256 MacFarlane: Quantifying Urban Saw Timber Data Analysis Tree Wood Volume Estimation Stem measurements were used to estimate the total merchant- able saw timber round wood volume (m3) in each sample tree from 0.15 cm (0.06 in) stump height to an approximate 20 cm (8 in) top DOB with Smalian’s formula (Avery and Burkhart 1994). An individual taper model for each tree derived from its top diameter and dbh was used to account for stem taper during volume calculations (change in stem diameter over log length was extrapolated to predict stump diameter outside bark for each tree). A species-level constant bark factor model, predicting wood volume inside bark from wood vol- ume outside bark, was used to estimate solid wood and bark volumes from total volume (Smith 1985). Exotic tree species were assigned a bark factor of a species in the same genera with an equivalent bark type. Recovered sawn lumber volume in standing trees was computed using the tree’s taper model and the International 1 ⁄4 in Board-foot rule for variable length logs (Freese 1973) so that recovered saw lumber could be compared with cubic round wood volume estimates (i.e., ac- counting for losses attributable to sawing). Crown wood board-foot volume was estimated using a model relating the basal area (BAi, ft2) and the number of merchantable 8 ft (26.4 m) saw logs (Li) in the crown of a tree to its Interna- tional 1 ⁄4 Li)0.74 derived from felled and dissected trees on Michigan timberlands (MacFarlane, unpublished). in Board-foot rule volume (VSi): VSi19.30 (BAi Scaling Up Individual Tree Estimates to the 13-County Region Average saw timber volume per hectare (m3 and bd ft) was estimated from the number of sample trees 20 cm (8 in) dbh or greater on a sample plot with an area ai. The contribution of each sample tree to per hectare estimates was weighted according to its selection probability, which was proportional to the size of the variable area plot on which it occurred (Shiver and Borders 1996); the variance of sample means was also weighted in the same way. Estimates from each of the LULC classes were then combined to estimate the overall urban condition for the 13-county region using typical pro- cedures for combining stratum in stratified sampling (Shiver and Borders 1996) with contributions of plots from each LULC weighted by the fraction of urban area they comprised (Table 1). RESULTS Overall, 76 urban QQSs were surveyed and 1887 stems and stumps 20 cm (8 in) or greater were measured translating into a mean density of 12.8 [±2.1 (standard error of mean) stems and stumps ha−1 (5.8 [±0.8] ac−1) across the 13-county urban area; 89.7% were healthy, live trees, 6.3% were classed as dying, 3.7% were stumps, and 0.3% were dead, standing ©2007 International Society of Arboriculture trees. Estimated density values for LULCs were 9.5 [±3.1], 13.7 [±3.5], 18.8 [±4.2], and 20.3 [±3.7] stems and stumps ha−1 for high-intensity urban, low-intensity urban, parks and golf courses, and roads and paved areas, respectively. At least 68 species (with 20 cm [8 in] or greater) representing 36 genera were found (some trees were only identified to their generic scientific name and species could not be identified for all stumps); each was assigned to a species-product class (see Appendix) based on U.S.D.A. Forest Inventory and Analysis groupings (Miles et al. 2001). Urban Wood Volume Grade and Species-Products The mean urban (round) wood volume across the 13-county area in tree stem sections 20 cm (8 in) or greater dbh was estimated to be 7.9 [±1.3] m3/ha−1 (117.2 ± 19.9 ft3/ac−1), ≈31% of which was graded as having no saw timber value (grade 0; Table 2) as a result of major rot, defects, and other problems (see Rast et al. 1973). Approximately 56% of all graded (not including crown wood) softwood volume per acre was deemed as having no saw timber value, whereas only 35% of potentially commercial hardwood stems were graded as unfit for saw timber products (Table 2). Approximately 73% of all stems of “noncommercial” species (Table 1) were rated as unsuited for saw timber. Less than 5% of red oak (shingle oak, Quercus imbricaria; pin oak, Q. palustris; northern red oak, Q. rubra; black oak, Q. velutina), white oak (white oak, Q. alba; swamp white oak, Q. bicolor; bur oak, Q. macrocarpa; English oak, Q. robur), and black walnut (Ju- glans nigra) wood was rated as having no value, whereas a large proportion of hard maple (58%) (hedge maple, Acer campestre; black maple, A. nigrum; sugar maple, A. saccha- rum) and soft maple (42%) (boxelder, A. negundo; Norway maple, A. platanoides; red maple, A. rubrum; silver maple, A. saccharinum) wood was graded as having no saw timber value. Approximately 60% of mean urban wood volume was saw timber grade (grades 1 through 5; Table 2) amounting to 4.7 m3/ha−1 [±0.9] (67.7 [±13.3] ft3/ac−1). Mean saw timber (round) wood volume translated into 1364 bd ft per urban hectare (552 bd ft/ac−1) of sawn lumber using the Interna- tional 1 ⁄4 in rule (a conversion ratio of 290 bd ft per cubic meter of wood [8.2 bd ft/ft3]). Most (93%) of urban softwood saw timber volume assigned to the lowest class (grade 3). This likely was the result of the greatly increased size and density of branch knots in open-grown coniferous trees, which are reflected in softwood grading rules (DeBell et al. 1994; Uusitalo and Isotalo 2005). In general, a smaller pro- portion of urban hardwood saw timber volume was in higher grade classes than in lower grade classes (11% grade 1, 13% grade 2, 24% grade 3, and 48% grade 5) except for grade 4, construction grade, which comprised only 4%. The latter re- flects reservations by field technicians regarding the potential
July 2007
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