Simulation Of GPP Based On Solar-induced Chlorophyll Fluorescence And Its Response To Drought In Mongolian Plateau

The continuous global warming has changed the distribution of water resources,and the hydrological cycle in many regions has been severely affected by climate change,leading to frequent occurrences of natural disasters on a global scale.Ecologically fragile arid/semi-arid regions are very sensitive to climate change,and the frequency,intensity,and scope of droughts tend to increase.The Mongolian Plateau is located in a typical arid/semi-arid zone,with abundant vegetation resources,plays an important role in East Asia and even the global carbon cycle,and is an important ecological security barrier in northern of China.The negative impact of drought and water shortage on vegetation productivity is an important factor restricting the ecological construction and sustainable economic and social development of the Mongolian Plateau.Therefore,it is of great significance to improve the comprehensive monitoring capability of the Mongolian Plateau for drought and the estimation accuracy of regional-scale vegetation gross primary productivity(GPP),and to explore the response of the green vegetation productivity of the Mongolian Plateau to drought is of great significance to the sustainable development of the ecological environment in the region.From the perspective of“atmosphere-vegetation-soil”,this dissertation takes the response relationship between vegetation ecosystems and drought as the research topic.Firstly,the potential relationship between solar-induced chlorophyll fluorescence(SIF)and GPP is used to construct three SIF-based GPP estimates.The model is used to screen the optimal GPP estimation model through ground observation data,and based on the GPP dataset output by the model,dynamically analyze the spatio-temporal characteristics of the GPP in the study area,revealing the evolution of the vegetation ecosystem GPP;Secondly,the Copula function is used to effectively couple soil moisture and meteorological data to construct the MMSDI drought monitoring index,which systematically analyzes the intensity,scope and frequency of drought on the Mongolian Plateau from 1981 to 2018,as well as the spatio-temporal trends and abrupt changes of drought on different time scales.In addition,Discussed the influencing factors of drought from the perspective of regional environment and atmospheric circulation.Thirdly,from the perspectives of sensitivity,resilience,resistance,vulnerability,etc.,quantitatively evaluated the response of different vegetation ecosystems GPP on the Mongolian Plateau to multi-scale drought;Finally,based on the CMIP6 model data,the Mongolian Plateau drought and vegetation GPP under different future emission scenarios were estimated,and their temporal and spatial evolution laws were studied respectively.The research content has improved the accuracy of GPP and drought monitoring on the Mongolian Plateau,and solved the key issue of quantifying the impact of drought on the growth status of different vegetations in the study area.This research work can provide technical support and methodological references for exploring the interaction between climate and vegetation under the background of global change,and studying ecosystem service functions and global carbon cycle.The main content and results of the paper are as follows:(1)Research on SIF-based GPP simulation and its temporal and spatial evolutionThe downscaled GOME-2 SIF data is compared with the Tropomi SIF data.The results show that there is good spatial consistency between the two(R²=0.87);Compared with the GPP linear regression model based on SIF,the SIF-based CNN-BP convolutional neural network GPP regression model,which takes into account multiple factors affecting GPP,has advantages in the GPP simulation of the Mongolian Plateau,which can effectively improve the accuracy of the GPP simulation;From 2007 to 2018,the slope of GPP in the growing season of the entire study area showed an upward trend,and there was an obvious spatial differentiation law,that is,decreasing from northeast to southwest.Among them,the high-value areas of GPP were mainly concentrated in the eastern forest area of Hulunbuir.In spring and autumn GPP showed a slow increase at the rate of 0.44 g Cm^-2/a and 0.45 g Cm^-2/a,while in summer GPP showed a significant increase at a rate of 1.52 g Cm^-2/a.