104 Duryea et al.: Hurricanes and the Urban Forest, II force winds better. A group was defined as five or more trees each growing within 3 m (9.9 ft) of another tree (but not in a row), and we measured this site characteristic for Hurricane Jeanne. Trees growing in groups had 88% survival during Hurricane Jeanne compared with 78% for those growing as individual trees (P 0.0001). In addition, trees growing in groups had less branch loss than individual trees (19% versus 27%, P 0.09). Trees in groups also survived Hurricane Ivan better than individual trees (Duryea et al. 2007). Wood Characteristics Wood density was not related to survival or branch loss for tree species in Hurricanes Jeanne, Charley, Georges, or An- drew. Species with low wood densities (0.31 and 0.42 g/cm3) and high survival rates in these hurricanes are gumbo limbo and baldcypress. Australian pine and melaleuca with low sur- vival in all the hurricanes have high wood densities of 0.83 and 0.65 g/cm3. In Jeanne, for example, melaleuca with its relatively high wood density had the lowest survival (22%) and the highest branch loss (60%) of all the species. This lack of relationship between tree survival and wood density is in contrast to many studies in which species with denser wood are less likely to fail in hurricanes (Putz et al. 1983; Webb 1989; Zimmerman et al. 1994; Francis 2000; Duryea et al. 2007). Francis and Gillespie (1993) spoke about exceptions of these positive effects of wood density by citing examples of trees species (balsa, Ochroma pyramidale and yagrumo, Cecropia schreberiana) that have light weak wood but sur- vived better in Hurricane Hugo compared with pitch apple (Clusia rosea) and Australian pine that have strong wood but experienced trunk snap and major branch breakage. However, King (1986) noted that the flexibility and strength conferred by high wood density give trees the ability to resist winds. Here it is important to point out that different strategies are most likely at work for different species. Everham and Bro- kaw (1996) emphasize that species differences do exist and they are probably explained by difference in canopy archi- tecture, wood density, bole shape, rooting patterns, and sus- ceptibility to disease. Also they point out that these differ- ences may be obscured by difference in exposure, soil, or silvicultural (cultural) treatments. 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 (Reyes et al. 1992; Forest Products Laboratory 1999; Niklas 1999). Tree species with the highest modulus of elasticity (oaks and south Florida slash pine, all above 9,600 MPa) survived Hur- ricane Jeanne better and lost fewer branches than the one species with the lowest values (sand pine with 7,000 MPa) (P 0.05). However, the modulus of elasticity had no rela- tionship to tree species survival or branch loss in the stronger ©2007 International Society of Arboriculture 233 km/h (145 mph) winds of Hurricane Charley; two species with the lowest modulus of elasticity are gumbo limbo and baldcypress, and these species experienced the highest sur- vival and nearly the lowest branch loss. Modulus of rupture is a measure of the bending stress that wood can experience without mechanically failing (Forest Products Laboratory 1999; Niklas 1999). Again, sand pine with the lowest modulus of rupture experienced significantly (P 0.05) lower survival and higher branch loss than the other species (oaks and south Florida slash pine). In Hurri- cane Ivan, survival was also higher for those species with the highest modulus of rupture, and branch loss showed no rela- tionship (Duryea et al. 2007). In Hurricane Charley, there appeared to be no relationship with the lowest modulus of rupture for baldcypress (46,000 kPa) and the highest for live oak (82,000 kPa), whereas baldcypress had the highest sur- vival (95%) and lowest branch loss (18%) compared with live oak’s 78% survival and 43% branch loss. 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). In Hurricane Charley, dense crowns had significantly greater survival than moderate crowns (78% versus 64%; P0.02) but were not different than open crown survival (75%). Branch loss was also greatest for moderate crown density trees (45% versus 36% and 34% for dense and open, P 0.08 and P 0.02), and the recalculated survival (subtract- ing the trees with greater than 50% branch loss) was the poorest for trees with moderate crown densities (37%). In Hurricane Jeanne, again dense-crowned trees survived better (88%) than (79%) moderate, which survived better than open trees (47%, P ranged from 0.05 to 0.0001). Branch loss was the greatest for moderate (43%), then open (33%), and the least with dense-crowned species (19%) (P ranged from 0.05 to 0.0001). Dense-crowned trees include citrus, black olive, laurel oak, sand live oak, and live oak. Moderate density trees included sea grape, Florida scrub hickory, and melaleuca. Open-crowned trees include gumbo limbo, Australian pine, Norfolk Island pine, baldcypress, and the pines. In contrast to our results, Everham and Brokaw (1996) in their review of hurricanes and trees discuss the tendency for dense-crowned trees to be more damaged in hurricanes compared with trees with more open-foliaged crowns. In agreement with this study’s results, in our study of southeastern coastal plain species, dense crowns also survived better but lost more branches (Duryea et al. 2007). The growth form of a tree can be categorized as excurrent or decurrent. Excurrent trees have strong apical dominance
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