Abstract: [Objective] Urban green belt is one of the most important ecological infrastructure.In this paper,using the concept of urban forest,RS and GIS technologies,we took the second green belt in Beijing as an example and explored the temporal and spatial dynamics with the urban tree canopy (UTC) from 2002 to 2013,in order to evaluate achievements of the ecological construction,and to reveal the process of landscape pattern changes,trend and driving forces,and to provide experiences for deepened ecological construction and maintenance of ecological infrastructure in the region.[Method] Based on 2002 aerial photo and 2013 Worldview2 images with a resolution of 0.5 meter,the urban tree canopy (UTC) of 2002 and 2013 within the second green belt were interpreted by using the eCognition 9.0 software as the platform of object-oriented interpreting technology.The landscape pattern index,which include the patch number,mean patch size (MPS),Mean patch fractal dimension (MPFD),mean shape index (MSI),mean patch edge (MPE),Shannon diversity index (SDI) and Shannon evenness index (SEI),were employed.In the analysis of changing reasons,the related statistical data were used in indentifying driving forces.[Result] The UTC area within the second green belt increased from 28 839.84 hm² in 2002 to 63 709.95 hm² in 2013,and UTC rate increased by 21.4%.The UTC patch number were decreased from 343 605 to 189 168 over this 11 years,and the impervious ground surface and grassland increased by only 8.89% and 8.89% respectively.The landscape matrix was constituted by agriculture land and impervious ground surface in 2002,but in 2013 which were transformed into the UTC and impervious ground surface.The values of landscape pattern index,such as MPFD,MSI,SDI,and SEI,showed a downward trend.This indicated that the extents of naturalization of the landscapes were gradually increased in the study area.The most stable patch type was the impervious ground surface and UTC,of which the unchanged area ratios were 64.59% and 60.34% respectively,and more than 95% of the bare land patches type was changed.From the view of changes on patch classes,both the number and area were decreased in small patch,medium patch,large patch and extra large patch.Only the huge patches were increased in the patch number and patch area.All these changes were driven by several factors including the implementation of the second green belt plan from 2003 to 2010,overall urban planning (2004-2020),river course treatment and ecological construction of Yongding and Wenyu rivers,development of new rural areas,and the urban forestation project entitled "One Million Mu Plain Afforestation Project".[Conclusion] It can be seen from the landscape changes of 11 years that the construction effect of the second green belt is very significant.Compared with the initial stage of construction,the UTC and impervious ground surface are the two fastest growing LUCC types,in which the increasing ratio of UTC is faster than that of impervious surface.From the perspective of the landscape pattern,the degree of artificial influence on patches in the whole study area is slowing down,and the degree of naturalization of landscape patches is increasing.All of these changes show that the UTC has rapid expansion in the area,and its potential role in urban biodiversity conservation is becoming more and more important.

Key words: second green belt, urban tree, canopy dynamics, landscape pattern changes, Beijing