| Acceleration of urbanization has directly cased a shortage of ecological construction land.urban forest construction is devoted to solving problems including poor quality of urban greening,low ecological benefits and high maintenance costs.Therefore,it has important research value to study on optimization of urban tree structure,expansion of ecological space and improvement of forest quality from the standpoint of urban tree canopy and three-dimension green biomass.The green space of residential areass and public institutions in Beijing urban areas were taken as the research object.The 2013 World View 2 satellite imagery with a resolution of 0.5 meter of Beijing urban areas was used as data sources after the geometric and orthographical correction,as well as the raster and vector data of urban tree canopy in Beijing 2013.Combined with field investigation,urban forest structure and urban tree canopy were analyzed and discussed,including aspects of tree species composition,community diversity,forest density,DBH structure,crown structure and height structure from overall level,different loop,different region and different classification.Some suggestions for future urban forest construction were put forward.The results showed that:(1)Structure characteristics of urban forest.For urban forests of residential areass and public institutions,there are a significant high proportion of trees.The application frequency of conifer is low and native tree species account for 76%.The tree species application in different loop and different region is consistent with the whole city.Urban forest index of richness,Shannon-wiener,Simpson and Pielou in public institutions are all higher than residential areass.The diversity index and richness index of residential areass have a same change trend between different loops: 4th-5th ring road > 5th Ring Road-the outer edge of the 2nd Greenbelt > within 2nd ring road> 3rd-4th ring road > 2nd-3rd ring road.But Pielou index has a changing trend of 3rd-4th ring road > within 2nd ring road.Comparing the four indexes between different regions,Haidian district,Tongzhou district,Fangshan district are at the forefront.The BDH,height and average crown breadth of trees in public institutions are 19.97 cm,7.63 m and 5.34 m respectively.They're all higher than residential areas which of 17.58 cm,7.76 m and 5.31 m.Grade distribution of the three indexes is pyramid-shaped.In residential areass,the three indexes present a diminishing trend from city center to urban periphery.Old districts such as Dongcheng district and Xicheng district have high level.Residential areass and public institutions have both reached the level of urban forest either in whole level or each partition.However,the potential of urban forest become greater with the expansion of loop.Of the 20 dominant tree species,Sophora japonica,Fraxinus chinensis,Platanus hispanica have the largest contribution to urban tree canopy and green biomass of residential areass in Beijing,Juniperus formosana and Sabina chinensis have the lowest.(2)UTC of residential areass.The UTC of residential areass in Beijing urban areas is 29.67% with the highest of 54.06% and the lowest of 11.31%.The potential UTC is 0.22%.In Beijing urban areas,73.12% residential areass are of low or middle canopy cover.The UTC in different sorts of residential areass presents: subsidiary of residential areas(33.96%)> resettlement residential areas(30.06%)> commodity residential areas(20.77%);2006-2010(33.49%)> 1991-2000(30.64%)> after 2010(30.62%)> 1981-1990(29.78%)> before 1980(29.21%)> 2000-2005(26.86%).Residential areass with low plot ratio have high canopy cover.The UTC of residential areass in overall level presents that urban periphery is greater than the city center,the west is better than the east and the north is slightly higher than the south.The potential UTC within 2 ring is 0,including Dongcheng district,Xicheng district and Fengtai district,only suburban areass have potential UTC,but very small.(3)UTC of public institutions.The UTC of public institutions is 26.88% with the highest of 50.38% and the lowest of 0.The potential UTC is only 0.643%.64% of green space in public institutions in Beijing urban areas are of very low or low canopy cover.The UTC rates in different administrative levels present that: town government(38.13%)> public institutions under the State(29.19%)> district government(26.61%)> township government(21.61%)> public institutions under the city(20.83%)> sub-district(7.99%).Only public institutions under state and city have potential UTC.They are 1.05% and 0.02% respectively.The UTC in different social function sorts present that: public institutions with public service(28.14%)> public institutions with administrative function(23.59%).Only public institutions with public service have a potential UTC of 0.89%.The UTC of public institutions become higher with the expansion of loop.The UTC in center districts are very low,such as Xicheng districts,Dongcheng districts,and Fengtai districts.Only 2nd-3rd ring road and Haidian districts have potential UTC,which are 1.85% and 1.16% respectively.(4)Optimization suggestions of urban forest in Beijing urban areas.Optimization suggestions were put forward according to the result of urban forest investigation and the five principles including forest structure and function,biological diversity and stability,humanistic ecology of urban,sustainable management,aesthetic.The suggestions were of five aspects aiming at the problems of the shortage in ecological construction land and low quality of urban forest construction in Beijing.Firstly,more attention should be paid to the selection of the urban forest tree species;Secondly,density of forest should be controlled reasonably;Thirdly,the status of "person" in urban forest construction should be promoted;Fourthly,construction of forest with water should be improved;Fifthly,the maintenance and management of urban forest should be strengthened. |
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