Arboriculture & Urban Forestry 33(2): March 2007 105 with the main trunk present throughout the life of the tree (giving rise to cone-shaped crowns with a central trunk). Decurrent trees have lateral branches, which grow as rapidly as the central trunk; they have no dominant main leader (Har- ris et al. 2004). Excurrent and decurrent trees had equal sur- vival and branch loss in Hurricane Charley. When survival was recalculated (subtracting the trees that had 50% or greater branch loss), decurrent trees survived slightly better than excurrent trees (52% versus 45%, P 0.06). In Hurri- cane Jeanne, decurrent trees survived better (87% versus 56%, P < 0.0001) and lost fewer branches (21% versus 42%, P < 0.0001) than excurrent trees. Typical excurrent trees are Norfolk Island pine, baldcypress, melaleuca, Australian pine, and the pines. Decurrent trees include black olive, citrus, sea grape, strangler fig, Florida scrub hickory, and the oaks. Some authors have noted that crown shape may influence how trees respond to wind (Curtis 1943; Skatter and Kucera 2000; Niklas 2002). In a study of cherry trees, Niklas and Spatz (2000) stated that their results showed that stem taper and canopy size and shape appear to influence stress levels to trees from wind. Rooting Space Giving trees enough rooting space is important for both tree health and anchorage. In Hurricane Georges, we measured rooting space and compared it with survival and branch loss. Trees with the most rooting space (>7 m2) had the lowest branch loss and the greatest recalculated survival (Table 3). When discussing soil conditions, Everham and Brokaw (1996) summarize many papers on soil and wind by saying that soil conditions that restrict root development and anchor- age lead to more tree and forest damage in wind. Pruning After Hurricane Andrew, we reported that gumbo limbo, live oak, and black olive had better survival if they had previously been correctly pruned (Duryea et al. 1996). We reanalyzed these data using the dicot tree species that had a sample greater than 20 trees (including a total of 371 trees)—black olive, gumbo limbo, bottlebrush (Callistemon viminalis), royal Poinciana, live oak, West Indian mahogany, and white Table 3. Survival and branch loss in Hurricane Georges for trees growing in different amounts of rooting spacez Rooting space (m2 ) Sample size for survival (recalculated survival and branch loss) 0 to 3.5 270 (224) 3.6 to 7 133 (99) >7 98 (70) Survival (%) 90 a 90 a 98 a Recalculated survival (%) 64 a 73 a 91 b . Branch loss (%) 36 a 29 a 20 b zNumbers in columns with different letters are significantly different at P 0.05). cedar. Survival for pruned trees was 73% compared with 47% for unpruned trees (P 0.0001). The Survey Arborists, urban foresters, and scientists confirmed many of our results about wind resistance but also provided some new information about some species not so frequently seen and measured in the urban forest. Consistent with our results, queen palm was ranked by the experts as the palm with the lowest wind resistance (Table 4). Royal palm and coconut palm were intermediate, again consistent with our results. Sabal palm was ranked high, which is consistent with our results from the tropical and northern areas of Florida (Dur- yea et al. 1996; Duryea 1997; Duryea et al. 2007). Some of the species with little information from our studies that were ranked high by the experts include pond apple (Annona gla- bra), cocoplum (Chrysobalanus icaco), and lignum vitae (Guaiacum sanctum). Species with little research information that were ranked with low wind resistance include weeping banyan, jacaranda (Jacaranda mimosifolia), and golden trum- pet (Tabebuia chrysotricha). Species ranked with high wind resistance in agreement with our results were crapemyrtle (Lagerstroemia indica), dahoon holly (Ilex cassine), southern magnolia (Magnolia grandiflora), sand live oak, live oak, and both species of cypress (Taxodium distichum and T. ascen- dens). One perplexing species is West Indian mahogany, which fared reasonably well in Georges and Andrew (Table 1); however, the survey respondents ranked it with medium to low wind resistance. In agreement with our results but in contrast to the survey results, in another study of 24 species experiencing Hurricane Georges, West Indian mahogany had the best survival and the least branch loss (Francis 2000). IMPLICATIONS FOR ARBORICULTURE AND URBAN FORESTRY Taking the results from our studies and incorporating the survey results and the scientific literature, we have developed lists of relative wind resistance for tropical and subtropical tree species (Figure 4). These lists should be used with caution with the knowledge that no species and no tree is completely windproof and with the consideration of local soil conditions, tree age, structure and health, and other urban forest conditions. In their thorough review of forest damage from wind, Everham and Brokaw (1996) concluded that species differences do exist and can be explained by differences in wood density, canopy archi- tecture, rooting patterns, susceptibility to diseases, and bole shape. Yet these differences, they say, can also be masked by varied soil conditions, exposure, wind intensity, and cultural practices. Some of the other practices and conditions of the urban forest and their associated recommendations are: • One of the most important findings and therefore rec- ommendations from this study are the rooting space re- sults. It is clear that the more rooting space that a tree ©2007 International Society of Arboriculture
March 2007
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