Modeling Of Carbon And Water Fluxes Of A Planted Shrub Ecosystem In Desert Steppe Areas Of Yanchi County And The Impact Of Climate Change On It

Desert steppe is a special type of vegetation in transition from grassland to desert and is the main part of the arid and semi-arid regions,its vegetation community structure is simple and vulnerable to climate change and human interference.For sand prevention and control and ecological restoration,China has implemented a large number of ecological management projects in the degraded grassland areas of the northwest desert steppe,such as grazing prohibition,grassland replanting and artificial planting shrubs,which are the important reasons for the "greening" of the vegetation in northwest China.However,the changes in ecosystem structure and function caused by the strong artificial disturbance in the desert steppe area,especially the shrub planting,are the current hot spots in ecosystem ecology.The study of carbon and water flux characteristics of planted shrub ecosystem in desert steppe area,revealing the response mechanism of ecosystem carbon and water cycle during the invasion of planted shrub into desert steppe,is of great scientific significance to understand how human activities affect the carbon and water cycle process of ecosystems in desert steppe areas.At the same time,under the background of global climate change,considering different climate change scenarios and future climate change trends,the simulation of carbon and water fluxes of the planted shrub ecosystem in the desert steppe areas and the study of the response of the ecosystem to climate change can provide a theoretical basis for coping with global changes in arid and semi-arid regions.Of course,accurate observation and simulation of carbon and water fluxes of planted shrub ecosystem in desert steppe areas are also of great practical value for guiding local ecological restoration and proposing regional ecological protection and reconstruction policies.Therefore,this study takes the planted shrub ecosystem by the ecological management project in Yanchi County desert steppe area of Ningxia as the research object,and uses the Biome-BGC model,Parameter ESTimation(PEST)method and climate scenario simulation as the technical means,based on the carbon and water flux observation data of Yanchi County desert steppe ecosystem observation research station for many years.The simulations of carbon and water fluxes in planted shrub ecosystems in the Yanchi County desert steppe area and the effects of climate change on carbon and water fluxes were carried out.The main findings are as follows:(1)The model was driven by meteorological data and physiological and ecological data of shrub vegetation,and then the physiological and ecological parameters of the Biome-BGC model were optimized by measured data of the eddy covariance system in the Yanchi County desert steppe ecosystem locational observation site combined with the PEST model.The optimized Biome-BGC model was used to simulate the gross primary productivity(GPP)and evapotranspiration(ET)of the Yanchi County desert steppe encroachment ecosystem and the simulation results were verified by using the measured data.The results showed that after parameter optimization,both GPP and ET simulated by the model improved,especially the simulation of the growing season was closer to the observed values.This result indicated that the PEST could improve the simulation accuracy of Biome-BGC model for GPP and ET of the planted shrub ecosystem in desert steppe area.Meanwhile,it was found that after the optimization of PEST,the improvement effect of Biome-BGC model on ET was more obvious than that on GPP.(2)The PEST-based Biome-BGC model has strong universality,and the parameters optimized by PEST can be applied to the simulation of GPP and ET of the planted shrub ecosystem in the desert steppe area for a long time.In this study,the optimized physiological-ecological parameters were used to simulate the GPP and ET of the planted shrub ecosystem in Yanchi County desert steppe area for a long time series from 1986-2018,and the trends of GPP and ET of the planted shrub ecosystem in Yanchi County desert steppe area of Ningxia over the past 32 years were analyzed.It was concluded that GPP showed a slowly increasing trend of 1.47 gC m-2 a-1 from 1986 to 2018,but the interannual variation rate of ET was larger and without a significant trend.(3)Correlation analysis of GPP and ET of the planted shrub ecosystem in the Yanchi County desert-steppe area from 1986-2018 with each climate factor showed that there was a significant positive correlation between GPP and precipitation(r=0.40,P<0.05)and also a positive correlation with temperature(r=0.40,P<0.01).That is,the magnitude of primary productivity in planted shrub ecosystems in the Yanchi County desert steppe region is mainly controlled by precipitation and temperature.The correlation between ET and precipitation was more significant(r=0.51,P<0.01),the evapotranspiration of planted shrub ecosystem in the desert steppe area of Yanchi County is obviously influenced by precipitation,and the amount of natural precipitation determines the variation of total annual evapotranspiration.However,there was a significant negative correlation between GPP and solar shortwave radiation(r=-0.36,P<0.05).There is a significant negative correlation between ET and solar short-wave radiation(r=-0.53,P<0.01)and vapor pressure deficit(r=-0.54,P<0.01).The strong solar shortwave irradiation and vapor pressure deficit reduced the GPP and ET of the planted shrub ecosystem,and the climate drought driven by the strong radiant energy would have a negative impact on the GPP and ET of the Yanchi County.This means that drought driven by strong radiant energy will have a stressful effect on photosynthetic production and water cycle processes.(4)The simulation of GPP and ET under different climate change scenarios showed that an increase under temperature alone would weaken GPP of the planted shrub ecosystem in the Yanchi County desert-steppe region,while an increase in precipitation alone would enhance GPP;when both temperature and precipitation were increasing,the enhanced effect of GPP caused by increased precipitation would mitigate the weakened effect of GPP caused by increased temperature.However,when the increase of temperature will be 3℃,the GPP basically decreases to the minimum value regardless of the increase in precipitation,which may be the maximum limit of the planted shrub ecosystem in the Yanchi County desert steppe area to tolerate climate warming,and the productivity of its ecosystem may collapse under this enhancement ET increases in both the increasing precipitation scenario and the rising temperature scenario,but if the temperature continues to rise,the magnitude of ET increase decreases with increasing temperature.(5)The GPP and ET of the planted shrub ecosystem in the Yanchi County desert steppe region tended to increase in the early,middle,and late part of this century and under the two scenarios of SSP2-4.5 and SSP5-8.5,while the carbon and water fluxes of the planted shrub ecosystem in the Yanchi County desert steppe region might slowly decrease in the future in the SSP 1-1.9 scenario.Thus,the growth of planted shrub in the Yanchi County desert steppe area may be promoted with increasing CO2 concentration and precipitation in the context of global warming conditions.