258 MacFarlane: Quantifying Urban Saw Timber be 5.4 [±1.7] m3/ha−1, or 1540 [±485] bd ft/ha−1 accounting for conversion of round wood to dimensional lumber. There was considerable variation in saw timber volume both be- tween and within different urban LULCs. Mean graded saw timber volume was 3.0 [±1.2], 5.3 [±1.6], 7.8 [±2.1], and 7.4 [±1.7] m3/ha−1, respectively, for high-intensity urban, low- intensity urban, parks and golf courses, and roads and paved areas. Estimated crown wood saw timber volume for high- intensity urban, low-intensity urban, parks and golf courses, and roads and paved areas was 0.4 [±0.2], 0.7 [±0.3], 1.0 [±0.3], and 2.1 [±0.6] m3/ha−1, respectively. Scaled up to the 13-county region, this amounts to a total standing volume of 1.15 million m3 of urban saw timber (≈327 million bd ft of dimensional lumber) (Table 3). Accessibility To successfully recover saw timber from a tree, the tree must be accessible, i.e., able to be felled, cut in sections of mer- chantable length, and delivered to a sawmill (in urban areas, portable sawmills can ease the latter burden). Accessibility was not equal across all urban land types (Obviously, it should be much easier to harvest wood from street and parkland trees than from around homes and offices.). Ap- proximately 93.5% of all saw timber on parks and golf courses was considered easily accessible and less than 1% difficult to access. Almost 90% of saw timber along roads and paved areas was rated as easily accessible, although street trees were approximately four times (2.1% versus 0.5%) more likely to be rated as difficult to access than trees on parks and golf courses with the main complication being extracting wood from occasional large trees whose crowns are closely intertwined with utility wires. High-intensity ur- ban areas posed a greater challenge for extracting saw timber from trees, although less than 4% of this saw timber was considered difficult to access. By sharp contrast, approxi- Table 3. Saw timber volume estimates (m3 County Genesee Ingham Jackson Lapeer Lenawee Livingston Macomb Monroe Oakland Shiawassee St. Clair Washtenaw Wayne 13-county area Standard errors in parentheses. ©2007 International Society of Arboriculture Total standing mately half of all saw timber in low-intensity urban areas was rated as difficult to access. This reflects the close proximity of many large trees to hazards (sense Matheny and Clark 1994) such as homes or fences, in low-intensity urban areas, that would necessitate extraordinary measures to harvest trees in standard log lengths. Based on the weighted contribution of each of the four urban LULCs to total urban area (Table 1), it was estimated that ≈56% of all urban saw timber in the 13-county area was easily accessible, another 16% would require some additional measures to extract that would add additional costs (moderately accessible), and the remaining 28% difficult (for most intents and purposes considered in- accessible). Thus, of the total standing urban saw timber, ≈72% was considered accessible for extraction, amounting to 825,000 m3 of urban saw timber (≈235 million bd ft of di- mensional lumber) (Table 3). Annual Yield The 825,000 m3 of urban saw timber that is accessible in the 13-county areas includes all standing trees, virtually all of which would not be harvested until the trees that contain them were dead, or at minimum dying. Thus, to calculate the avail- ability of urban saw timber on an annual basis, it was nec- essary to estimate the rate at which trees would become avail- able. However, mortality rates and removal rates could not be directly assessed from the data collected for this study (stumps, e.g., represent death events from different years and may be ground up and seeded over and thus might not be tallied at all). Instead, recent estimates by Nowak et al. (2004) describing general trends and specific tree removal and mor- tality rates were combined with the data presented here and used to make reasonable estimates of urban saw timber avail- ability on an annual basis. Nowak et al. (2004) suggested that standing trees in ap- parently good condition die at a rate of ≈1.4% per year. The ) for urban portions of 13 counties in southeastern lower Michigan. Accessible 109,358 (34,428) 57,5 (18,124) 41,013 (12,912) 15,426 (4856) 22,094 (6956) 38,813 (12,219) 157,526 (49,591) 34,835 (10,967) 226,736 (71,380) 18,971 (5972) 36,594 (11,520) 60,017 (18,894) 327,415 (103,075) 1,146,368 (360,894) 78,738 (24,788) 41,450 (13,049) 29,529 (9296) 11,107 (3497) 15,908 (5008) 27,945 (8798) 113,419 (35,706) 25,081 (7896) 163,250 (51,394) 13,659 (4300) 26,348 (8295) 43,212 (13,604) 235,739 (74,214) 825,385 (259,844) Annual yield 1575 (496) 829 (261) 591 (186) 222 (70) 318 (100) 559 (176) 2,268 (714) 502 (158) 3,265 (1028) 273 (86) 527 (166) 864 (272) 4,715 (1484) 16,508 (5,197)
July 2007
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