Arboriculture & Urban Forestry 37(2): March 2011 MATERIALS AND METHODS Development of the EAB Cost Calculator To estimate costs of removal and treatment for an urban forest, the EAB Cost Calculator uses a range of input variables to de- fine initial forest composition, fee schedules for treating, remov- ing, and replacing ash trees, and a time schedule for applying insecticides, tree removal, and replacement. Input data used for the current case study of the City of Indianapolis is presented in Table 1. The size of the ash forest at risk is based on a street tree inventory (Peper et al. 2008), and indicated by the numbers of trees present within a user defined set of size classes expressed as diameters of trunk at breast height (DBH). Treatment costs are expressed as dollars per centimeter. Treatment fees are struc- tured to increase with tree size to account for research findings that indicate higher doses of insecticides per DBH are required to control EAB in larger trees (Herms et al. 2009; Smitley et al. 2010). This gives an average cost, not adjusted for inflation, of $2.42/cm DBH of treating all Indianapolis trees in the first year. Cost of tree removal is also based on DBH and increases with tree size to account for the increased labor, equipment, and dif- ficulties encountered in removing larger trees. Actual rates used in the Indianapolis case study (Table 1) were estimates pro- vided by the City Forester of Indianapolis (LP). The price for removing trees includes the cost of removing a tree and grind- ing the stump. Replacement costs ($300) include the cost of tree removal plus the cost of obtaining, planting, staking, and mulching a tree. Annual maintenance costs for new or existing ash trees are not included in the simulation model because the goal of the study was to focus on additional costs incurred by the arrival of EAB. Costs for removing and replacing trees are spread over five years under the assumption that target numbers of trees designated for this activity will be spread over five years. Tree Growth Model In order to predict the costs of treatment, removal, and the size of the managed forest 25 years into the future a growth model accounting for tree growth over time needed to be developed. The model was developed from data collected on DBH and time elapsed after planting white and green ash in Indianapolis (Peper et al. 2008). For ease of programming, the midpoint of each ash 75 size category was regressed as a linear function of the average time after planting for a tree to reach this size class, rather than using a second order polynomial which gave a slightly better fit (R2 0.999). The slope the line is biased in that it [y = 1.16x + 3.73; R2 = = 0.994; where y = tree size (cm) DBH, and x = time in years] over- estimates the growth of young and old trees but underestimates the growth of middle aged trees. For Indianapolis, this would un- derestimate the growth of a median sized tree (age = 15 yrs; DBH = 19.1 cm) by 5.1% (2.38 cm) over the 25-year simulation. Clear- ly, many other factors such as transplant shock, tree condition, and site conditions, are likely to cause individual tree growth to deviate from the actual growth rate predicted by this model. How- ever, because forest size was estimated from the numbers of trees in size categories, the study authors lacked the necessary infor- mation to make this model more realistic. Thus, this simple linear model as is used as the best estimate of tree growth over time. Using Tree Inventory Data to Project Forest Size To approximate the growth of ash trees present in an actual ur- ban forest, the calculator estimates the starting size of each tree from the initial tally of trees grouped by size class. Poten- tial starting sizes are determined by dividing the total number of trees in a size class by the number of trees in the category. Each tree is assigned to the midpoint of each of those size ranges. For example, if 100 ash trees in an inventory have a DBH be- tween 15 and 30 cm, trees are assigned to one of 100 size classes, 0.152 cm apart, with starting values of anywhere from 15.32 to 30.40 cm. For computational efficiency, this algorithm was used for up to 500 trees. When more than 500 trees were in a category, the additional trees were assigned to one of the existing starting size categories until there are no trees remaining. Every surviving ash tree grows from its starting size, accumulating 1.16 cm per year. The size of remaining ash forest is determined as the sum of the DBH for all living trees in a given year. The study authors specified each replacement tree to have a 5 cm caliper at planting. To simplify the model it was assumed that trees designated as ash replacements will grow at the same annual rate as ash trees. Calculating Discount Rates To account for the time value of money, the calculator allows the user to define a discount rate to discount all current expens- es into the future by the following formula Vn = Vo (1 + i)n, Table 1. Inventory of Indianapolis ash trees under care by the City of Indianapolis in 2007. Fee schedule used to estimate costs of a single application of insecticide for emerald ash borer, and removing an ash tree and grinding the stump. Tree size DBH (cm) 0–7 7–15 15–30 30–45 45–61 61–76 76–91 91–107 >107 Ash Trees (% of total) Initial Total DBH Rates per DBH for tree removal and stump grinding used in the EAB calculator simulation between 25 and 30 cm were $13.25. Similarly, rates for trees with DBH between 102 cm and 107 cm were $25.00. Average Cost /cm DBH for treating all trees in first year z ©2011 International Society of Arboriculture Ash trees (count) 1310 1783 3275 2020 1140 626 340 152 122 Insecticide application cost ($) / cm DBH 1.57 1.57 1.57 2.36 2.36 3.15 3.15 3.94 3.94 10,768 (9.2) 3,401 m $2.42 Tree removal and stump grinding cost ($)/cm DBH 3.62 3.62 3.62z 5.22 5.22 7.28 7.28 7.28z 9.84
March 2011
| 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
