Arboriculture & Urban Forestry 33(3): May 2007 221 Table 1. Summary of data collection in cities using 0.04 ha circular plots. City Atlanta, GA Baltimore, MD Boston, MA Calgary, Alberta Freehold, NJ Jersey City, NJ Minneapolis, MNz Moorestown, NJ Morgantown, WV New York, NY Philadelphia, PA San Francisco, CA Syracuse, NYy Toronto, Ontariox Washington. DCw Woodbridge, NJ zNowak et al. 2006b. yNowak and O’Connor 2001. xKenney et al. 2001. wNowak et al. 2006c. tors to yield dry weight biomass. These conversion factors, derived from average moisture contents of species given in the literature, averaged 0.48 for conifers and 0.56 for hard- woods (Nowak 1994). Open-grown, maintained trees tend to have less above- ground biomass than predicted by forest-derived biomass equations for trees of the same diameter at breast height (Nowak 1994). To adjust for this difference, biomass results for open-grown urban trees were multiplied by a factor of 0.8 (Nowak 1994). No adjustment was made for trees found in more natural stand conditions (e.g., vacant lands, forest pre- serves). Because deciduous trees drop their leaves annually, only carbon stored in woody biomass was calculated for these trees. Total tree dry weight biomass (above- and below- ground) was converted to total stored carbon by multiplying by 0.5. Multiple equations developed for a single tree species were combined to produce one predictive equation for a wide range of diameters for each species. The process of combining the individual formulas (each 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 mul- tiple equations). Formulas were combined to prevent dis- jointed sequestration estimates that can occur when calcula- tions switch between individual biomass equations. If no biomass equation could be found for an individual species, the average of results from equations of the same genus was used. If no equations for the genus were found, the Year 1997 1999 1996 1998 1998 1998 2004 2000 2004 1996 1996 2004 2001 2000 2004 2000 Number of plots 205 200 217 350 144 220 110 206 136 206 210 194 197 211 201 215 average of results from all broadleaf or conifer equations was used. Standard errors given for carbon report sampling error rather than error of estimation. Estimation error is unknown and likely larger than the reported sampling error. Estimation error also includes the uncertainty of using biomass equations and conversion factors, which may be large, as well as mea- surement error, which is typically small. Urban Tree Growth and Carbon Sequestration Average diameter growth from the appropriate land use and diameter class was added to the existing tree diameter (year x) to estimate tree diameter in year x+1. For urban trees in forest stands, average dbh growth was estimated as 0.38 cm/ year (0.15 in/year) (Smith and Shifley 1984); for trees on land uses with a park-like structure (e.g., parks, cemeteries, golf courses), average dbh growth was 0.61 cm/year (0.24 in/year) (deVries 1987); for more open-grown trees, dbh class- specific growth rates were based on Nowak (1994). Average height growth was calculated based on formulas from Fleming (1988) and the specific dbh growth factor used for the tree. Growth rates were adjusted based on tree canopy condi- tion. Adjustment factors were proportional to percent crown dieback (i.e., the greater the crown dieback, the slower the growth rate) and the assumption that less than 25% crown dieback had a limited effect on dbh growth rates. For trees with fair to excellent condition (less than 25% dieback), no adjustment was made to the growth rate; for poor condition trees (26% to 50% dieback), growth rates were multiplied by 0.76; critical trees (51% to 75% dieback) by 0.42; dying trees (76% to 99% dieback) by 0.15; and dead trees by 0. The difference in estimates of carbon storage between year x and year x+1 is the net amount of carbon sequestered annually. Tree death leads to the eventual release of stored carbon. To estimate the net amount of carbon sequestered by the urban trees after decomposition, carbon emissions resulting from decomposition after tree death must be considered. To calculate the potential release of carbon resulting from tree death and decomposition, estimates of annual mortality rates by condition class were derived from a study of street-tree mortality (Nowak 1986). Annual mortality was estimated as 1.9% for trees 0 to 3 in dbh in the good–excellent condition class (less than 10% dieback); 1.5% for trees greater than 3 in dbh in the good–excellent condition class; 3.3% for trees in fair condition (11% to 25% dieback); 8.9% for poor condi- tion; 13.1% for critical condition; 50% for dying; and 100% for dead. Two types of decomposition rates were used: 1) rapid re- lease for aboveground biomass of trees that are projected to be removed and 2) delayed release for standing dead trees and tree roots of removed trees. Trees that are removed from urban sites are not normally developed into wood products that provide for long-term carbon storage (i.e., removed trees ©2007 International Society of Arboriculture
May 2007
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