84 Duryea et al.: Hurricanes and the Urban Forest, I pears to be predictable on a large scale (for example, the windward versus leeward sides of a mountain) but inconsis- tent at smaller topographic scales (Everham and Brokaw 1996). When trying to explain differences in damage, many stud- ies have found greater damage with increasing hurricane in- tensity (Tanner et al. 1991). Increased rainfall associated with a hurricane and saturated soils results in more tree mortality and especially the uprooting of trees (Cremer et al. 1982; Peterson and Pickett 1991; Tanner et al. 1991). Several stud- ies have found a negative correlation between wood density and mortality during hurricanes (Putz et al. 1983; Webb 1989; Zimmerman et al. 1994). The size, shape, and openness of the crown may also be related to the extent of damage (Curtis 1943). Foster (1988) found that full crowns were more susceptible to wind compared with trees with a more tapered shape. Dense-crowned trees have been found to be more easily damaged compared with open-foliage crowns (Curtis 1943; Everham and Brokaw 1996). This corresponds to our Hurricane Andrew results in which pruned trees (with more open and well-distributed crowns) were more wind- resistant than unpruned trees (Duryea et al. 1996). On a stand level, Everham and Brokaw (1996) report that thinned forests (especially recently thinned) are more vulnerable to high winds because of increased wind flow in the canopy and the absence of neighboring trees. Barry et al. (1993) summarize wind resistance as being dependent on tree species and then on five factors: wood strength, crown shape and size, extent and depth of the root system, soil moisture conditions, and shape of the stem. Studies of urban tree response to hurricanes are not as common yet have yielded interesting and useful results. After Hurricane Hugo struck Puerto Rico in 1989, Francis and Gillespie (1993) found that trees were more damaged with increasing wind gust speed and increasing diameter. Palms were significantly more resistant to wind than broad-leaved trees. After Hurricane Georges in Puerto Rico, Francis (2000) found a correlation between tree size and defoliation and crown loss. He also found differences in extent and type of damage for 24 species in the University of Puerto Rico Bo- tanical Garden. Trees with denser wood and greater branch flexibility were less likely to snap or uproot (Francis 2000). A windstorm that struck Guangzhou City in subtropical south China in 1995 with winds up to 80 km/h (50 mph) damaged over 1% of the tree canopy (Jim and Liu 1997). Tree species differed in their susceptibility to the storm damage; vulner- able species tended to have lower wood density and wide crowns with dense foliage. Shallow root systems also predis- posed trees to uprooting in the winds (Jim and Liu 1997). Hurricane Andrew with its 265 km/h (165 mph) winds took 38% of the urban trees in the Miami–Dade County area. In addition to species differences in wind resistance, native trees survived better than exotics (Duryea et al. 1996). Tree prun- ©2007 International Society of Arboriculture ing, an important cultural practice, improved wind resistance of three common species (live oak [Quercus virginiana], black olive [Bucida buceras] and gumbo limbo [Bursera si- marouba]). Observations from Hurricanes Camille (1969), Frederick (1979), and Hugo (1989) have resulted in the de- velopment of species lists based on flood tolerance, breakage, uprooting, salt, and deterioration by insects and disease (Tou- liatos and Roth 1971; Swain 1979; Barry et al. 1993). The objectives of this study were to analyze the effects of hurricane force winds on tree species growing in urban areas in the southeastern United States coastal plain: (1) to deter- mine if there was species-specific damage over the varied wind speeds, and (2) to determine if damage was related to tree attributes (such as size, leaf loss, wood density, and crown shape) and site characteristics (trees growing in groups). METHODS Urban Tree Damage Measurements Urban tree damage was measured after (within 3 to 6 days) each hurricane that struck the Florida panhandle (Erin, Opal, Ivan, and Dennis) (Figure 1). We also report the hurricane response of coastal plain species such as live oak and sabal palm (Sabal palmetto) that occur throughout Florida and were impacted by Hurricanes Andrew, Charley, Frances, and Jeanne. Hurricane Andrew results were collected in a survey of 128 homeowners in Dade County, Florida, who reported the impacts of the hurricane on trees in their yards (Duryea et al. 1996). The methodology for the other eight hurricanes was the same and is as follows. Neighborhoods at the point of landfall of the hurricane were randomly chosen on the strong Figure 1. Urban trees were measured after the eight hur- ricanes striking Florida and one hurricane in Puerto Rico. For each hurricane, the arrow points to the location of landfall. The maximum sustained wind speed (km/h and mph) and year are included.
March 2007
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