Responses Of Carbon Fixation Process Of Alpine Vegetation To Snow Changes Over The Tibetan Plateau

Due to the impact of global climate change,the alpine vegetation ecosystem and the snow cover environment on the Tibetan Plateau have undergone significant changes.As the main carbon pool in this region,alpine vegetation ecosystem not only plays a key role in the regional carbon cycle,but also has an important impact on the ecological,climate,and hydrological processes of the surrounding areas through the interaction between the climate,soil,and vegetation.On the other hand,snow cover,one of the most active elements on the Tibetan Plateau,also has an important impact on the carbon fixation process of alpine vegetation by changing the soil temperature and humidity.However,the current understanding of the effects of snow changes on the vegetation carbon fixation process is still superficial.Therefore,it is necessary to study the complex responses of vegetation carbon fixation process to snow changes,which not only helps to deepen our understanding of the alpine vegetation ecosystem,but also improves the accuracy of the model's calculation of material circulation and energy flow within the ecosystem.At the same time,this study is also of great significance for accurately assessing the response mechanism of alpine vegetation ecosystems and actively responding to global climate change.To this aim,the phenology module of the Biome-BGC was first modified by the remotely sensed phenology,and then the ecophysiological parameters of three typical vegetation types were calibrated by combining the flux-measured GPP data.Subsequently,the calibrated Biome-BGC model was used to simulate the carbon fixation capacity of alpine vegetation from 1982 to 2015.Finally,the response mechanisms of vegetation carbon fixation process to climate change and snow change were analyzed respectively.The conclusions are as follows.(1)The original meteorological-based phenology module in the Biome-BGC was found to significantly delayed the SOS of alpine vegetation,while the modified Biome-BGC whose phenology module was modified by the remotely sensed phenology can more accurately simulate the phenological parameters of alpine vegetation.In addition,after using the simulated annealing algorithm to calibrate the sensitive ecophysiological parameters selected by the Morris and Sobol'method,the Biome-BGC model can more accurately simulate the carbon fixation capacity of alpine vegetation as compared to the original model.(2)Due to the effects of topographic conditions and hydrothermal gradients on the Tibetan Plateau,the carbon fixation capacity of alpine vegetation exhibited obvious spatial heteorogenity,which showed a decreasing trend from southeast to northwest.In addition,according to the statistical results,the mean annual GPP from 1982 to 2015was 537.7 g C/m~2/yr,the mean annual NPP was 289.1 g C/m~2/yr,and the mean annual NEP was 17.8 g C/m~2/yr.At the same time,they all showed a significant increasing trend from 1982 to 2015,with a rate of 3.81 g C/m~2/yr,1.97 g C/m~2/yr,and 1.03 g C/m~2/yr,respectively.(3)The carbon fixation process of alpine vegetation had different responses to the changes in different meteorological factors.Among them,temperature and precipitation played a leading role in the variation of vegetation carbon fixation capacity,while vapor pressure deficit(VPD)and solar shortwave radiation played a relatively weak role.In addition,in most areas,there was a significant positive correlation between vegetation carbon fixation capacity and temperature as well as precipitation,while for VPD and solar shortwave radiation,only a small number of regions have shown significant correlations.Furthermore,the area difference between the significant positive correlation region and the significant negative correlation region was small.(4)For different geographical regions and different vegetation types,the response of vegetation carbon fixation process to snow changes is different.Among them,there was a significant correlation between the vegetation carbon fixation process and the snow changes in the southeastern part of the Tibetan Plateau,Changtang Plateau,and Kailas Range,and significant negative correlations accounted for most of these areas.In addition,according to the response results of different vegetation types,there was a significant correlation between the carbon fixation process and snow changes for alpine steppes,while the correlation was not statistically significant for other vegetation types.On the other hand,there were also obvious differences in the response of vegetation carbon fixation process to snow changes in different altitudes,temperature and precipitation gradients as well as different ecological zones.