Spatiotemporal Change Research Of Ecosystems Productivity In Regions Of Central Asia Based On Improved BEPS Model

In recent years,global climate change has intensified,and the ecological security problems in the arid regions of Central Asia have become more and more prominent,posing a realistic challenge to the green “Belt and Road” construction.Fragile ecosystems,unique natural geography,and weak ground monitoring networks have led to large uncertainties in productivity estimates in the arid regions of Central Asia based on the light energy utilization model.Based on the theory and technology of RS,GIS,using improved Degree-Day Mode,Nash unit hydrograph algorithms,hydrological analysis and other methods,this research combines the ecological environment characteristics of the study area to improve the BEPS(Boreal Ecosystem Productivity Simulator)model based on remote sensing data.According to the data of regional meteorology,vegetation,soil,topography and snow,the regional estimation of ecosystem productivity in the arid regions of Central Asia was realized.It also studies the characteristics of time and space changes in productivity,analyzes the impact of climate change,and provides reference information for assessing the risks of the ecological environment in Central Asia and ensuring the construction of green “Belt and Road”.The main research processes and conclusions are as follows:(1)The improvement and verification of BEPS model.In the BEPS carbon cycle sub-model,the mountain range was extracted based on the mountain shape attribute,and the calculation method of the mountain unit area was adjusted according to the topographic factor;the NDVI index was introduced to give accurate vegetation growth for the maximum photosynthetic carboxylation rate parameter.In the water circulation sub-model,the hydrological analysis,the improved Degree-Day Mode and the Nash unit hydrograph algorithms are used to estimate the replenishment of ice and snow melt water in the mountain basin.In the stomatal conductance sub-model,the calculation parameters of the daily average temperature are adjusted according to the meteorological data of the hourly scale;the vegetation condition index was used to improve the influence of agricultural irrigation and water use strategy of desert vegetation roots on leaf water potential stress parameters.The improved BEPS model was proved to be feasible and reliable in arid area of Central Asia by site observation data and remote sensing data.(2)Spatial and temporal variations of ecosystem productivity in arid regions of Central Asia.The spatial distribution,intra-annual variation,inter-annual variation and trend of total primary productivity(GPP),net primary productivity(NPP)and net ecosystem productivity(NEP)in Central Asia ecosystems from 2001 to 2015 were quantitatively analyzed by linear correlation and spatial statistics.The results show that the spatial distribution of GPP and NPP is "high in the middle mountains and low in the East and West sides".The carbon sinks of NEP are mainly distributed in the central mountainous areas,northern Kazakhstan and oasis.In the five-year scale of spatial pattern change,Xinjiang is the only administrative region with an increased carbon sink area.During the year,GPP,NPP and NEP showed a single peak distribution with peaks in summer.GPP and NPP showed a significant downward trend in inter-annual change,while NEP showed a significant downward trend with periodic change characteristics.On the trend of change,GPP,NPP and NEP in Western Kazakhstan showed a significant downward trend.(3)Analyze the impact of changes in hydrothermal conditions on productivity.Based on the photosynthetic model of vegetation,the functional relationship between water and heat conditions and GPP of vegetation was established,and the comprehensive effects of water and heat factors on vegetation growth were studied,so as to improve the correlation between water and heat conditions and productivity,and then the effects of changes of water and heat conditions on ecosystem productivity were analyzed.Moran's I index was used to characterize the spatial pattern of precipitation,temperature and GPP,and to explore the relationship between GPP and climate change in spatial pattern.The results show that in the arid region of Central Asia,the decrease of precipitation resulted in the decrease of GPP and NPP,but did not affect the carbon sequestration efficiency(NPP/GPP)of vegetation,and also reduced soil respiration,which led to the unclear effect of the change of hydrothermal conditions on NEP.In spatial pattern,the low precipitation and high temperature regions correspond to the low GPP region,while the high precipitation and low temperature regions correspond to the high GPP region.In the process of pattern change,precipitation and temperature changes lead to frequent changes in the GPP pattern.However,in mountainous and farmland,the GPP pattern remains relatively stable.