Potential Leaf Area Index Analyses for the City of Toronto’s Urban Forest
One commonly used measure of urban forest structure is canopy cover, or simply the proportion of the city that is covered by tree canopies as visualized from above. This measure is intuitive and relatively easy and inexpensive to measure. However, canopy cover only represents the urban forest in two dimensions and fails to recognize differences in species and tree condition. Many of the benefits that cities derive from their urban forest can vary directly with the total leaf area of the forest, for example, reduction in air temperature, sequestering gaseous pollutants, and carbon sequestration (Nowak, 1994a). In addition, urban forests play a role in moderating urban forest climate through shading buildings, people, and hard surfaces, and through evapotranspirational cooling and the windbreak effect (McPherson et al., 1988; Akbari and Taha, 1992; McPherson and Rowntree, 1993; Brown and Gillespie, 1995). Increasing leaf area will increase shading and evapotranspiration, and could also have a direct impact on the windbreak effect. Similarly, storm water attenuation (Xiao et al., 1998) will be affected by leaf area, as well as other factors.
Consequently, an estimate of the total leaf area of the urban forest would be more informative than simply an estimate of its canopy cover. The most commonly used measure of leaf area is the leaf area index (LAI), defined as the total leaf surface area per unit land area. While LAI offers some indication of the urban forest’s ability to provide services to the community as outlined above, urban forest management and planning requires some indication of the potential that exists to expand leaf area. A low canopy cover, in itself, says little about the potential for that area to support a tree canopy. For example, a portion of a city may be found to have a canopy cover of 20%; urban forest managers may consider expanding this to say 30%. Without some indication of the carrying capacity or potential to support additional trees within the city as a whole or in specific areas, managers are unaware of the practical possibilities of reaching their goals. The potential leaf area index (PLAI) is a measure of the carrying capacity of an urban area and is a function of the amount of available growing space and its configuration (Kenney, 2000). Armed with an estimate of the LAI and PLAI, urban forest managers and planners can more effectively address the protection and enhancement of this important resource. This chapter reports on a study whose purpose was to estimate the LAI and PLAI for the City of Toronto, Ontario.
KeywordsLeaf Area Geographical Information System Canopy Cover Urban Forest Total Leaf Area
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