36 Vander Vecht and Conway: Comparing Species Composition and Planting Trends Acer I or D or S Probable Cause ↓ I Adelgids: Cooley spruce gall adelgid, Pine bark adelgid, Hemlock adelgid D Anthracnose: Apiognomonia; Cylindrosporium; Marssonia; Glomerella. 11 I Aphids - OTHER: Braggia spp., Aphis sp., Sitobion sp. Euthoracaphis, Dilachnus D Armillaria root rot or Oak root fungus. 111 I Armored scales: Greedy; Latania; Oleander; San Jose; Oystershell; Walnut I Ash plant bug; Ash tingid (Leptoypha minor) 1 I Asian Ambrosia Beetle, Xylosandrus crassiusculus 11 1 I Asian Longhorned Beetle Anoplophora glabripennis 11 11 11 1 I Bark beetles - Dendroctonus 1 D Beech Bark Disease Netcria fungi; Beech Bark Scale 1 D Black Knot 1 I Boxelder bugs. 11 I Bronze Birch Borer 1 D Cedar-apple rust 1 S Clearwing borers 11 11 I Cynipid gall wasps. 1 D Cytospora Canker 11 11 111 11 D Dematophora root rot. 1 S Diplodia canker, Sphaeropsis sapinea = Diplodia pinea 1 I Dogwood Borer (Synanthedon scitula ) 1 D Dutch elm disease. 1 I Elm leaf beetle. 1 I Emerald Ash Borer (Agrillus planipennis) 1 I Eriophyid mites, aka "gall mites", Aceria and Vasates spp. 11 I European elm bark beetle. 1 1 1 1 11 1 1 1 1 1 111 11 1 11 Maple (Acer spp.) Box elder (Acer negundo) Ash (Fraxinus spp.) Honey locust (Gleditsia triacanthos) Spruce (Picea spp.) Acer neg. Fraxinus GleditsiaPicea Pinus 2PopulusPrunusThuja Pine: scots, nigra, sylvestris, resinosa sugar, brutia, jeffrey Poplar, Cottonwood, Aspen Cherry, Plum, Prune, Peach, Nectarine (Populus spp.) (Prunus spp.) Ulmus amer.AcerBetulaCercisCornus Cedar(Thuja occidentalis) Elm: American + European (Ulmus spp.) Euonymus Maple (Acer spp.) Pest count → 12 785379919 12 Birch (Betula spp.) Redbud (Cercis spp.) Dogwood (Cornus spp.) Euonymus (Euonumus spp.) FagusGinkgoGleditsia Beech (Fagus spp.) Maiden-hair tree (Ginkgo biloba), Ginkgo Honey locust (Gleditsia triacanthos) PopulusQuercus decSyringa Poplar, Cottonwood, Aspen (Populus spp.) Oak - deciduous (Quercus spp.) 8 5832059 11 TaxusTilia Thuja Lilac (Syringa spp.) Yew (Taxus spp.) Linden (Tilia spp.) Cedar(Thuja occidentalis) 5 031 11 1 111 1 11 11 11 11 11 1 11 111 1111 11 1 Figure 3. Pest Vulnerable Matrix for non-street trees comparing current species composition with planting trends. Refer to Figure 2 for legend. tial losses in structural value have been estimated at CAD $4 billion (City of Toronto 2011). Lymantria dispar will also likely continue to be a problematic pest in Toronto and require coordinated intervention from the city (through aerial spraying and egg-mass removal) every few years (J. Ric, pers. comm., July 10, 2013). Lymantria dispar and A. glabripennis are especially problematic because they are polyphagus and feed on multiple tree families, making it diffi- cult to avoid all susceptible genera in new plantings. An additional concern stems not from known threats, but from the fact that some already common genera continue to be planted regularly. Though Tilia and Gleditsia have few pests to which they are sus- ceptible (see Figure 2), and currently have no severe interactions, they are quite common in the street tree population and continue to be planted with high frequency. In the constantly changing landscape in ©2015 International Society of Arboriculture which urban forests now exist, the potential for new pests with strong impacts on these genera cannot be eliminated or necessarily predicted. As such, efforts should be made to maintain proportions of these genera at or below 10% throughout the city. Where the genus is predominated by a single species, such as Gleditsia, the proportion should be kept in-line with the 5% guideline for single species. While few genera are above the 10% threshold in Toronto’s urban forest, there is still cause for working to increase the diversity in both the street and non- street tree populations. Plantings of Acer should be scaled back as they dominate the landscape and also appear to be the most commonly planted and sold genera in the city. As the inverse SDI values show, there is room for improvement for genera diversity in both populations, but particularly for trees. Reducing reliance on Acer and substituting street
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