350 Nowak et al.: Assessing Urban Forest Structure and Ecosystem Services weight/m2 of leaf area. Shrub leaf biomass is calculated as the product of the crown volume occupied by leaves (m3) and mea- sured leaf biomass factors (g/m3) for individual species (e.g., Winer et al. 1983; Nowak 1991). Shrub leaf area is calculated by converting leaf biomass to leaf area based on measured species conversion ratios (m2/g). As a result of limitations in estimating shrub leaf area by the crown-volume approach, shrub leaf area is not allowed to exceed a LAI of 18. If there are no leaf-biomass- to-area or leaf-biomass-to-crown-volume conversion factors for an individual species, genus or hardwood/conifer averages are used. For trees in more forest stand conditions (higher plant com- petition), LAI for more closed canopy positions (CLE0–1) is calculated using a forest leaf area formula based on the Beer- Lambert Law: LAI = ln(IIo)−k where I light intensity beneath canopy; Io light intensity above canopy; and klight extinction coefficient (Smith et al. 1991). The light extinction coefficients are 0.52 for conifers and 0.65 for hardwoods (Jarvis and Leverenz 1983). To estimate the tree leaf area (LA): LA =[ln(1 − xs)−k]× r2 where xs is average shading coefficient of the species and r is the crown radius. For CLE 2–3: LA is calculated as the average of leaf area from the open-grown (CLE 4–5) and closed canopy equations (CLE 0–1). Estimates of LA and leaf biomass are adjusted downward based on crown leaf dieback (tree condition). Trees are assigned to one of seven condition classes: excellent (less than 1% die- back); good (1% to 10% dieback); fair (11% to 25% dieback); poor (26% to 50% dieback); critical (51% to 75% dieback); dying (76% to 99% dieback); and dead (100% dieback). Condi- tion ratings range between 1 indicating no dieback and 0 indi- cating 100% dieback (dead tree). Each class between excellent and dead is given a rating between 1 and 0 based on the midvalue of the class (e.g., fair 11% to 25% dieback is given a rating of 0.82 or 82% healthy crown). Tree leaf area is multiplied by the tree condition factor to produce the final LA estimate. Species Diversity A species diversity index (Shannon-Wiener) and species rich- ness (i.e., number of species) (e.g., Barbour et al. 1980) are calculated for living trees for the entire city. The proportion of the tree population that originated from different parts of the country and the world is calculated based on the native range of each species (e.g., Hough 1907; Grimm 1962; Platt 1968; Little 1971, 1976, 1977, 1978; Viereck and Little 1975; Preston 1976; Clark 1979; Burns and Honkala 1990a, 1990b; Gleason and Cronquist 1991). Structural Value The structural value of the trees (Nowak et al. 2002a) is based on methods from the Council of Tree and Landscape Appraisers (CTLA 1992). Compensatory value is based on four tree/site characteristics: trunk area (cross-sectional area at dbh), species, condition, and location. Trunk area and species are used to de- termine the basic value, which is then multiplied by condition and location ratings (0 to 1) to determine the final tree compen- ©2008 International Society of Arboriculture satory value. Local species factors, average replacement cost, and transplantable size and replacement prices are obtained from ISA publications. If no species data are available for the state, data from the nearest state are used. Condition factors are based on percent crown dieback. Available data required using location factors based on land use type (International Society of Arbori- culture 1988): golf course 0.8; commercial/industrial, cem- etery, and institutional 0.75; parks and residential 0.6; transportation and forest 0.5; agriculture 0.4; vacant 0.2; wetland 0.1. Insect Effects The proportion of leaf area and live tree population and esti- mated compensatory value in various susceptibility classes to gypsy moth (Liebhold et al. 1995; Onstad et al. 1997), Asian longhorned beetle (e.g., Nowak et al. 2001), and emerald ash borer (ash species) are calculated to reveal potential urban forest damage associated with these pests. Biogenic Emissions Volatile organic compounds can contribute to the formation of O3 and CO (e.g., Brasseur and Chatfield 1991). The amount of VOC emissions depends on tree species, leaf biomass, air tem- perature, and other environmental factors. This module estimates the hourly emission of isoprene (C5H8), monoterpenes (C10 ter- penoids), and other volatile organic compounds by species for each land use and for the entire city. Species leaf biomass (from the structure module) is multiplied by genus-specific emission factors (Nowak et al. 2002b) to produce emission levels stan- dardized to 30°C (86°F) and photosynthetically active radiation (PAR) flux of 1000 mol/m2/s. If genus-specific information is not available, then median emission values for the family, order, or superorder are used. Standardized emissions are converted to actual emissions based on light and temperature correction fac- tors (Geron et al. 1994) and local meteorological data. Because PAR strongly controls the isoprene emission rate, PAR is esti- mated at 30 canopy levels as a function of above-canopy PAR using the sunfleck canopy environment model (A. Guenther, Nat. Cent. for Atmos. Res., pers. comm., 1998) with the LAI from the structure calculations. Hourly inputs of air temperature are from measured National Climatic Data Center (NCDC) meteorological data. Total solar radiation is calculated based on the National Renewable Energy Laboratory Meteorological/Statistical Solar Radiation Model with inputs from the NCDC data set (Maxwell 1994). PAR is calculated as 46% of total solar radiation input (Monteith and Unsworth 1990). Because tree transpiration cools air and leaf temperatures and thus reduces biogenic VOC emissions, tree and shrub VOC emissions are reduced in the model based on air quality model- ing results (Nowak et al. 2000). For the modeling scenario ana- lyzed (July 13–15, 1995), increased tree cover reduced air tem- peratures by 0.3°C to 1.0°C resulting in hourly reductions in biogenic VOC emissions of 3.3% to 11.4%. These hourly reduc- tions in VOC emissions are applied to the tree and shrub emis- sions during the in-leaf season to account for tree effects on air temperature and its consequent impact on VOC emissions. Carbon Storage and Annual Sequestration This module calculates total stored carbon and gross and net carbon sequestered annually by the urban forest. Biomass for
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