230 McPherson: Selecting Reference Cities for i-Tree Streets Arboriculture & Urban Forestry 2010. 36(5): 230-240 Selecting Reference Cities for i-Tree Streets E. Gregory McPherson Abstract. The i-Tree Streets (formerly STRATUM) computer program quantifies municipal forest structure, function, and value using tree growth and geographic data from sixteen U.S. reference cities, one for each of sixteen climate zones. Selecting the reference city that best matches a subject city is prob- lematic when the subject city is outside the U.S., lays on the border between two climate zones, has a different climate, or tree species composition because of differences in elevation, urban morphology, and environmental quality. A systematic process for selecting the best match is described and illustrated for Lisbon, Portugal. Selection criteria are tree species composition, heating and cooling degree days, and annual precipitation. Raw and difference values for each criterion are normalized to range from 0 to 10 using linear interpolation. The coefficient for each criterion is weighted to reflect its relative importance. The Root Mean Square Error (RMSE) is calculated and the reference city with the lowest value is the best match for the subject city. The state of Califor- nia’s reference cities of Modesto (RMSE = 2.41) and Claremont (2.71) proved to be the best match for Lisbon when coefficients were unequally weighted. Key Words. Benefit-Cost Analysis; i-Tree Streets; Municipal Forests; Street Tree Inventory; Urban Forest Valuation. The USDA Forest Service released STRATUM (Street Tree Re- source Analysis Tool for Urban Forest Managers) to the public in August 2006 as one component of the i-Tree software suite. Based on 20 years of urban forest science, STRATUM (hereafter re- ferred to as i-Tree Streets) was developed for urban foresters, mu- nicipal arborists, tree organizations, landscape architects, contrac- tors, planners, environmental consultants, and others interested in analyzing the benefits and costs of municipal forests (McPher- son 1992; McPherson and Simpson 2002; Maco and McPherson 2003; McPherson et al. 2005). The computer program helps users understand the structure, function, and management needs of their street trees, calculate the environmental and aesthetic benefits the trees provide, and determine the dollar value for those benefits. i- Tree Streets was designed to help users improve tree management, and to show decision-makers and residents alike that urban for- ests are an essential part of healthy, well-balanced communities. The U.S. was divided into sixteen national climate zones by aggregation of 45 Sunset climate zones (Figure 1) (Brenzel 1997). Sunset zones were aggregated based on factors that influence plant distribution, such as length of growing season and mini- mum temperature, as well as building energy use patterns (i.e., number of days with highs of 32°C or higher). Also, ecoregions developed by Bailey (2002) and Breckle (1999) were consulted to delineate climate zone boundaries. Termed “reference city,” one city was selected for intensive study within each climate zone. Criteria for selection included: 1) an updated tree inventory (20,000 to 100,000 street/park trees), 2) accurate information on planting dates for aging a sample of approximately 900 trees by the city forester, and 3) large, old trees present in the community. In each reference city, 30 to 60 trees from each of the 22 ma- jor tree species were aged and measured [e.g., diameter at breast height (dbh), height, crown diameter]. Crown volume and leaf ©2010 International Society of Arboriculture area were estimated from computer processing of tree-crown im- ages obtained by using a digital camera. The method has shown greater accuracy than other techniques (±20% of actual leaf area) in estimating crown volume and leaf area of open-grown trees (Peper and McPherson 2003). Linear regression was used to fit predictive models with dbh as a function of age for each of the 20 sampled species. Predictions of leaf surface area, crown diam- eter, and height metrics were modeled as a function of dbh using best-fit models. Geographic data were collected for use in i-Tree Streets’ numerical models such as temperature, precipitation, air pollutant concentrations, and fuel mix for energy production. The program uses this background reference city data to mod- el the growth, benefits, and costs of street trees. Users import data collected in a sample or complete inventory conducted by profes- sionals or trained volunteers, and enter community specific infor- mation (e.g., program management costs, city population, price of residential electricity) to customize the benefit-cost data. The program calculates the costs associated with planting and manag- ing the trees and quantifies the value of annual benefits includ- ing carbon dioxide sequestration, energy conservation, air quality improvement, stormwater control, and increase in property value (Scott et al. 1998; Xiao et al. 2000; Simpson 2002). Results are pre- sented as easy-to-read and easy-to-use graphs, charts, and tables. i-Tree Streets is offered free to the public and supported by contributions from the i-Tree (2009) public/private partnership: USDA Forest Service, State and Private Forestry; Davey Tree Expert Company; International Society of Arboriculture, Society of Municipal Arborists; Arbor Day Foundation, and Casey Trees (www.itreetools.org). Results generated by Streets were used by New York City’s forestry director to demonstrate the social, economic and environmental value of increasing investment in the municipal forest. The result was an impressive USD $380
September 2010
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