Arboriculture & Urban Forestry 41(1): January 2015 are reductions in vulnerability, while some issues will remain. For severe interactions, there are several concerns that persist from the current population into the planting trends. Specifically, vulnerability to Xylosandrus crassiusculus (Asian ambrosia beetle), Anoplophora glabripennis (Asian longhorned beetle), and Lymantria dispar (gypsy moth) continue. How- ever, anthracnose complexes, Agrilus planipennis (emerald ash borer), Dematophora root rot, and Cronartium ribicola (white pine blister rust) have no or low vulnerability in the planting trends despite representing severe vulnerability in the current assemblage. Vulnerability to moderate severity pests and diseases also sees a significant decrease in the planting trends from 25 interactions down to eight. The pest vulnerability for non-street trees is given in Figure 3. There is generally less vulnerability to pests in the planting trends versus the current assemblage, although there are still notable areas of concern. With respect to severe pests, there are more interactions of concern than in the street tree population. Vulnerability to Dematophora root rot, anthracnose, X. crassiusculus, A. glabripennis, and L. dispar will continue based on the planting trends. Additionally, beech bark disease and Agrilus anxius (bronze birch borer) are two severe pests emerg- ing in the planting trends. For moderate severity interactions and issues particular to Toronto (those cells with double-lined black boxes), there is a decrease in vulnerability in the planting trends (from 43 interactions to 24), similar to street trees. DISCUSSION The common tree genera in Toronto resemble the genera that are common across many cities in North America: Acer, Fraxinus, Quercus, Gleditsia, Ulmus, Malus, Prunus, and Tilia (Dreistadt et al. 1990; Raupp et al. 2006). This supports the gen- eral observation that urban environments, includ- ing vegetation, tend to be more homogenous than their non-urban or wild counterparts (McKinney 2002). These common genera are likely prevalent for many reasons, including their historic ability to thrive to maturity in urban environments (i.e., tolerance of soil compaction, pollution, limited space, and road salt; Richards 1983), and potentially because they are also readily available. This latter point should not be overlooked given the strong 33 influence the horticultural industry can have on the species compositions of vegetation communities in urban environments (Pincetl et al. 2013). Like currently common genera, the trees identi- fied as commonly planted (or sold) in this research are not unique to Toronto. D’Amato and colleagues (2002) found that the most requested tree genera by urban foresters in Ohio included Acer, Quercus, Fraxinus, Amelanchier, Tilia, Ulmus, Pyrus, Malus, Syringa, and Prunus trees—many genera that are rep- resented in the survey and interview responses here. The patterns of tree planting in Toronto have the potential both to improve species diversity in the urban forest in certain respects while also maintain- ing the predominance of already common genera. This is particularly true in Toronto’s municipally owned street trees; Acer, Tilia and Gleditsia continue to be commonly planted by the city, though in the case of Acer, the specific species now planted (Acer freemanii) is relatively uncommon in the current street tree population. The remaining commonly planted street tree genera are relatively rare in the current assemblage (such as Celtis, Gymnocladus, and Ginkgo) and have the potential to increase diversity. In the broader urban forest, there is limited overlap between current composition and planting trends. Again Acer is both well established in the population and commonly being planted. Beyond Acer and a few genera common only in one or two management areas in Toronto (e.g., Gleditsia), the currently common and frequently planted genera in non-street trees are fairly divergent (Table 7). Table 7. Currently common and frequently planted gen- era for street and non-street tree populations. Street trees current Acer Tilia Gleditsia Picea Malus Fraxinus Pinus Quercus Street trees planting trends Acer Tilia Gleditsia Ulmus Celtis Gymnocladus Ginkgo Non-street tree current Acer Thuaj Gleditsia Ulmus Prunus Fraxinus Pinus Populus Picea Non-street tree planting trends Acerz Tilia Gleditsia Quercusz Ginkgo Populus Syringa Hydrangea Buxus Cornus Euonymus Thuja Betula Cercis Fagus Taxus z Genus was indicated by more than one non-municipal actor. ©2015 International Society of Arboriculture
January 2015
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