6 McPherson: Benefit-Based Tree Valuation Table 2. Appraised values and related information for each pistache tree. Tree 1 Tree 2 dbh (cm) Trunk area (cm2 Basic tree cost ($) Condition (%) ) Appraised value ($) 21.0 347 4,319 56 1,451 24.3 462 5,693 56 1,913 Tree 3 22.2 388 4,812 64 1,848 Unit cost $11.94/cm2 TA, installed cost $345, species 90%, location 66.7%. dbh, trunk diameter at breast height. future costs from benefits. Additionally, the cost-effective- ness of removing and replanting the pistache trees in a nearby shrub bed, where they would not damage the sidewalk and curb, is examined. Benefit Estimation Benefits were calculated using the same numeric modeling methods described earlier. Tree growth data, however, were for pistache street trees in Modesto, California, a Central Valley city with a climate similar to Davis (Figure 1; Peper et al. 2001). Environmental benefits were calculated by dbh class for pistache in Modesto using local geographic data (e.g., air pollutant concentrations, hourly climate, building types) (McPherson et al. 1999). Benefits were monetized based on information specific to Davis (Table 1, Maco and McPherson 2003). For more detailed information on model- ing and monetizing benefits in Modesto and Davis, see McPherson et al. (1999) and Maco and McPherson (2003). Cost Estimation Past and future expenditures for tree planting and care were based on the cost of street tree services in Davis (Loux 2002; Maco and McPherson 2003; R. Cain, pers. comm.). Planting costs for a #15 tree averaged $75 per pistache, and replanting included $50 per tree to prepare the shrub bed (Table 3). Table 3. Costs for tree maintenance and sidewalk, curb, and gutter repair. Activity Plant pistache Plant replacement (add $50 for site prep of shrub bed) Prune small tree (4 year cycle) Prune 15–30 cm dbh (8 year cycle) Prune 31–45 cm dbh (8 year cycle) Prune 46–60 cm dbh (8 year cycle) Remove pistache (15–30 cm dbh at 15 years) Grind sidewalk (5 squares at years 10, 15, 30, and 35) Root prune (at years 20 and 40) Remove and replace sidewalk (at years 20 and 40) Remove and replace curb/gutter (at years 20 and 40) dbh, trunk diameter at breast height. ©2007 International Society of Arboriculture $/tree 75 125 36 113 163 225 150 20 30 576 600 Small trees (<15 cm dbh [6 in]) were pruned on a 4 year cycle at a cost of $36 per tree. Larger trees were pruned on 8 year cycles (Table 3). Removal costs for the 15 year old pistache trees (23 cm dbh [9 in]) were $150 per tree. Program expen- ditures could not be directly determined by size class for storm cleanup, liability (i.e., trip-and-fall and property dam- age payments), or administration (i.e., supervisory and cleri- cal staff salaries, training, supplies). To estimate costs per centimeter of dbh for these categories, first, the total annual expenditures were divided by total tree numbers to derive average annual costs per tree ($1.21 for storm cleanup/ liability and $3.33 for administration). Next, it was assumed that per-tree costs for these services increase proportionally to dbh. Therefore, average annual costs per centimeter dbh were calculated as $0.04/cm ($0.11/in) for storm cleanup/liability and $0.12/cm ($0.31/in) for administration by dividing by the population’s average dbh of 27.6 cm (11.4 in). Expenditures for curb and sidewalk repairs resulting from tree damage caused by the pistache trees were obtained from Davis’s Public Works Department (S. Knopf, pers. comm.). Grinding the joints of lifted sidewalk squares reduces the tripping hazard and is less expensive than removal and re- placement. Grinding is a temporary measure because it can only be repeated several times before the concrete becomes too thin to serve as a reliable walking surface. The joints of sidewalks near the pistache trees were ground 5 years ago (year 10) and will be ground again this year (year 15). This cycle was assumed to be repeated with five joints ground (one per tree) at years 30 and 35 (Table 3). The sidewalk and curb were projected to be removed and replaced at years 20 and 40 as a result of heaving by the pistache trees. Sidewalk grinding costs were $20 per 1.2m(4 ft) joint, and typically one joint was heaved and ground per tree (Table 3). Tree roots were pruned before repouring the curb and side- walk at a cost of $30 per tree during years 20 and 40. Each sidewalk square measured 1.2 × 1.2 m (4 × 4 ft) and the cost for removal and replacement was $128/m2 ($12/ft2) or $576 per tree assuming three squares were replaced per tree (S. Knopf, pers. comm.). Curb and gutter replacement costs were $162/m ($50/ft) or $600 per tree assuming 3.7 m (12.2 ft) per tree. Tree 4 22.9 410 5,068 60 1,824 Tree 5 24.9 486 5,973 48 1,720 Average 23.1 419 5,173 57 1,751
January 2007
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