30 Vander Vecht and Conway: Comparing Species Composition and Planting Trends ited number of trees. However, these businesses do reflect important sources of vegetation in an urban center and should be included in future research. Information about how the city manages tree plant- ings was obtained through a series of e-mail commu- nications (occurring from February to October 2013) with R. Vendrig, the supervisor of Tree Nursery and Natural Resource Management in the Urban Forest Renewal section of the Urban Forestry Department. These communications culminated in a semi- structured interview that took place in June 2013. Pest Vulnerability Methods To compare current and future pest vulnerability, a straightforward, previously developed matrix that visualizes interactions between tree pests and tree genera was used. Laćan and McBride (2008) devel- oped the pest vulnerability matrix (PVM) for north- ern California, U.S., by obtaining and aggregating pest and host information for tree families com- monly found in California and assigning severity classes to pests and diseases (Laćan and McBride 2008). Pests and diseases were organized into com- plexes to increase the visual readability and prac- ticality of the matrix (Laćan and McBride 2008). The PVM was obtained from its original authors and contained information on more than 100 tree pests and more than 150 tree genera (Laćan and McBride 2008). Interactions between relevant pests and tree genera had been assigned to severity classes based on the management importance of the pest. Low severity indicates problems that typi- cally cause little damage and rarely require inter- vention; moderate severity indicates problems that are not usually lethal but oſten require interven- tion due to damage that is unsightly or that might lead to problems over time; and severe are those that always require management action, typically because of the lethal nature of the insect or disease, and less frequently where problems are not lethal but are unacceptable to people and indirectly lead to mortality through premature tree removal (Laćan and McBride 2008). The matrix also includes emerging problems to encompass potential upcom- ing threats, primarily invasive non-native species and those pest species with expanding ranges. Using the original matrix as a base, the PVM was modified for Toronto, drawing on information from several sources: the Ontario Ministry of Natural ©2015 International Society of Arboriculture Resources (OMNR 1991), the City of Toronto’s For- est Health Care website (City of Toronto 2013b), the University of California’s Integrated Pest Manage- ment website (www.ipm.ucdavis.edu), and relevant resources and literature (Sinclair et al. 1987; Haack and Acciavatti 1992; Waldron 2003; Czerwinski et al. 2006; Schall and Davis 2009; Natural Resources Can- ada 2011; Dykstra and Sabourin 2012; Plant Disease Diagnostic Clinic 2013.) A semi-structured inter- view with J. Ric, the supervisor of the Forest Health Care (FHC) unit in Toronto’s Urban Forestry Depart- ment, was also conducted to gain a sense of general and specific pest concerns in the City of Toronto, as well as for feedback on the relevancy and accuracy of the PVM for Toronto’s pest and tree interactions. In the interest of making the matrix cleaner and easier to interpret, some tree genera columns were omitted from the completed matrices. This was done to account for genera that were noted in surveys but are not classically considered trees or woody shrubs; using Farrar’s (1995) Trees in Canada as a reference, only trees and shrub genera included in this text were retained in the matrix for analysis. This ultimately affected only a few genera noted in the garden center surveys. When comparing the current composition to the planting trends for each tree population, results obtained from Toronto’s Urban Forestry Department stood in as street tree planting trends, and the results from landscape architects, NPOs, and garden center surveys were taken to be the trends for non-street trees. RESULTS Current Species Composition The street tree population in Toronto has rela- tively limited diversity, with 60% of the over 500,000 street trees in the database represented by five common genera (Table 1). Acer is by far the most common street tree, representing one- third of all street trees. This is also the only genus that exceeds the 10% threshold, with Gleditsia, Tilia, Picea, and Fraxinus all representing be- tween 5% and 10% of the street tree population. Five genera also comprise more than 5% of the non-street tree urban forest (Table 1), with three in common with the street tree genera. Again, Acer is the most common genera, while Thuja is also above the 10% threshold. Thuja, however, is not par-
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