90 Duryea et al.: Hurricanes and the Urban Forest, I Table 3. Survival and branch loss for tree species in Hurricane Ivan for different modulus of elasticity classesz Modulus of elasticity (MPa) 10,000 9,000 to 9,999 8,000 to 8,999 7,000 to 7,999 <7,000 Sample size (number of trees) 659 590 157 213 126 Survival (%) 77 a 75 a 74 a 74 a 63 b zNumbers in columns with different letters are significantly different at P 0.05. et al. (1991) noted that the strong wood of live oak resulted in lost branches only and little windthrow during Hurricane Hugo in South Carolina. Putz et al. (1983) found that tree species with shorter, thicker stems and denser wood tended to uproot instead of break and that species with low-density wood had higher mortality. Two other measurements of wood strength are the modulus of elasticity (Young’s elastic modulus) and the modulus of rupture. The modulus of elasticity measures the wood’s stiff- ness; after applying a certain weight (in MPa), it measures whether the wood recovers to its original position (Forest Products Laboratory 1999; Niklas 1999). Tree species with the highest modulus of elasticity (>7,000 MPa) survived Hur- ricane Ivan better than those with the lowest values (<7,000 MPa) (Table 3). Examples of species with the highest modu- lus of elasticity are slash pine, longleaf pine, sand live oak, and live oak (10,500, 11,000, 10,900, and 10,900 MPa, re- spectively). Those with the lowest modulus of elasticity are southern redcedar and spruce pine (6,410 and 6,900 MPa). Branch loss did not appear to be related. Niklas (1999) com- pared young black locust (Robinia pseudoacacia) with older trees and found that younger trees were less stiff and strong as a result of the different proportions of primary and sec- ondary tissues as trees age. Modulus of rupture is a measure of the bending stress that wood can experience without mechanically failing (Forest Products Laboratory 1999; Niklas 1999). Results from Hur- ricane Ivan show that survival was highest for those species with the highest modulus of rupture (70,000 kPa), whereas branch loss showed no relationship (Table 4). Examples of species with low survival rates that have a low modulus of rupture were tulip poplar, spruce pine, and sycamore (41,000, 41,000, and 45,000 kPa). However, some species with high survival also have a low modulus of rupture (southern mag- nolia [47,000 kPa] and sweetgum [49,000 kPa]). Live oak and sand live oak represent the highest modulus of rupture category with 82,000 kPa. In a study with Scots pine (Pinus sylvestris), Skatter and Kucera’s (2000) results supported the modulus of rupture as important to a tree’s susceptibility to wind. Crown Characteristics Crown density is an estimate of the openness of the crown or the ratio of positive and negative space within the crown (Hightshoe 1988; Gilman 2005). We determined from the literature and observation the density class of each species and then compared open with moderate with dense crowns for branch loss and survival (Hightshoe 1988; Gilman 2005). Dense crowns had significantly greater survival than moder- ate or open crowns (84% versus 74% versus 67%) (P < 0.0001). When we recalculated survival (excluding trees with 50% or greater branch loss), the relationship was still the same and significant (75%, 68%, and 62%) (P < 0.0001). In contrast to our survival results, Foster (1988) concluded that after a catastrophic wind in New England, tree species with full crowns and shallow roots were more susceptible than those with a vertical distribution of canopy, flexible branches, and tapering shape. In their literature review, Everham and Brokaw (1996) summarized that there is a tendency for dense-crowned trees to be damaged and open-foliage crowns Table 4. Survival and branch loss for tree species in Hurricane Ivan for different modulus of rupture classesz Modulus of rupture (kPa) 70,000 60,000 to 60,999 50,000 to 50,999 <50,000 Sample size (number of trees) 240 486 722 246 ©2007 International Society of Arboriculture Survival (%) 94 a 74 bc 69 c 75 b zNumbers in columns with different letters are significantly different at P 0.05. Survival (%) (recalculated by subtracting trees with 50% branch loss) 87 a 70 b 60 c 68 b . Branch loss (%) 20 b 19 b 23 a 19 b Survival (%) (recalculated by subtracting trees with 50% branch loss) 72 a 66 a 73 a 66 a 52 b . Branch loss (%) 19 ab 24 a 12 c 20 b 22 ab
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