84 dence of ash in western Canada was supported by two sources: 1) Google Maps counts of street trees at 16 random locations in four Manitoba communities indicated that there were about twice as many street trees in Manitoba than Ontario, and 2) street tree composition data for the city of Saskatoon, Saskatchewan (Geoff McLeod, pers. obs.) indicated that about 25% of street trees were ash, approximately 4× that of eastern Canada. This pattern is perhaps not surprising given the more limited num- ber of tree species that can tolerate the somewhat more extreme climate found in the prairies region (McKenney et al. 2007). Predicting EAB Spread The Canadian Forest Service Forest Bioeconomic Model (CFS- FBM) was used as the basic modeling framework for projecting EAB spread over time. The model shares conceptual similarities with the spread model described by Yemshanov et al. (2009a), Yemshanov et al. (2009b), and Koch et al. (2009). Briefly, CFS- FBM provides a grid-based modeling framework for simulating a variety of processes in a spatial setting, including the spread, establishment, and impact of alien species. For example, the model has been used to examine potential wood supply im- pacts from Sirex noctilio, an invasive alien wasp species (Koch et al. 2009; Yemshanov et al. 2009a; Yemshanov et al. 2009b). A simplified version of CFS-FBM was used to obtain a coarse depiction of how EAB might spread across the country. The approach required a spread probability-density function, or ‘kernel,’ which determined the probability of EAB spread as a function of the geographic distance to locations with known EAB infestations. Published EAB spread rates vary by more than two orders of magnitude, reflecting the highly variable spread of EAB under different conditions. The smallest report- ed value (30 m/yr) was for a new infestation starting from a single source (a pile of infested logs) with many ash trees in the near vicinity (Mercader et al. 2009). In contrast, Kovacs et al. (2010) reported an average spread rate of 16 km/yr based on spread data in Michigan, U.S., over the period 1994–2009. Similarly, Smitley et al. (2008) reported a rate of 10.6 km/yr for the spread rate of detectable symptoms for an outbreak in southeastern Michigan over the period 2003–2006. These larger estimates are based on data that include natural long distance dispersal events that may be induced by high popula- tion density and/or low host availability, as well as regional- scale, human-assisted movements (such as trade and trans- portation). Based on comparison to observed rates of spread in southern Ontario, the spread rate reported by Smitley et al. (2008) was adopted as a baseline value for the current study. The spread model simulations covered an area extend- ing from maritime Canada in the east to Alberta in west- ern Canada with a map cell resolution of ~1 km2 . The model employed a negative exponential function to deter- mine the probability, p that a cell would become infested as a function of its distance, d from the nearest infested cell: [1] p = e-0.0943d The value of the exponent in Equation 1 (i.e., 0.0934) was determined such that the mean distance defined by the equation is 10.6 km (i.e., the desired average spread rate as previously outlined). To address the wide variation in potential spread rate, ©2012 International Society of Arboriculture McKenney et al.: Cost of EAB in Canadian Municipalities the model was run with three different maximum spread values to represent slow, medium, and fast linear rates of spread cor- responding to approximately 10, 30, and 50 km/year. The maxi- mum spread value truncates the negative exponential probabili- ty-density function, thus placing an upper limit on the extent of annual spread – a key factor controlling overall spread rates and patterns (Koch et al. 2009; Yemshanov et al. 2009a; Yemshanov et al. 2009b). This approach produced a uniform spread pattern that predicted consistent arrival times that were not influenced by rare (and highly uncertain) long-distance dispersal events. The model was run over a 30-year time horizon to generate expected arrival times for EAB at each map cell in the study area. The model was initiated from known Canadian and U.S. EAB occurrence locations as of 2009 (USDA-APHIS 2011). An im- plicit assumption was that any cell that fell within the study area contained at least some ash that could be a host (and hence path- way) for colonization. This assumption was necessary because, as previously noted, detailed spatial data of ash abundance were not available in Canada; furthermore, ash is considered relatively common throughout its native Canadian range (Farrar 1995). Unit Cost Estimates for EAB Damage Four types of costs were explicitly incorporated into this study: removal costs, replacement costs, treatment costs, and what were termed as community overhead costs (Ta- ble 4). All cost estimates are in year 2010 Canadian dol- lars and based on a combination of published values from the United States. (Kovacs et al. 2010) and personal communi- cations with City Foresters in Windsor, Toronto, Oakville, London, Ottawa, and Thunder Bay, Ontario; and Saskatoon. It was assumed that all ash street trees, as defined in this study, required either removal or treatment. Removal costs vary widely according to tree size (height and diameter), lo- cation (e.g., proximity to buildings, and power and telephone lines), and contractor rate; the cost estimates attempted to de- scribe an average cost for small, medium, and large trees (Ta- ble 4). Replacement costs are also highly variable and depend on the size and source of the planting stock; the estimate of CAD $400 is representative of the per tree costs incurred by municipalities when planting well established (i.e., ~ 4 cm in diameter) saplings. It was posited that only a certain percent- age of removed trees would actually be replaced; in lieu of data on this subject, a 50% replacement rate was assumed. Insecticide treatments were incorporated into the model as an alternative to cutting large and medium sized trees. Three plausible treatment scenarios were considered: 1) no treatments; 2) a modest treatment rate, where 10% of large and medium trees were treated; and 3) a high treatment rate, where 50% of large and medium trees were treated. Cur- rently, the main product used in Canada for protecting trees against EAB attack is TreeAzin™ (McKenzie et al. 2010). Treated trees were tracked in a separate cost stream that re- ceived ongoing biannual treatments for the remainder of the simulation; cost estimates were based on reported costs as- sociated with TreeAzin for large and medium trees (Table 4). Community overhead costs are intended to represent consider- ations such as staff time to manage and coordinate the response, communication costs, monitoring and surveillance costs, and dis- posal operations for tree waste. Based on discussions with city
May 2012
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
