Arboriculture & Urban Forestry 38(3): May 2012 smaller municipalities) were felled or required significant prun- ing or removal following the storm (Brian Head, Acting Man- ager Streets and Parks Division, City of St. John’s, pers. comm.). 93 Figure 1. Hurricane Igor storm track, September 19–21, 2010. Source: Environment Canada. The island of Newfoundland is located within the Bo- real Shield Ecozone of Canada (Wiken et al. 1996). The bo- real forest is dominated by coniferous softwood trees, such as black spruce [Picea mariana (Mill.) B.S.P.] and balsam fir [Abies balsamea (L.) Mill.], as well as occasional hardwoods such as white birch (Betula papyrifera Marsh). The greater St. John’s area lies within the provincial Maritime Barrens ecore- gion (Damman 1983). Although described as a “barrens,” the ecoregion has many pockets of forest that are dominated by balsam fir. In the urban region, planted non-natives include sycamore maple (Acer pseudoplatanus L.), Norway maple (Acer platanoides L.), and linden (Tilia americana L.) (Fry et al. 2008) as well as numerous other ornamentals and exotics. Given the rarity of a hurricane in an urban region within a boreal setting, researchers set out to determine whether there were factors that influenced which trees in the greater St. John’s area were damaged by Hurricane Igor. Research in the tropics and sub-tropics on hurricane damage in urban areas has shown that windthrow causes two types of damage: uprooting or stem (i.e., snapping of the trunk). This damage is related to tree spe- cies (Francis 2000; Duryea et al. 2007a; Duryea et al.2007b), tree size, measured by both height and diameter at breast height (Dur- yea et al. 2007a; Duryea et al.2007b), the degree to which tress grew in clusters (Duryea et al. 2007a; Duryea et al.2007b), soil saturation (Fan and Su 2008), and soil type (Francis 2000). The variation in uprooting and stem breakage (collectively referred to as “stem failure”) by species is believed to be related to wood strength (Francis 2000; Duryea et al. 2007a; Duryea et al.2007b), with trees with denser wood less likely to snap or be uprooted, al- though this was not observed in all hurricanes in Florida (Duryea et al. 2007a; Duryea et al.2007b). Direct tests of wood strength have been shown to be correlated with stem failure in Hawaii, U.S. (Asner and Goldstein 1997). Asner and Goldstein (1997) also showed that native trees were more likely to remain stand- ing, but when they fell, were more likely to be uprooted than snapped. On the other hand, non-native trees were more likely to be snapped than uprooted and had a higher percentage of damage overall (Asner and Goldstein 1997). This is similar to the pattern observed in Dade County, Florida, following Hurricane Andrew (Duryea et al. 1996) and for tropical species across Florida fol- lowing Hurricanes Jeanne and Charley (Duryea et al. 2007b). However, Duryea et al. (2007a) did not see a difference in dam- age rates between native and non-native coastal plain tree species across Florida following Hurricanes Erin, Opal, Ivan, Dennis, Charley, Frances, and Jeanne or for tropical tree species follow- ing Hurricane Georges (Duryea et al. 2007b). The variation in the tree properties and damage rates between different hurricanes in urban areas across the same study area observed by Duryea et al. (2007a) and Duryea et al. (2007b) suggests that the properties of the storm itself appear to influence patterns of tree damage. Research on tree properties and hurricane damage in non-ur- ban tropical and sub-tropical forests shows that damage appears to be a factor of species (Gresham et al. 1991; Zimmerman et al. 1994), wood density (Zimmerman et al. 1994), DBH (Lugo et al. 1983; Gresham et al. 1991; Walker 1991; Zimmerman et al. 1994; Kane 2008), and location [e.g., distance from coast, topographic position, stand density (Lugo et al. 1983), and aspect (Lugo et al. 1983; Walker 1991)]. A study of stand-level varia- tion in damage in response to Hurrican Juan (the most recent hurricane to hit Atlantic Canada prior to Igor) showed that stand density (those that had undergone pre-commercial thinning) and average slenderness of trees in the stand were the only significant factors (Cameron 2004). Analyses of temperate and boreal forest response to non-hurricane windstorm events (including thunder- storms and tornados) suggest that damage is explained in part by tree size (Peterson 2007; Valinger and Fridman 2011), stand composition and age (Ulanova 2000), wood strength (Peterson 2007), soil properties (Ulanova 2000), and amount of thinning (Kane 2008). However, as with patterns of hurricane damage, specific damage patterns are more complex (Webb 1989; Boose et al. 1994; Kane 2008). Much of the research on wind damage in boreal forests has focused on patterns at stand and landscape levels (e.g., Boose et al. 1994; Ulanova 2000; Boose et al. 2001; Zeng et al. 2009; Valinger and Fridman 2011) and has made use of simulation models of wind storms (e.g., Achim et al. 2005; Zeng et al. 2009; Zeng et al. 2010). Experimental tree-pulling of boreal species has suggested that DBH and height (Peltola et al. 2000) and soil properties (Elie and Ruel 2005) are important fac- tors in determining susceptibility to blowdown in a wind event. It was hypothesized that for the current urban boreal study area, factors likely to influence tree fall (either uprooting or snap- ping) were species, age, size (height and DBH), slenderness, and location (within dense stands or in more open, wind-exposed ar- eas). Specifically, the study authors predicted that native species would be less likely to be damaged, even though trees in this part of the world have not had the opportunity to evolve with hur- ricane exposure. Most of the native trees are conifers, and hur- ricane research in the southeastern United States and elsewhere has not shown clear trends of higher susceptibility of conifers to hurricane damage, despite their less-dense wood (Gresham et al 1991; Everham and Brokaw 1996; Duryea et al. 2007a). Non- native species in the urban St. John’s area have a tendency to be Sycamore maple, Norway maple, and linden, which are also not native to hurricane-prone areas, and thus are not expected to ©2012 International Society of Arboriculture
May 2012
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