Arboriculture & Urban Forestry 33(2): March 2007 89 Andrew in which 90% of the south Florida slash pine (Pinus elliottii var. densa) that fell broke (Duryea et al. 1996). Van Hooser and Hedlund (1969) also found that pines tended to snap compared with uprooting for broadleaf species. In tropi- cal species, Putz et al. (1983) found that species with shorter, thicker stems and denser wood tended to uproot rather than to snap. However, in our study, when it did fall (only 9% of the live oaks), live oak with its dense wood (0.8 g/cm3) equally uprooted or broke. Property Damage Twenty percent (20%) of the trees that fell damaged property in Hurricane Ivan. Of these, 74% damaged major property (house, power lines, screened enclosures, automobiles, and so on) and 26% damaged minor property (fences, sheds, side- walks). Ten percent of the total damage was to power lines. The tree species causing the most damage were pines, pecan, tulip poplar, and laurel and water oaks. In Hurricanes Erin and Opal, laurel oak, longleaf pine, sand pine, and slash pine caused the most damage (between 11% and 18% damage) and smaller species such as laurelcherry (Prunus caroliniana) and southern redcedar caused the least damage (Duryea 1997). What Makes a Tree More Wind-Resistant? The goal of this research project is to attempt to determine what biological and cultural factors make trees more or less wind-resistant. By evaluating these factors, we can under- stand the differences between species (i.e., whether they have dense wood or defoliate quickly in wind) and between certain practices (such as planting trees in groups compared with individual tree plantings), which could result in a healthier and more wind-resistant urban forest. Defoliation There were distinct species differences in defoliation during Hurricane Ivan. Species such as sand live oak, crapemyrtle, and dogwood lost an average of 94%, 88%, and 86%, respec- tively, of their leaves compared with southern redcedar, wax myrtle, slash pine, longleaf pine, and loblolly pine, which lost 32%, 31%, 29%, 19%, and 11% of their leaves, respectively (least significant difference17%). Leaf loss had a positive relationship (P < 0.0001) with both survival and recalculated survival (trees with 50% branch loss excluded); in other words, losing leaves during the hurricane meant higher sur- vival. Francis and Gillespie (1993), reporting on urban trees in Puerto Rico after Hurricane Hugo in 1989, also found that crown damage appeared to be avoided if the crown surface area was reduced quickly with leaf and twig loss during the hurricane. There are some exceptions to defoliation being a strategy for survival; southern magnolia, American holly, and sabal palm are all excellent survivors but they only lost 43%, 34%, and 27% of their leaves, respectively. Native and Exotic Species In the coastal plain area, exotic tree species made up 8% of the trees in the urban forest. The major exotic species were crapemyrtle, Chinese tallow (Sapium sebiferum) (a prohibited invasive species), camphor tree (Cinnamomum camphora) (an invasive species), Bradford pear (Pyrus calleryana), and palms such as pindo palm (Butia capitata) and Washington fan palm (Washingtonia robusta). As a group, native trees survived the same as exotic trees (73% versus 77%, not sig- nificantly different [NS] and lost the same amount of branches (20% versus 15%, NS) and leaves (58% versus 60%, NS). In contrast, after Hurricane Andrew struck south Florida, native trees survived winds better than nonnative trees (Duryea et al. 1996). Other studies have shown trends toward increased wind damage of exotic species in rural plan- tation forests (King 1945; Everham and Brokaw 1996). Wood Characteristics In a comparison of wood density and wind resistance, we found that tree species with higher wood density (g/cm3) had greater survival in Hurricane Ivan (Table 2). When trees with 50% or greater branch damage were also included as nonsur- vivors and a new percent survival was calculated, this recal- culated survival was also greater with higher wood density. Differences in branch loss for various wood densities were not as apparent (Table 2). In a study with 24 tropical species in Puerto Rico, Francis (2000) also found that trees were less likely to fail by uprooting or breakage if they had higher wood density. In addition, similar to our study in which live oak with a wood density of 0.8 has a high survival rate, Hook Table 2. Survival and branch loss for tree species in Hurricane Ivan for different wood density classesz Wood density (g/cm3 ) 0.8 to 0.89 0.6 to 0.69 0.5 to 0.59 0.4 to 0.49 Sample size (number of trees) 240 197 703 607 Survival (%) 94 a 78 b 73 bc 69 c Survival (%) (recalculated by subtracting trees with 50% branch loss) 87 a 73 b 66 c 62 c zNumbers in columns with different letters are significantly different at P 0.05. No tree species occurred in the 0.7 to 0.79 class. ©2007 International Society of Arboriculture . Branch loss (%) 20 ab 18 b 23 a 19 b
March 2007
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