Arboriculture & Urban Forestry 34(6): November 2008 Arboriculture & Urban Forestry 2008. 34(6):347–358. 347 A Ground-Based Method of Assessing Urban Forest Structure and Ecosystem Services David J. Nowak, Daniel E. Crane, Jack C. Stevens, Robert E. Hoehn, Jeffrey T. Walton, and Jerry Bond Abstract. To properly manage urban forests, it is essential to have data on this important resource. An efficient means to obtain this information is to randomly sample urban areas. To help assess the urban forest structure (e.g., number of trees, species composition, tree sizes, health) and several functions (e.g., air pollution removal, carbon storage and sequestration), the Urban Forest Effects (UFORE) model was developed. Data collection variables and model methods are detailed and urban forest structure results are compared among 14 United States cities with average tree density ranging between 22.5 trees/ha (9.1 trees/ac) in Casper, Wyoming, U.S. to 275.8 trees/ha (111.6 trees/ac) in Atlanta, Georgia, U.S. Advantages and disadvantages of this ground-based method of assessing urban forest structure, functions, and values are discussed. Key Words. Air pollution removal; carbon sequestration; tree measurement; urban forest monitoring; urban forest sampling. Urban vegetation, particularly trees, provides numerous benefits that can improve environmental quality and human health in and around urban areas. These benefits include improvements in air and water quality, building energy conservation, cooler air tem- peratures, reductions in ultraviolet radiation, and many other environmental and social benefits (e.g., Dwyer et al. 1992; Kuo and Sullivan 2001; Westphal 2003; Wolf 2003; Nowak and Dwyer 2007). Statistically sound data on the urban forest struc- ture are required to properly assess the magnitude of these ben- efits. To optimize forest benefits, information on costs associated with vegetation management should also be assessed. Forest structural data (e.g., number of trees, species composition, tree size, health, tree location) provide the basis to estimate total leaf area, tree and leaf biomass, and quantify numerous forest func- tions (ecosystem services). Accurate measures of urban forest structure are critical for proper urban forest planning to help sustain or enhance environmental quality and human health and well-being in cities. The most precise way to assess urban forest structure is to measure and record information on every tree. A complete cen- sus may work well for relatively small populations (e.g., street trees, small parks) but is cost-prohibitive for larger tree popula- tions. Thus, random sampling can provide a cost-effective means to assess urban forest structure and functions for large-scale assessments. A limited number of assessments of entire urban forest ecosystems across a city based on ground sampling of individual trees has been conducted. Various studies in the past have attained information on urban forest structure and factors affecting structure, but these assessments focused on relatively small areas, subsets of the landscape, or tree cover attributes (e.g., Jones 1957; Derrenbacher 1969; Hyams 1970; Duncan 1973; Schmid 1975; Numata 1977; Sukopp et al. 1979; Kunick 1982; Boyd 1983; Iizumi 1983; Sanders 1983; Santamour 1983; Dorney et al. 1984; Moran 1984; Profous 1984; Richards et al. 1984; Rowntree 1984; Whitney 1985; Profous et al. 1988; Gil- bert 1989; Jim 1989). More recently, increasing numbers of comprehensive assessments of urban forest structure have been conducted using sampling techniques (e.g., McBride and Jacobs 1976, 1986; Miller and Winer 1984; Nowak 1991, 1994b; Mc- Pherson 1998; Nowak and O’Connor 2001; Nowak et al. 2002b, 2006b, 2006c, 2006d, 2007b, 2007c, 2007d; Ham et al. 2003; Lozano 2004; Yang et al. 2005; Escobedo et al. 2006; McNeil and Vava 2006; Buckelew Cumming et al. 2007). The Urban Forest Effects (UFORE) model was developed to aid in assess- ing urban forest structure, functions, and values (Nowak and Crane 2000). This model contains protocols to measure and monitor urban forests as well as estimate ecosystem functions and values. The UFORE model has been used in approximately 50 cities across the globe (approximately one-third outside of the United States) to assess urban tree populations using a standardized approach (e.g., Nowak and O’Connor 2001; Nowak et al. 2002b, 2006b, 2006c, 2006d, 2007b, 2007c, 2007d; Ham et al. 2003; Lozano 2004; Yang et al. 2005; Escobedo et al. 2006; McNeil and Vava 2006; Buckelew Cumming et al. 2007). Many of these cities were analyzed in cooperation with local institutions. Some cities have published reports, whereas others have used the model outputs without producing reports or have reports cur- rently in production. An understanding of the UFORE model operation and its ad- vantages and disadvantages are critical to understanding the ac- curacy and purpose of the model as well as its strengths and limitations. Through this understanding, the model can be more fully used to improve urban forest assessments and enhance planning and management to sustain ecosystem services in urban and urbanizing areas. This article reviews the data collection required by the model and then details the methods of how structure and functions are estimated, including a discussion of the advantages and disadvantages of the approaches used. The article concludes with a discussion of how UFORE results can be integrated within long-term management plans. METHODS The basic premise behind the UFORE model is that urban forest structure affects forest functions and values. By having an ac- curate assessment of urban forest structure, better estimates of ©2008 International Society of Arboriculture
November 2008
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