Aaron Pothier

When development practices emphasize infill and densification, it is vitally important that the value of urban ecosystems be understood by city planners. A well-managed urban forest delivers essential environmental services to the city in which it grows. Few entities are better positioned to provide consistent broad-scale maintenance and protection of urban trees than are large downtown institutions. In this study, we investigate the structure and function of an urban forest growing on a large institutional property in the city centre of Toronto, Canada. In total, 584 trees comprising 41 species were inventoried and their characteristics modeled using i-Tree ECO and STREETS. Five tree species were determined to dominate this forest (66%), with Green Ash (highly vulnerable to the Emerald Ash Borer) accounting for 16% of all trees and delivering the highest value of ecological services. The annual worth of environmental services provided by the entire urban forest was estimated at 15,453 USD in 2009, and its compensatory value assessed at 543,367 USD. An economic benefit-to-cost analysis of investment in tree maintenance produced a ratio of 1.32:1; should the aesthetic value of trees be monetized the proportion of benefit would increase. Our approach to the design of this study provides an easily replicated roadmap for other institutions to undertake accounting of the ecological benefits flowing from their current (and potential future) tree population.