364 Bangkok (on the basis of tree population) are angsana (Ptero- carpus indicus), tamarind (Tabebuia rosea), golden shower (Cassia fistula), Honduras mahogany (Swietenia macrophylla), and bullet wood (Mimusops elengi). All of these were recorded in samples taken in Bangkok and three were among the top five in terms of frequency of occurrence. In Beijing, the five most common species are Japanese pagoda tree (Sophora japonica), Chinese white poplar (Populus tomentosa), Chinese juniper (Ju- niperus chinesis), black locust (Robinia pseudoacacia), and Chi- nese red pine (Pinus tabulaeformis) (Yang et al. 2005). The survey of Beijing using the method presented in this article found four of these five species among the top five species on the basis of their frequencies of occurrence. All five species were recorded by the method. The species area curve procedure used by botanists to deter- mine the size of a sample needed to identify the number of plant species in an area was applied to the data obtained on tree spac- ing and the dimensions of tree wells. In this procedure, the sample size is doubled and the number of species is plotted over each sample size (Mueller-Dombois and Ellenburg 1974). Curves plotted in this way level off and the point at which they begin to level off determines the size of the sample needed. This plotting procedure was used to see if the number of samples was adequate to capture the variety of tree spacings and tree well dimensions in the cities studied. Curves for three cities, repre- sentative of the patterns of curves for all of the cities studied, are presented here (Figure 4). Most cities followed the pattern illus- trated by Bangkok and Johannesburg with regard to tree spacing. This pattern is characterized by a leveling off in the number of tree spacings recorded when the number of samples was in- creased for 16 to 32. This suggests that the sampling intensity was sufficient to capture 90% to 95% of the spacings between trees used in the cities. In contrast, the curve produced for Sin- gapore does not illustrate a leveling off of the number of tree spacings observed. In this situation, a sample size greater than 32 would be required. Bangkok and Singapore showed a leveling off of the number of tree well sizes when the number of samples was increased from 16 to 32, suggesting that a sufficient number of samples had been taken in each city to record most of the range in tree well dimensions. Data from Johannesburg, how- ever, suggest more than 32 samples are needed to encompass the range of tree well dimensions. During the data collection, it was obvious that some cities had strict control over the spacing of street trees and the size of tree wells, whereas other cities ex- hibited a much wider range in these design characteristics. GRAPHICALLY RECORDED INFORMATION The method presented here records information on urban forest structure in a graphic form. Such characteristics as the horizontal and vertical extent of tree canopies in relations to streets, side- walks, and adjacent buildings can be quickly sketched but are difficult to summarize. The study of 33 of the world’s urban forests generated hundreds of street and boulevard cross-sections sketches and street plans. They are very valuable in character- izing the structures of the urban forests. Unlike numeric data, this graphic information cannot be easily averaged nor compared using statistical analysis. One might morph the street cross- section sketches recorded in a particular urban environment to produce a “typical” or “average” cross-section using existing computer programs. These morphed cross-sections might turn ©2008 International Society of Arboriculture McBride: Characterizing Urban Forest Composition and Structure out to be very characteristic for different cities. The individual cross-section sketches and street plans can be very meaningful to landscape architects and urban planners who often work with visual information. It was not within the scope of this study to “test” the usefulness of the information collected for designers, but I am convinced, through my work with landscape architects and urban planners, that they would find the structural informa- tion recorded graphically revealing and useful. CONCLUSIONS The method described in this article provides an efficient way to record information that is of interest to design professionals con- cerned with the composition and structure of urban forests. Larger sample sizes (greater than 40 sample sites), than the ones used in a study of the world’s urban forests on which this article is based, will be needed to more accurately determine species composition and the range tree spacing and tree well dimension in most cities. However, the method provides a rapid way of recording data that can be used to calculate the relative fre- quency of occurrence of tree species, document dimensions of tree spacing, and planting sites. The graphic representation of additional urban forest structural information can provide a use- ful record for landscape architects and urban planners who nor- mally work with both quantitative and visual data. LITERATURE CITED Arnold, H.F. 1993. Trees in Urban Design. Van Nostrand Reinhold, New York, NY. Barnard, E.S. 2002. New York City Trees. New York: Columbia Uni- versity Press. Beijing Annals Editorial Board. 2000. Landscape plants. In: Beijing Annals Editorial Board (Eds.), Beijing Annals, Vol. 16 Municipal Administration, Coy 52, Gardens and Afforestation Annals. Beijing Publishing House, Beijing, China, pp. 337–384 (in Chinese). Gangotena, et al. 1990. Tree species along major arterial and residential streets of Quito. Quito: Department of Parks and Gardens (in Span- ish). In: Murray S. 1999. “Human Nature,” the shaping of the urban ecosystem in spontaneous settlements of Quito, Ecuador. PhD Dis- sertation. University of California, Berkeley. Gruffydd, B. 1987. Tree Form, Size, and Colour: A Guide to Selection, Planting, and Design. E. & F. N. Spon, New York, NY. Jacobs, A.B., E. MacDonald, and Y. Rofe. 2002. The Boulevard Book. The MIT Press, Cambridge, MA. Kazakov, L. 1999. Trees Growing in Murmansk. Polar Arctic Botanical Garden, Apatity, Russia. Littlewood, M. 1988. Tree Detailing. Butterworth Architecture, London, UK. McBride, J.R. 2000. Urban Forestry: What we can learn from cities around the world. Proceedings of the XIIth National Urban and Com- munity Forest Conference, Seattle, WA. pp. 39–43. ———. 2003. Palo Alto Tree Study. Report to the City of Palo Alto Planning Department, Palo Alto, CA. McPherson, E.G., G. Gonzalez, G. Monfette, and R. Lorenzen. 2003. Expanding street tree canopy cover and repairing sidewalks in the City of Los Angeles. Western Arborist 29:1–4. Miller, R.W. 1997. Urban Forestry. Prentice Hall, Upper Saddle River, NJ. Mueller-Dombois, D., and H. Ellenburg. 1974. Aims and Methods of Vegetation Ecology. J. Wiley, New York, NY. Nowak, D.J. 1994. Urban forest structure: The state of Chicago’s urban forest, pp. 3–18. In: McPherson, E.G., D.J. Nowak, and R.A. Rown- tree (Eds.). Chicago’s Urban Forest Ecosystem: Results of the Chi-
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