order;in July of the rising period,the above-ground biomass of grassland in the study area In July,the highest percentage of areas with significant correlation between above-ground biomass and relative humidity was 42.01%.The areas affected by climatic factors are mainly located in the western,northwestern,southwestern and central regions of the study area;the above-ground biomass of grassland in the affected areas is mainly positively correlated with relative humidity and rainfall,and negatively correlated with sunshine duration and temperature.The degraded areas of grassland above-ground biomass were significantly influenced by climatic factors,while the improved areas of grassland biomass were not significantly influenced by climate.