26 Vander Vecht and Conway: Comparing Species Composition and Planting Trends Arboriculture & Urban Forestry 2015. 41(1): 26–40 Comparing Species Composition and Planting Trends: Exploring Pest Vulnerability in Toronto’s Urban Forest Jennifer Vander Vecht and Tenley M. Conway Abstract. Urban forests represent a valuable resource for cities but are not without costs. These costs can include time, money, and the loss of beneficial services as results of pest infestations. Knowledge of an urban forest’s tree species composition and vulnerability to pests is needed to help managers enhance services delivered, while minimizing expenses over the long-term. Recent research has explored the impacts of individual pests on urban forests, but less attention has been given to the overall pest vulnerability. In this research, tree genera currently prevalent and commonly planted in Toronto, Ontario, Canada, were analyzed using a pest vulnerability matrix to explore how the city’s urban forest species composition and pest vulnerability may be changing. Current tree species composition was derived from existing inventory data, while the planting trends of a variety of local actors were determined through surveys and interviews. Results indicate there is somewhat limited diversity in current street and non-street tree populations, as well as a number of common tree species that have severe pest vulnerabilities. While new plantings replicate some current composition and pest vulnerability issues, several less common species are also being planted. As a result, overall pest vulnerability should decrease in the future, while some ongoing management concerns remain. Key Words. Canada; Ontario; Pest Vulnerability; Pest Vulnerability Matrix; Planting Trends; Species Diversity; Survey; Toronto; Tree Pests; Urban Forests. Urban forests make significant, positive contribu- tions to cities and their residents through myriad health, social, environmental, and economic benefits (Tyrväinen et al. 2005). Managing this resource, however, is not without costs. Urban forests are associated with disservices, including the mone- tary, time, labor, and carbon costs of tree planting, removal, and care (Escobeder et al. 2011). Effective and strategic management of urban forests can help to minimize these costs while maximizing the ben- efits trees bring to a city. An important element of good management is accounting for species diver- sity, as the composition impacts associated services and disservices. In general, higher diversity is asso- ciated with improved ecosystem service provision (Gamfeldt et al. 2012), and decreased risk of major losses due to insect and disease outbreaks (Raupp et al. 2006; Laćan and McBride 2008). Pest vulnerability is an important issue to address in urban forests because these forests generally have an over-dominance of a select number of tree species ©2015 International Society of Arboriculture and genera (Laćan and McBride 2008). The danger with this limited diversity is that large sections of an urban forest can be rapidly lost in the event of an outbreak affecting a highly prevalent species or genera (Laćan and McBride 2008). Losses due to pest outbreaks not only decrease the structural and func- tional value of an urban forest, they also increase management costs as diseased, dying, and dead trees have to be treated, removed, and/or replaced. Investigations into urban forest species compo- sition are oſten limited to street tree populations (Sun 1992; Raupp et al. 2006; Sjöman et al. 2012; Subburayalu and Sydnor 2012). However, species composition on private property is likely different from public lands as a result of differences in the knowledge, goals, and actions of key actors engaged in private property tree planting. The literature exam- ining urban forest species composition has also given limited consideration to how the composition may be changing based on what species are currently being planted and how the relevant local actors influ-
January 2015
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