CASA Model Was Used To Evaluate The Effect Of Poplar Deforestation On Net Primary Productivity Of Vegetation In Dongting Lake Wetland

Net primary productivity(NPP)of vegetation is the total amount of CO2 absorbed by vegetation into organic dry matter per unit area and per unit time,which is a direct reflection of the efficiency of vegetation carbon sequestration.Monitoring the changes of NPP is helpful to assess and manage the change of regional eco-environmental quality and carbon sequestration function.Due to the impacts of human activities and global climate change,wetlands are the most sensitive areas to CO2 emissions and greenhouse effect.Therefore,it is important to understand the impacts of human-induced wetland changes on NPP to reveal the mechanism of carbon sequestration in wetlands.Dongting Lake wetland is one of the largest freshwater wetland ecosystems in China.Since the end of the last century,the demand for pulp wood has led to a large increase in poplar planting area,which has led to the destruction of the original wetland cover and serious damage to the ecological environment of Dongting Lake.To restore Dongting Lake wetland ecosystems,China implemented the Poplar Ecological Retreat Project(PERP).The PERP provided an effective observation condition for revealing the effects of wetland change on NPP.This study took Dongting Lake wetland as the research area,and based on remote sensing and GIS technology,we analyzed the data before and after PERP(before PERP:2008-2013;After PERP:2013-2018),wetland cover change,landscape pattern evolution,and NPP spatial and temporal trends.Pearson correlation analysis,random forest regression and systematic clustering analysis were used to distinguish the driving forces of landscape pattern,climatic factors,vegetation cover and other factors on NPP.We further understood the evolution trend of NPP before and after PERP in Dongting Lake wetland,clarified the role of PERP in the evolution of carbon sequestration function in Dongting Lake wetland,provided theoretical basis for improving carbon sequestration capacity of Dongting Lake wetland,and provided reference for similar ecological projects in the future.The results showed that:(1)The implementation of PERP changed the evolution trend of wetland cover in Dongting Lake.Changes in the proportion of wetland cover area before PERP were as follows:moss marshland(MML)increased from 20.3%to 27.6%,forest beach(FB)from 8.1%to 9.4%,reed beach(RD)from 41.9%to 34.3%,moss marshland(MML)from 8.2%to 6.0%,shallow water(SW)from 15.2%to 15.9%,deep water(DW)increased from 6.3%to 6.6%.Changes in the proportion of wetland cover area after PERP were as follows:moss marshland(MML)from 27.6%to 16.4%,Forest beach(FB)from 9.4%to 5.5%,reed beach(RD)from 34.3%to 39.1%,Mudflats(ML)from 6.0%to 8.2%,shallow water(SW)from 15.6%to 20.2%,deep water(DW)decreased from 6.6%to 4.9%.A total of 13895.3ha of poplar returning area(PRA)was produced in PERP forest beach(FB)contributed the most to PRA(36.11%),but because poplar seedlings planted in reed beach(RD)and moss marshland(MML)were also cleared.moss marshland(MML,30.48%)and reed beach(RD,32.07%)also contributed to poplar retreat.PERP inhibited the expansion of Moss marshland(MML)and forest beach(FB),and promoted the expansion of reed beach(RD),moss marshland(MML),and shallow water(SW).(2)PERP had little effect on total NPP of Dongting Lake wetland,with the total NPP decreased by only 2.21%after PERP,which was mainly due to the expansion of reed beach(RD)which made up for the loss of total NPP after poplar deforestation.Before and after PERP,reed beach(RD)played a dominant role in NPP,accounting for about 60%of the total NPP in Dongting Lake wetland.After PERP,the contribution of RD to NPP was further increased to 66.6%.After PERP,the contribution of forest beach(FB)to NPP decreased from about 20%to 12.3%,and that of moss marshland(MML)decreased from about 14%to 8.7%.The contribution of Poplar returning area(PRA)generated by PERP to total NPP was only 3.2%.(3)Before and after PERP,the contribution rate of landscape pattern factors decreased from 51.29%to 38.77%,while that of meteorological factors increased from 33.19%to 40.63%,and that of vegetation cover increased from 15.58%to 20.60%.Because PERP inhibited the fragmentation and homogenization trend of Dongting Lake wetland,and promoted the development of diversity,patch irregularness,and landscape heterogeneity,the effects of landscape pattern factors on NPP were more uncertain.This study is helpful to further understand the contribution of anthropogenic activities,climate change and other factors to wetland NPP in the implementation of ecological retreat project,provide scientific evidence to reveal the mechanism of wetland carbon cycle,optimize the environmental management mode of Dongting Lake wetland,and promote the restoration and development of wetland ecosystem function in Dongting Lake.