Arboriculture & Urban Forestry 34(6): November 2008 Arboriculture & Urban Forestry 2008. 34(6):359–365. 359 A Method for Characterizing Urban Forest Composition and Structure for Landscape Architects and Urban Planners Joe R. McBride Abstract. A method combining numeric data collection with the preparation of street tree cross-sections and plans, based on surveys of 33 urban forests around the world, is reviewed. The combination can provide design professionals with graphic information on urban forest structure not collected by more traditional methods for urban forest inventories. Key Words. Composition; planting space; species frequency; street cross-section; street plan; structure; tree spacing. An urban forest can be characterized in terms of composition, structure, and function (Rowntree 1984). Composition is usually characterized by identifying the species that are present and the contribution (number of individuals or percent) of each species to the total tree population. Structure is usually defined in terms of tree size, canopy cover, and the spacing of trees along streets and in parks. These characteristics are useful in describing an urban forest and for understanding the functional roles of urban forests. However, they do not provide information on other structural characteristics that are of interest to landscape archi- tects concerned with urban forest design. This article reports on a method developed to collect information on urban forest com- position and structure of interest to landscape architects. The specific information addressed includes 1) species frequency; 2) tree spacing; 3) dimensions of planting spaces; 4) tree height in relation to adjacent building height; 5) height to live crown; 6) radial spread of trees in relation to the adjacent streets, side- walks, and buildings; and 7) tree planting patterns along streets. BACKGROUND OF STUDY The method reported here was developed for a study of the urban forest composition, structure, and function in 33 of the world’s cities (McBride 2000). It investigated the relationships between urban forest characteristics and the biomes in which cities oc- curred. One hypothesis of the study was that structural charac- teristics of an urban forest might be influenced by the charac- teristics of the biome. For example, would one expect to find a greater frequency of trees with horizontally spreading branches in cities in the desert and savanna biomes where shading of streets and sidewalks would be a valuable structural dimension? Arborists have systematically collected structural information on urban forests as a part of street and park tree inventories (Miller 1997). Tree diameter, height, live crown ratio, crown diameter, and tree condition are commonly recorded in these surveys. Street tree inventories are usually conducted for an entire city or subsection of a city. Sample inventories have been developed to more efficiently acquire information about compo- sition and structure. The distribution of sample sites has followed simple random sampling, randomized block sampling, system- atic random sampling, and two-staged sampling (Panahi et al. 2003). A few studies have attempted to determine the sample size necessary to estimate various compositional and structural characteristics of an urban forest. Nowak et al. (2003) suggested a minimum sample of 200 plots is needed to determine percent species composition in large cities. Trees have been used as design objects in urban environments since the establishment of cities. Their use as aesthetic objects has ranged from the individual specimen tree that resembles sculpture to large numbers of trees unifying and enhancing the visual properties of streets, boulevards, and parks. Countless outdoor spaces in cities are defined by trees. As objects of de- sign, they provide variation in size, shape, color, and texture. The use of trees in urban design has been documented by Arnold (1993). He describes the design qualities of trees along avenues, streets, and in parks and public spaces other than parks (e.g., parking lots, traffic islands, medians). Arnold contrasts eight geometric patterns grouping trees in small parks using examples from the United States and Europe. He also reviews the design element of trees used in larger urban parks using Central Park in New York City, U.S., as an example. Jacobs et al. (2002) pro- vides an account of how trees have been used as design elements along boulevards in cities around the world. These and other works (Gruffydd 1987; Littlewood 1988; Tsuru 1992; Zion 1995; Trowbridge and Bassuk 2004) treating the use of trees in urban design should be consulted for details of tree species se- lection, spacing, and planting patterns in common use. METHOD A block-long unit of a streetscape was used as a sampling unit in this study. The streetscape may be defined as the area between buildings along a street. It includes the facades of the buildings, front yards (in the case of residential neighborhoods), sidewalks, driveways, curbside planting strips, and the street. It may be modified as an environment for trees by stormwater drainage features, utility lines, transit stops, and parking areas. The block- long streetscape was chosen as a sample unit because it usually represents a unit of a design plan for a larger section of a street or boulevard. This block-long unit may be similar to adjacent blocks for the entire length of a street or individual blocks may differ as one moves along a street. Most city blocks exhibit some ©2008 International Society of Arboriculture
November 2008
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