Study On Community Structure And Carbon Fixation Of Urban Forest In Shanghai, China

Cities are important places of human activity. Although urban areas are account for only a small part of the Earth's surface, they play a significant role in changing the world's carbon circulation process, and its role is more and more important nowadays. Furthermore, its impact is not limited to city boundaries. In this sense, urban ecosystem is the important carbon source/sink in the global carbon circulation. Known as the "City's lung", urban forest is an essential part of urban ecosystem, which plays a central role in maintaining the urban environment and making it better. In recent years, the increasing of concentrations of greenhouse gases in the atmosphere leads to global climate change. Therefore, the carbon sequestration of urban forest has received increased attention of scholars. Study on community structure is the foundation of the study on the ecological benefits and management of urban forest. The differences of urban forest's structure will affect its overall ecological benefits in a large extent. In the current background of shortage of urban land resources and the high cost of new development of green space, how to improve urban forest's community structure to increase the urban ecosystem carbon sequestration and carbon storage capacity is very important.In this study, based on RS image digitization, investigation into representative community, measurement on physiological variables of dominant species, the overall structure characteristics of urban forest in shanghai were studied. Also, with CITYgreen model, carbon fixation of urban forest in Shanghai was evaluated. Furthermore the effects of community structure on carbon fixation of urban forest were studied.The results and conclusions are as follow: 1) Overall structure, community characteristics and distribution pattern of the urban forests in ShanghaiBy the end of 2007, in 688 km2 of the central city, the area of urban green space was 13910 hm2, the coverage rate of the green space was about 20.4%. Urban green space primarily concentrated in the executive office area, residential area and public open space. Forest communities are accounts for 72% of the total area of green space in Shanghai. Medium area communities of 500～3000 m2 were mainly area types, accounting for the total urban forest area 43.7%.Shanghai urban forest tree species common in about 68 species of small trees or shrubs of about 105 species, of which frequency of occurrence than 2% of the common species there are 18 species of common communities are camphor, sycamore, cedar, fir, willow, Ginkgo biloba, Magnolia grandiflora and other 17 kinds of communities, generally speaking, the Shanghai urban forest tree species composition of a single kind of obvious advantages. Urban forest canopy mainly 40% to 70%, averaging about 53% canopy density; trees, the average diameter of about 22 cm, about 66% of the trees for the 10～30 cm in the diameter wood; trees overall good health, the health level of 95% of the trees in three or more. For different functional areas, public space, urban forest in the trees, average canopy density, community average DBH, tree grade, and the health of trees such as higher than other functional areas.The urban forest by the construction division of group life form species of vegetation types, mainly in Shanghai urban forest evergreen broad-leaved forest, deciduous broadleaf forest, evergreen coniferous forest, deciduous conifer forest vegetation types 4, mainly mixed only small percentage.4 types of forest vegetation, in addition to deciduous conifer forest vegetation types other than 3, the average canopy density in the community, community density, average tree height, diameter, crown width and diameter at breast height basal area and other community characteristics are rather close to. 2) Carbon storage status, carbon fixation capacity and distribution pattern of urban forest in ShanghaiThe total carbon storage of urban forest in Shanghai was 478472 t, the average carbon density was 47.80 t·hm-2, the total amount of annual carbon sequestration 6256 t·a-1, the average carbon sequestration rate of 0.625 t·hm-2·a-1.As for different functional areas in the urban forest, the overall carbon storage and annual carbon uptake by the contribution of the order of from high to low was: administration area>residential area>public space>road area; by urban forest carbon density order:public space> residential area≈administration area≈road area; by city rate of forest carbon sequestration order:public space≈residential area≈administration area>road area.17 species common in forest communities, the carbon density, respectively the largest and the smallest communities and the acacia community of poplar, the average carbon density was 57.10 t·hm-2 and 27.81 t·hm-2; carbon sequestration rates of the largest and the smallest difference community and the community for privet locust, the average carbon sequestration rate was 1.08 t·hm·a-1 and 0.37 t·hm-2·a-1. According to the carbon sequestration rate density and size, can be a common community in Shanghai urban forest is divided into four categories:(1) carbon sequestration rate density and the larger community, (2) higher density of carbon, carbon sequestration rate in small communities, (3) lower carbon density, carbon sequestration rates of the larger community, (4) carbon sequestration rate density and smaller communities.For different types of forest vegetation communities, Shanghai urban forest carbon storage and carbon absorption in the overall distribution are:evergreen broad-leaved forest> deciduous forest> deciduous coniferous forest> evergreen coniferous forest, and evergreen broad-leaved forest of the total carbon storage of 47% to 63% of the total annual carbon uptake contributed 50% to 63%. From the perspective of carbon sequestration, carbon density by beginning with the largest community order:evergreen coniferous forest> evergreen broadleaf forest> deciduous forest> deciduous coniferous forest; by community in carbon sequestration rates from large to the small order:deciduous conifers> evergreen broadleaf forest> deciduous forest> evergreen coniferous forest.3) Effects of community structure on carbon fixation of urban forest in ShanghaiUrban forest canopy, the average diameter at breast height, density, community, hierarchy, community structure and community type of carbon sequestration capacity factors are all caused by different degrees. First, the urban forest canopy density and its carbon sequestration rates have played a decisive role, canopy density and community density and carbon sequestration rates of carbon were tested significantly positive correlation, and with the canopy density increased, Community sequestration of carbon density and a significant increase in the rate; Second, the average diameter in a sense represents the community's age structure, increase in tree growth and biomass accumulation also accelerated the decline and leaves off the trees under the double impact, the average diameter at breast height and community carbon density was positively correlated with carbon fixation rates were negatively correlated; again, for the community density, its urban forest carbon sequestration rate showed a significant positive correlation with the carbon density of not significant, considering the stand age, canopy degree of interaction and community density can still be found, low-density communities with high diameter ratio, the high-density communities with a higher carbon density; finally, from the type of tree species composition and structure of point of view, mixed forest carbon density than pure, multi-storied plantation carbon density and carbon sequestration rates are higher than the single forest, and carbon sequestration capacity of the differences to some extent by the average diameter at breast height, density and other factors affect the community.