Arboriculture & Urban Forestry 36(5): September 2010 Normalizing the Values To normalize the values, ranges were calculated for each criterion and 10 equal intervals calculated (Table 5). For example, HDD values ranged from 0 to 4,354. Ten in- tervals, each 435 in size, were established and assigned the appropriate normalized value ranging from 0 to 10. Linear interpolation was used to calculate the normal- ized value for each difference value (Table 5). For example, the HDD difference value for Fort Collins is 2,168 (3,252– 1,084). This value falls within the normalized value range of 5 to 4 (1,742–2,177). After linear interpolation the normal- ized value is 4.98, close to the value of 5.0 because the dif- ference value 2,168 is close to the interval value of 2,177. Accounting for Importance In this step, the coefficient for each criterion is weighted to re- flect its relative importance. The analyst’s best judgment is used to assign weighted values. If the criteria are equally impor- tant, 0.25 is assigned to each of the four coefficients. Unequal weighting can be done to accentuate or diminish the relative importance of individual criterion. The rationale for a weighting scheme applied in the Lisbon example is discussed as follows. Table 5. Normalized interval values (first column) and cor- responding difference values for each of the four criterion (subsequent columns). Value % Tree Match 10–9 <10 9–8 8–7 7–6 6–5 5–4 4–3 3–2 2–1 1–0 10–15 15–20 20–25 25–30 30–35 35–40 40–45 45–50 >50 HDD (18°C) CDD (18°C) > 3919 >2234 Precip. (mm) >1243 3483–3919 1986–2234 1105–1243 3048–3483 1737–1986 967–1105 2612–3048 1489–1737 829–967 2177–2612 1241–1489 691–829 1742–2177 993–1241 1306–1742 745–993 871–1306 435–871 <435 496–745 248–496 <248 Matching tree species composition To calculate Tree Match, the percentage of each reference city population that matches Lisbon’s population at the species and genus levels is summed. A species-to-species match is most desirable, but frequently a different species of the same genus will have similar growth rate, habit, and mature size. For ex- ample, European hackberry (Celtis australis) is the most abun- dant species in Lisbon but was not measured in any reference cities. However, Chinese hackberry (Celtis sinensis), common hackberry (Celtis occidentalis), and sugarberry (Celtis laevi- gata), were measured in reference cities and can be considered to match European hackberry at the genus level because they all grow at a moderate rate to be large, deciduous shade trees. The two reference cities that best match Lisbon in terms of species composition are Minneapolis, MN, and Fort Collins, CO (Table 3). They are the only two cities with matches for both hackberry and basswood, which account for 34% of Lisbon’s street tree population. Green ash (Fraxinus pennsylvanica), the most common street tree species in both Minneapolis and Fort Collins, can match with narrowleaf ash (Fraxinus angustifolia) 552–691 414–552 276–414 138–276 <138 235 in Lisbon because both are medium-stature shade trees. Minne- apolis has five species of maple that match boxelder (Acer ne- gundo), which comprises 7% of Lisbon’s population. In Fort Col- lins, three species of maple match with Lisbon’s box elder. Fort Collins’ Great Plains cottonwood (Populus sargentii) can match with Lisbon’s Carolina, white, and black cottonwoods (Populus × Canadensis, P. alba, P. nigra). Also, Fort Collins’ plums (Prunus spp.) can match with Lisbon’s cherry plum and sweet cherry (Prunus cerasifera, P. avium). The total percentage of Lisbon’s tree population matched by the tree species measured in Minne- apolis and Fort Collins is 68.5% and 46%, respectively (Table 3). The three California reference cities have measured trees that match at the species and genus levels, as well as climates that are more similar to Lisbon’s because their winter sea- sons are warmer. Modesto has nine matches accounting for 37.2% of Lisbon’s street tree population. Claremont (6 match- es, 16%), and Santa Monica (1 match, 2.3%) have matches as well (Table 3). Charleston, SC, has three tree matches (2.1%). Tree Match is weighted 0.1 in this assessment, and considered the least important variable. The rationale is that Lisbon’s tree population is a mix of deciduous species from northern Europe and broadleaf evergreens from the Mediterranean region. Although Minneapolis and Fort Collins provide the most matches, their pop- ulations lack the Mediterranean species. Perhaps more important- ly, their tree grow rates are unlikely to be a close match because of their much shorter growing seasons. By selecting a low weight for Tree Match the effect of these confounding factors is minimized. Matching HDDs and CDDs The large HDD numbers and corresponding differences between Lisbon and reference cities can be misleading. Effects of trees on heating savings are small per HDD and much greater per CDD. The magnitude of this difference is on the order of 10, largely because trees can increase winter heating loads by ob- structing irradiance, as well as reduce heating loads. To obtain a rough estimate of how this difference influences tree energy savings, divide HDDs (base 18°C) by 1,000 and CDDs by 100. For Lisbon, HDDs convert from 1,084 to 1.08, and CDDs con- vert from 474 to 4.74 (Table 3). Annual cooling savings from trees will be approximately 4 to 5 times greater than heating savings on an average per tree basis. This conversion is a very rough approximation because actual results will be influenced by tree sizes, locations, building vintage, types of heating and cooling equipment, and the prices of electricity and natural gas. Because of Lisbon’s relatively mild climate, energy sav- ings from trees will not be large, so this benefit should not dominate selection. Claremont and Santa Monica provide the best match when both HDDs and CDDs are considered, and Santa Monica is closer than Claremont when consider- ing CDD. Although CDDs are relatively more important than HDDs in Lisbon, and Fort Collins and Minneapolis provide the best match in terms of CDDs, their HDDs are so much greater than Lisbon’s that heating savings would be grossly overestimated. So as not to overstate heating savings, the HDD variable is weighted 0.3, which accentuates the dif- ference between cities with HDDs that are much greater than Lisbon’s and those with HDDs closer to Lisbon’s. The CDD variable is weighted 0.2 because even air condition- ing savings will be relatively small in this benign climate. ©2010 International Society of Arboriculture
September 2010
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