88 McKenney et al.: Cost of EAB in Canadian Municipalities can be substantial (Nowak et al. 2010), there is significant difficulty finding reliable estimates of ash density for them. Furthermore, it is not clear what percentage of ash trees in the park\woodland set- ting would pose safety risks and thus require management action. These estimates also ignore costs associated with ash trees in smaller towns and rural residential settings—again, due to data availability. Finally, it should be recognized that ash trees do exist in urban cen- ters outside the native range of ash, which could significantly add to the cost of EAB in Canada [e.g., 5.3% of municipal trees in Van- couver, British Columbia, are ash (McManus, pers. comm.)]. How- ever, pathways and spread rates into these areas are highly uncertain. Many other benefits have been attributed to urban trees, including home value premiums, energy savings, pollution and runoff reduc- tion, and human health benefits (Dwyer et al. 1992). These benefits have been quantified for various locations, allowing approximate economic values to be attached to urban trees (e.g., McPherson et al. 2007). Including the loss of these benefits would clearly increase the economic impact attributed to EAB. In fact, EAB cost estimates provided by Sydnor et al. (2007) and Sydnor et al. (2011) approxi- mately doubled when these “landscape values” were included in their calculations. These benefits were not incorporated here because widely accepted values do not exist for Canada and published values are situation dependent (McPherson et al. 2007). Nevertheless, the incorporation of such values could be the subject of future efforts. Losses in timber sales would also be expected as a result of an Figure 3. Sensitivity of EAB economic impact estimates to model parameter values for: a) eastern Canada, b) western Canada, and c) the entire study area. The sensitivity coefficients were gener- ated using a regression approach; larger values indicate more influence on the impact estimates. communities in four midwestern states, with costs increasing to $13.4–$26 billion when the extended benefits were considered. Even after accounting for population size differences, their re- moval and replacement estimates are about three times higher than those reported here. Again, this is partly explained by the inclusion of all street, private, and park trees in their estimates. Another ma- jor difference is that Sydnor et al. (2007; 2011) did not incorporate spread dynamics, and hence economic discounting considerations, into their estimates. As demonstrated here, discounting can have a major impact on cost estimates. This variation demonstrates the wide range in projected costs that can result from (sometimes subtle) methodological differences between impact studies and underlines the importance of exploring multiple approaches to such work. The estimates provided here are conservative in a number of ways, focusing on direct financial costs associated with street (and backyard) tree management. While this represents an important and more readily quantifiable portion of EAB impacts, there are a number of other direct financial considerations that warrant mention. These estimates do not include costs related to trees in parks and urban woodlands. Though the number of ash in these land use categories ©2012 International Society of Arboriculture EAB invasion (Schwan and Elliott 2010). However, detailed spatial data on ash volumes are not available for much of Canada, making it very challenging to estimate potential harvest losses in the natural forest setting. Ash also plays an important ecological role in many southern Canadian ecosystems. For instance, ash is a common ripari- an species and its loss will likely effect water quality for both wildlife and humans (Kreutzweiser 2010). Furthermore, the loss of ash could have a major impact on biodiversity in agricultural landscapes of southern Ontario, where it is often a key component of remnant for- est woodlots (Schwan and Elliott 2010). While these ecosystem ser- vices are extremely challenging to include in an economic analysis, they are mentioned here to emphasize the extent to which this assess- ment underestimates the full impact of this invasive alien species. For the three lower discount rates, costs increased as treatment rate increased; this pattern was reversed at the 10% discount rate. This finding suggests, for example, municipalities/homeowners that have high borrowing costs should consider treating a portion of their trees because this results in a series of smaller, delayed payments compared to large scale removal and replacement ef- forts (McKenney and Pedlar 2012). Even at lower discount rates, the opportunity to spread removal costs over time through the use of treatments may be appealing to some municipalities. These re- sults provide a simple approximation of how overall costs may vary under different treatment rates. However, it is important to note that decisions to treat versus remove trees can be complex and involve not only relatively straightforward considerations such as treatment, removal, and replacement costs, but also more subtle factors like the influence of tree cover on property values, energy budgets, and pollution control. Recent studies have examined this topic from the perspective of both individual homeowners (McK- enney and Pedlar 2012) and municipalities (Sadof et al. 2011). Additionally, these results may be roughly interpreted to sup- port slow-the-spread efforts against EAB. If the medium or fast spread rate models are deemed to be more indicative of likely outcomes, then the cost differences between the slow versus me-
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