Arboriculture & Urban Forestry 33(4): July 2007 259 latter rate was used to describe mortality in the “live” cat- egory in this study, 1.4% of the 89.7% of urban stems per acre or 1.3%. Trees with crown deterioration, equating roughly to “dying” trees in this study, had a mortality rate of ≈6.4% (Nowak et al. 2004), which equates to 0.4% more of the trees in this study. Ignoring the stumps, another 0.3% can be tallied from dead standing trees that have not yet been removed. All totaled, it can be expected that ≈2% of the accessible volume would come available annually, which translates into ≈16,500 m3 (or ≈4.7 million bd ft) of urban saw timber per year available in the 13-county study area (Table 3). DISCUSSION The methods presented here allowed for a regional estimate of urban saw timber to be developed and extrapolated through urban land area estimates derived from satellite photography. Data describing urban land cover are generally widely avail- able (e.g., the entire United States; Nowak et al. 2006); thus, these methods could be replicated almost anywhere. To the extent that average per hectare estimates derived from urban areas in southeastern lower Michigan are representative of broader regional species composition and urban tree demo- graphic structure, these specific estimates could be further extrapolated outside of this specific region. However, the overall weighted estimates are also sensitive to the relative makeup of urban areas (e.g., a different ratio of high- versus low-intensity urban areas) such that per hectare estimates for urban LULCs would need to be reweighted accordingly. Over 16,000 m3 of urban saw timber is estimated to come available each year in the 13-county study area. To put this number in perspective, small modern sawmills process ≈3000 to 10,000 m3 of wood per year annually (Pascal Kamdem, Michigan State University, pers. com.). Assuming a mini- mum of 3000 m3 to remain viable, all of the potentially available wood in the 13 counties that comprise southeastern lower Michigan could support the minimum annual needs of five of these mills. The 4.7 million bd ft of lumber annually available in urban trees in this region is equivalent to the amount of wood used to build 362 average-sized homes (Falk 2002). The quality of wood in urban softwoods was generally low based on the grading standards applied, which was not sur- prising given the importance of maintaining small branch knots along the main stem of (coniferous) trees to softwood quality; a condition most likely to be met when trees are forest grown (DeBell et al. 1994; Uusitalo and Isotalo 2005). However, most urban saw timber (≈90%) inventoried came from commercially viable hardwood timber species, 60% of which was considered saw-grade quality. Whereas noncom- mercial species comprise a trivial proportion of large trees, wood from exotic species did comprise a substantial propor- tion of urban wood (e.g., Siberian elm, Norway maple, and horsechestnut), raising potential concerns regarding their uti- lization (e.g., commercial kiln-drying procedures have not been developed for them). However, wood from many of these species are already commercially viable (Norway maple is considered a valuable hardwood in Germany; Jurek and Wihs 1998), and some North American vendors have been able sell wood from exotic tree species at a premium (www. urbantreesalvage.com). One aspect of urban wood quality not addressed by this study is that of the mechanical properties of urban wood. Mackes et al. (2005) found that the modulus of rupture and modulus of elasticity were both lower in open-grown trees, primarily attributable to a greater quantity of juvenile wood, which suggests a potentially lower strength for “urban” wood. Further research would have to be done to specifically measure wood properties of urban versus forest-grown trees. The estimates of urban saw timber presented here are likely conservative based on the definitions of “urban” area used in this study. The use of remotely sensed land use/land cover imagery to define urban areas likely underestimates the num- ber of trees in urban areas relative to definitions based on political boundaries such as city limits or census districts (e.g., Nowak et al. 2006), which, if used, would have in- cluded wood from trees growing in forested areas within urban zones. It also likely underestimates the total amount of urban area. In a recent study, Fang et al. (2006) demonstrated that land use maps were more likely to misclassify urban areas (in Chicago) as forested than the reverse, because of the fact that many houses were beneath a canopy of trees. Estimates of urban saw timber availability were also likely conservative based on definitions of what portion of urban wood qualified as extractable sawn wood products. Low- intensity urban areas comprised almost half of all urban area in the 13-county region (Table 1) and almost 50% of the wood in these areas was rated as difficult to access attribut- able to a frequent close proximity of large trees to potential hazards (e.g., homes). Sherrill (2003) proposed reasonable guidelines for safely extracting urban wood and commercial arborists to safely remove such trees all the time; thus, a larger proportion of wood from these trees may actually be accessible. Advances in sawing technology might also allow portions of some of the wood rated as grade 0 to be used for saw timber. Typical modern sawmills often dissect logs into a variety of component parts of different grades such that the visually based whole log or tree stem grading rules used here (e.g., Rast et al. 1973) may be overly conservative. Estimates also did not include the solid wood products potential of trees smaller than 20 cm (8 in) diameter. Ad- vances in wood technology have greatly expanded the poten- tial for smaller trees (LeVan-Green and Livingston 2001). However, by extrapolating volume estimates for trees of dif- ferent sizes measured in this study, it was estimated that trees less than 20 cm (8 in) contribute only ≈3% to the total un- ©2007 International Society of Arboriculture
July 2007
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