The concept of urban forestry was created as early as in 1894 in the United States, and developed in 1960s quickly in North America and Europe. The concept of urban forestry was introduced to china in the 1980s, and Chinese scholars paid more attention on the aspects of urban forest functions and benefits previously, but rarely studied their service ability were, which was concerned more by foreign scholars. In China, the current criterion that have evaluated the service ability of urban forest often employed some statistical indices such as coverage rate, coverage area per capita, and numbers of urban forest as well. Whether to acquire the "natural services" of urban forests is the basic requirements of urban forest functions, a guide line of life quality of urban residents, and an important aspect of city modernization and international competitiveness as well. The access to urban forests for urban residents could be used as an index to evaluate the implementation of urban forest functions.As the main element of the ecological landscape, urban forests are the principal part of ecological constructions in urban regions. The blocks were an essential composition of an urbanized area. The network analyst model in ArcGIS9.2 was used to analyze the accessibility between the blocks and urban forest patches within Outer Ring Highway in Guangzhou, and the service capacity of urban forest patches was examined as well. The differences among the blocks were analyzed in combination with tree canopy coverage by interpretation of aerial photos.
The results showed that 73% of the blocks could reach large green patchs in a 2km walk distance, and about 46% in a 1km walk distance, but only 18% of blocks could reach large green patchs in a 0.5km walk distance. Urban forest patches accessibility was the highest in Yuexiu District,while the provision and accessibility were the most inadequate and worst in Fangcun and Haizhu Districts. Among blocks that could not reach large green patch in a 0.5km walk distance but tree canopy coverage beyond 30%, the percent of institutions block was predominant, while the coverage of most of the resident blocks was under 10%, and even 64% of those under 5%.
Though the actual distance might be over-estimated or under-estimated due to the use of centroids as the blocks, the losses and gains were balanced within the 2169 blocks and the uncertainty was assumed to be negligiblein this analysis. The results indicated that the total service area of urban forests was smaller than the area derived by the conventional simple buffering method, and this was due to the fact that the actual pedestrian routes were not direct and affected by many factors. GIS-based network analyses offered a more realistic insight into actual provision of urban forest and were able to reliably identify the location of lacking urban forest services and inform the local planning process. Combining the actual pedestrian accessibility rather than buffers or straight lines to urban forests with the tree canopy coverage of blocks, the results derived from the Network Analyst Model using block units provided valuable information for urban forest landscape pattern optimization at a small scale in Guangzhou.