(2)Analysis of the characteristics of spatial and temporal changes in the Mongolian Plateau droughtComparing three kinds of soil moisture grid data(ERA-5,GLDAS,FLDAS)with the data of the actual measurement site,it is found that the ERA-5 product has an advantage in capturing the soil moisture characteristics of the Mongolian Plateau.The Copula joint distribution function was used to effectively couple the ERA-5 soil moisture data and the meteorological data,and the revised MMSDI index was established,and the historical disaster data and the SPEI index were used to verify the time-space ratio,which proved that the MMSDI index is suitable for a large area of the Mongolian Plateau.Drought monitoring.Using the MMSDI drought index to carry out the analysis of the spatial and temporal characteristics of the drought in the Mongolian Plateau,it was found that the overall MMSDI of the Mongolian Plateau showed a significant downward trend from 1981 to 2018(MMSDI-12 slope-0.397/10a).Through the MK mutation test,a mutation occurred in 1998,and the degree of drought increased after the mutation;Spatially,the drought in other regions except the western part of Inner Mongolia has an increasing trend.The drought in the Mongolian Plateau is getting more and more serious,and the drought situation in Mongolia is more severe than that in Inner Mongolia;The degree of drought intensification in different seasons is in the order of summer>spring>autumn>winter;The main factors affecting the short-term drought in the Mongolian Plateau are soil moisture,precipitation and VPD in order.(3)Research on the response of Mongolian Plateau GPP to droughtThe sensitivity of GPP to dry and wet environments is different in the growing season.Grassland types are most sensitive to dry and wet environments,followed by shrub,cropland,and sandy vegetation,and coniferous forests and broad-leaved forests are less sensitive;Using ARx autoregressive model analysis,from arid to semi-humid ecosystems,the vegetation resilience has a logarithmic relationship with the AI index(R²=0.88),indicating that the vegetation resilience of the Mongolian Plateau increases as the ecosystem becomes humid.The vegetation resistance index presents a parabolic-shaped fitting line in arid to semi-humid regions(R²=0.79),and reaches a peak around0.28 in the AI index.This shows that the vegetation in the arid and semi-humid areas of the Mongolian Plateau has better resistance to drought,while the vegetation in the semi-arid areas is weaker;Pearson correlation analysis is used between GPP and MMSDI,and MMSDI dominates the grassland on a short time scale.The fluctuation of type GPP,grassland type can be used as an"indicator"for the occurrence and development of drought.The GPP fluctuations of shrubs,agricultural vegetation and sandy vegetation are mainly affected by the MMSDI on the medium time scale.The GPP of coniferous forest and broad-leaved forest is mainly affected by the long-term MMSDI,and is negatively correlated with the short-term MMSDI;from the analysis of the vegetation GPP loss rate curve under drought stress,the GPP loss rate fluctuates in the summer grassland type Larger.The loss rate of GPP in forest areas is in the range of 0-40%,while the loss rate of GPP in shrubs,cropland and sandy vegetation(0-60%)is between grassland types and forest areas;During 2007?2018,the typical grassland GPP in the Mongolian Plateau suffered the most severe drought loss,with a cumulative total of 1.4×10⁷g Cm^-1.(4)Estimation and analysis of Mongolian Plateau GPP under future drought scenariosBased on the CMIP6 global climate simulation data,the MMSDI index under different emission scenarios in the future was obtained and analyzed after screening and verification.The annual average MMSDI value of the Mongolian Plateau area continued to decrease as the emission concentration of the three same scenarios increased.In terms of space,the area with a decreasing trend shows a gradually expanding trend from the east to the west of the study area.The frequency of droughts in the early and late 21st century is greater than that in the mid-term,while the frequency of droughts in the western part of the study area is greater than that in the eastern part.A GPP estimation model based on the multi-scale MMSDI index was established to estimate the future GPP of the Mongolian Plateau.The average values of all emission scenarios are higher than the historical average of GPP.In terms of time,as the concentration of greenhouse gas emissions increases,the rate of decrease of GPP increases;Spatially,as the concentration of greenhouse gas emissions increases,the range of vegetation GPP in the Inner Mongolia area has gradually expanded,and the vegetation GPP in the western and central areas of the Mongolian Plateau has decreased more than the eastern area.