Effects Of Urbanization On Carbon Storage And Sequestration In The Built-up Area

Urbanization is the most important aspect of land use and land cover change globally and dramatically altered the ecosystem process and function.With the increase of scientific and political interest in regional aspects of the global carbon cycle,the missing of inventory data of urban area has already bottleneck the evaluation of regional carbon balance.There are two opposite forces drive the carbon sequestration of the plants within the urban areas.On one hand,urban land use reduced the coverage of vegetation with impervious surface.On the other hand,GS within urban location may have higher productivity than surrounding wildland due to garden management,heat island,ozone restriction etc.Those opposite driving forces leave uncertainty of the net effect of urban land use on regional carbon sequestration variation.Using comprehensive field survey of the distribution and growing conditions of vegetation in the young city-Taizhou and the historical city-Hangzhou in the Southeastern China,our objective was to assessing the carbon sequestration and storage by estimating the biomass and net primary productivity(NPP) of urban vegetation including trees,shrubs and lawns in the greenspace in both streets and parks with the consideration of garden management(pruning and mowing) effect.The annual carbon fixation of vegetation in Hangzhou built-up area is 132 388.2 tC in which trees accounted for about 74%lawn accounted for 24%and shrub only 2 %.The carbon sequestration of Hangzhou built-up area is 4.3 tC·hm^-2·yr^-1 the carbon storage of vegetation in Hangzhou is 554 689.8 tC in which trees contributed 503 937.0 tC,lawn contributed 41 895.0 tC and shrub contributed only 8 857.8 tC.The annual carbon fixation of vegetation in Taizhou built-up area is 23 949.9 tC in which trees accounted for about 64%lawn accounted for 27%and shrub only 9%.The carbon sequestration of Taizhou built-up area is2.1tC·hm^-2·yr^-1 the carbon storage of vegetation in Hangzhou is 91 763.1 tC in which trees contributed 66 467.7 tC,lawn contributed 9956.7 tC and shrub contributed only 15 338.7 tC. We also found that due to the urban environment and human activities effects,the trunk growth rate of individual urban tree is two times of that in the native forest.This shows that urban vegetation is different with wild vegetation is samilar in the humid subtropical regions with the other climatic zones.In urban area,pruning contributes 30%of tree NPP.This means the previous work should revaluate there results.By comparing with natural forest,the carbon sequestration ability of Hangzhou built-up area has already met the level of native evergreen broad-leaved forest and we infer that the carbon sequestration ability of Taizhou built-up area can meet the level of native evergreen broad-leaved forest with green coverage rising from 23%up to 46%.This help government and the planning department to find way to fulfill "carbon neutral" in the future management and decision-making.Taizhou was very young thus the urban trees are quite small.However,because of the reduction of green land and trees by frequent turnover,urban trees in Hangzhou also show smaller ages comparing with the age of the city.Therefore,in the future, urban greening should not only increase the area and proportion,but also should pay more attention to protect the existing green space,especially the old greenspace.