Arboriculture & Urban Forestry 34(6): November 2008 each measured tree is calculated using allometric equations from the literature (see Nowak 1994c; Nowak et al. 2002b). Equations that predict aboveground biomass are converted to whole tree biomass based on a root-to-shoot ratio of 0.26 (Cairns et al. 1997). Equations that compute fresh weight biomass are multi- plied by species- or genus-specific conversion factors to yield dry weight biomass. These conversion factors, derived from av- erage moisture contents of species given in the literature, aver- aged 0.48 for conifers and 0.56 for hardwoods (see Nowak et al. 2002b). Open-grown, maintained trees tend to have less aboveground biomass than predicted by forest-derived biomass equations for trees of the same dbh (Nowak 1994c). To adjust for this differ- ence, biomass results for urban trees are multiplied by a factor of 0.8 (Nowak 1994c). No adjustment is made for trees found in more natural stand conditions (e.g., on vacant lands or in forest preserves). Because deciduous trees drop their leaves annually, only carbon stored in wood biomass is calculated for these trees. Total tree dry weight biomass is converted to total stored carbon by multiplying by 0.5 (Forest Products Laboratory 1952; Chow and Rolfe 1989). The multiple equations used for individual species were com- bined to produce one predictive equation for a wide range of diameters for individual species. The process of combining the individual formulas (with limited diameter ranges) into one more general species formula produced results that were typically within 2% of the original estimates for total carbon storage of the urban forest (i.e., the estimates using the multiple equations). Formulas were combined to prevent disjointed sequestration es- timates that can occur when calculations switch between indi- vidual biomass equations. If no allometric equation could be found for an individual species, the average of results from equations of the same genus is used. If no genus equations are found, the average of results from all broadleaf or conifer equations is used. To estimate monetary value associated with urban tree carbon storage and sequestration, carbon values are multiplied by $22.8/ tonne of carbon ($20.7/ton of carbon) based on the estimated marginal social costs of carbon dioxide emissions for 2001 to 2010 (Fankhauser 1994). Urban Tree Growth and Carbon Sequestration To determine a base growth rate based on length of growing season, urban street tree (Fleming 1988; Frelich 1992; Nowak 1994c), park tree (deVries 1987), and forest growth estimates (Smith and Shifley 1984) were standardized to growth rates for 153 frost-free days based on: standardized growth measured growth × (153/number of frost-free days of measurement). Average standardized growth rates for street (open-grown) trees were 0.83 cm/year (0.33 in/year). Growth rates of trees of the same species or genera were then compared to determine the average difference between standardized street tree growth and standardized park and forest growth rates. Park growth averaged 1.78 times less than street trees, and forest growth averaged 2.29 times less than street tree growth. Crown light exposure mea- surements of 0 to 1 were used to represent forest growth condi- tions; 2 to 3 for park conditions; and 4 to 5 for open-grown conditions. Thus, the standardized growth equations are: Standardized growth (SG) 0.83 cm/year (0.33 in/year) × number of frost free days/153 and for: CLE 0–1: Base growth 351 SG/2.26; CLE 2–3: base growthSG /1.78; and CLE 4–5: base growth SG. Base growth rates are adjusted based on tree condition. For trees in fair to excellent condition, base growth rates are multi- plied by 1 (no adjustment), poor trees’ growth rates are multi- plied by 0.76, critical trees by 0.42, dying trees by 0.15, and dead trees by 0. Adjustment factors are based on percent crown die- back and the assumption that less than 25% crown dieback had a limited effect on dbh growth rates. The difference in estimates of carbon storage between year x and year x + 1 is the gross amount of carbon sequestered annually. Air Pollution Removal This module quantifies the hourly amount of pollution removed by the urban forest, its value, and associated percent improve- ment in air quality throughout a year. Pollution removal and percent air quality improvement are calculated based on field, pollution concentration, and meteorologic data. This module is used to estimate dry deposition of air pollution (i.e., pollution removal during nonprecipitation periods) to trees and shrubs (Nowak et al. 1998, 2000). This module calculates the hourly dry deposition of ozone (O3), sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), and particulate matter less than 10 m (PM10) to tree and shrub canopies throughout the year based on tree-cover data, hourly NCDC weather data, and U.S. Environmental Protection Agency pollu- tion concentration monitoring data. The pollutant flux (F; in g/m2/s) is calculated as the product of the deposition velocity (Vd; in m/s) and the pollutant concentra- tion (C; in g/m3): F = Vd × C Deposition velocity is calculated as the inverse of the sum of the aerodynamic (Ra), quasilaminar boundary layer (Rb), and canopy (Rc) resistances (Baldocchi et al. 1987): Vd =(Ra + Rb + Rc)−1 Hourly meteorologic data from the closest weather station (usu- ally airport weather stations) are used in estimating Ra and Rb. In-leaf, hourly tree canopy resistances for O3, SO2, and NO2 are calculated based on a modified hybrid of big leaf and multilayer canopy deposition models (Baldocchi et al. 1987; Baldocchi 1988). Because CO and removal of particulate matter by vegetation are not directly related to transpiration, Rc for CO is set to a constant for in-leaf season (50,000 sec/m [15,240 sec/ft]) and leaf-off season (1,000,000 sec/m [304,800 sec/ft]) based on data from Bidwell and Fraser (1972). For particles, the median de- position velocity from the literature (Lovett 1994) is 0.0128 m/s (0.042 ft/s) for the in-leaf season. Base particle Vd is set to 0.064 m/s (0.021 ft/s) based on a LAI of 6 and a 50% resuspension rate of particles back to the atmosphere (Zinke 1967). The base Vd is adjusted according to actual LAI and in-leaf versus leaf-off sea- son parameters. Bounds of total tree removal of O3, NO2, SO2, and PM10 are estimated using the typical range of published in-leaf dry deposition velocities (Lovett 1994). Percent air qual- ity improvement is estimated by incorporating local or regional boundary layer height data (height of the pollutant mixing layer). More detailed methods on this module can be found in Nowak et al. (2006a). ©2008 International Society of Arboriculture
November 2008
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
