260 MacFarlane: Quantifying Urban Saw Timber processed volume, thus smaller diameter urban trees would likely contribute very little additional lumber. Because most saw timber is harvested from forest-grown trees, it may be useful to compare the quality and availability of urban saw timber with that available from forests. Based on data from the U.S.D.A. Forest Inventory and Analysis program (Unit 4, Michigan, cycle 6, 2004), 5055 bd ft/ac−1 (12,486 bd ft/ha−1) of graded saw timber is available on an average forested acre in southern lower Michigan, which is approximately nine times what was estimated for an average urban acre (552 bd ft/ac−1). Ignoring softwoods in our study, which were not graded into all five U.S.D.A. Forest Services tree grades (see “Methods”), ≈59% of all forest-grown bd.ft. were in factory-grade classes (1, 2, and 3) and 41% in the construction and local use classes (grades 4 and 5), compared with 54% and 46%, respectively, for urban hardwood saw timber volume (Figure 1). Within the upper saw grades, for- est- versus urban-grown hardwood saw timber volume was 14% versus 10% in grade class 1, 19% versus 12% in grade class 2, and 27.0% versus 32% for grade class 3, respectively. Thus, in an absolute sense, there are clearly many more saw timber quality trees in forested versus urban areas, but these results suggest only a small difference in wood quality rela- tive to the large (900%) decrease in wood quantity when one harvests hardwood trees in urban versus forested areas. Despite the potential availability of urban saw timber dem- onstrated here, there are still outstanding logistical problems regarding successful capitalization of it, including costs of extraction, handling, and variation in supply. One fact regard- ing urban trees, however, is the inevitability of their removal once they become dead or diseased, because they represent a liability (a hazard, sense Matheny and Clark 1994) and di- minish aesthetic value (Scott and Betters 2000). Nowak et al. (2004) estimated that 17% of all dead standing trees in Bal- timore, Maryland, were removed over a 2-year period, or ≈8.5% per year. This suggests some consistent level of urban wood will be available for harvest over time. One caveat regarding capitalizing on urban wood made available through tree sickness and mortality is the need to properly sanitize diseased or infested trees to prevent further dissemination of the pests and pathogens that injured or killed them. A primary vector for spread of emerald ash borer, e.g., is through transportation of infested firewood to new areas (Poland and McCullough 2006). Because emerald ash borer, and most other pests and pathogens of tree stems, live and feed in the phloem, debarking trees and stripping off a small bit of the outermost wood can be a simple and highly effec- tive treatment so long as these residual materials are properly disposed of (this has been examined for sanitizing emerald ash borer-infested ash logs in Michigan). Other methods sug- gested include treating infested logs with pesticides (Nzokou et al. 2006a) or preservatives (Nzokou et al. 2006b), but there ©2007 International Society of Arboriculture Figure 1. Comparison of saw timber quality grades (grade 1 is the highest grade) between urban- and for- est-grown trees in southern lower Michigan. are challenges to implementing such treatments at large scales. Although it may not be realistic to expect arborists and small, private landowners to bear the logistical or financial burden of harvesting urban saw timber, clearly public entities could expand the current level of efforts. For example, the Community and Urban Forest Inventory and Management (computer) Program was recently created to help communi- ties in California to inventory tree volume and calculate value for their urban forests (Pillsbury and Gill 2003). It has already been demonstrated on smaller scales that internal utilization of dead and dying municipal trees can offset the costs of tree removal and allow reduced infrastructural costs such as the purchase of wood for park benches and picnic pavilions (Bratkovich 2001). Training municipal foresters to grade
July 2007
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