Spatiotemporal Evolution Of Land Use In The Process Of Urbanization And Its Impact On Urban Forest Carbon Storage And Carbon Cycle

Urban forests are closely related to urban landscape construction,park management,and urban planning.It alleviates the decline in environmental quality caused by urban development by affecting changes in the physical space and biological environment of the urban.,having important functions in CO₂ emission reduction,air purification,and noise reduction,Climate improvement and other aspects in the urban.However,with the acceleration of urbanization,urban land use has undergone dramatic changes,especially forest changes,which will affect the urban’s carbon storage and the spatial and temporal pattern of carbon cycle,thus affecting forests’improvement of urban ecology.Therefore,monitoring the spatiotemporal dynamics of land use in the process of urbanization,analyzing evolution laws and evaluating their impact on urban forest carbon storage and carbon cycle,and all aspects of urban planning,environmental management,urban forest construction,and evaluation of urban forest ecological service functions have important theoretical and practical significance.This study takes Hangjiahu as an example,uses Landsat TM and OLI remote sensing data,and uses C5.0 decision tree to classify the land use in Hangjiahu,and analyzes the dynamic trends of land use during the urban expansion in the past 20 years（2000-2018）;On this basis,the random forest model and the In TEC model are used to estimate the urban forest aboveground carbon storage（AGC,Aboveground Carbon）and simulated net ecosystem productivity（NEP,Net ecosystem productivity）;finally,analyzing the impact of land use changes in the urbanization process on forest AGC and NEP spatial and temporal distribution patterns.Through research,the following three conclusions are obtained:1.The C5.0 decision tree accurately extracted the land use information of Hangjiahu in the past 20years,which laid an important foundation for the analysis of the spatial and temporal evolution of land use and its impact on forest AGC and NEP during the urbanization process.The overall accuracy of image classification in different periods is more than 90%,and the Kappa coefficient is more than 0.87;the results of urbanization dynamic analysis show that from 2000 to 2018,the land use in the Hangjiahu has undergone dramatic changes during urbanization,and the urban area accounts for the study area increased from 5.84%in 2000 to 21.32%in 2018.During the urban expansion,the forest area decreased by approximately 27,900 ha,and 71.05%of these reduced forest areas were caused by urban expansion.2.The optimized random forest model realizes high precision estimation of urban forest AGC.The research shows that the correlation index R² between the estimated AGC and the measured AGC of the random forest model in different periods is between 0.69 and 0.73,and the RMSE is relatively small,between 3.18 Mg·ha^-1 and 4.84 Mg·ha^-1.The analysis shows that during the past 20 years,the forest carbon stocks and total carbon stocks in the Hangjiahu have gradually increased.The average forest carbon stocks increased from 10.14 Mg·ha^-1 in 2000 to 44.59 Mg·ha^-1 in 2018.Carbon stocks increased from 641.38 Mg C in 2000 to 2472.51 Mg C in 2018,which is about 2.85 times,but local carbon stocks have decreased.The reductions occurred mainly in central Huzhou,Fuyang District,and south of Hangzhou,and the reduction value mainly concentrated between 10-20 Mg·ha^-1.3.Research shows that the InTEC model can better model the Hangjiahu urban forest NEP,the site-verified model R² is above 0.88,and the RMSE is 59.87g C·m^-2·yr^-1;NEP spatiotemporal analysis shows that from 2000 to 2015,the total amount of forest NEP in Hangjiahu increased from 2.52Tg·C·yr^-1 in2000 to 4.06 Tg·C·yr^-1 in 2018,about 1.61 times.However,in some plains and urbans with small forest area but suffered serious urbanization,forest NEP has also decreased,such as Pinghu,Haiyan,Tongxiang,Haining and Deqing,Yuhang,Hangzhou and Xiaoshan City.At the same time,forest NEP has also decreased around the main urban areas of Anji,Linan and Fuyang.4.The analysis shows that the urban expansion in Hangjiahu has led to a decline in forest carbon storage in Huzhou,Deqing,Yuhang,Fuyang District,Hangzhou Municipality,southern Xiaoshan City and southern Yuhang District.The total carbon storage decreased by about 415,193 Mg,which is about3285.36 ha;on the other hand,urban expansion has led to a reduction in forest NEP in Fuyang District,Hangzhou municipality and southern Xiaoshan District.The reduction in forest NEP due to urban expansion is about 21732.39 ha,about total NEP of 0.07 Tg·C·yr^-1;the impact of the buffer zone on the urban expansion of Hangzhou,Jiaxing and Huzhou and the spatial changes of forest carbon storage and NEP shows that the forest AGC and NEP in the main urban area are large during the urban expansion The reduction has had a serious impact on the urban forest carbon sink function.