242 Hauer et al.: Assessment of Tree Debris Following Urban Forest Ice Storms restoration funding based on the severity of the storm. The vast majority of the model sites in this study were declared FEMA disaster sites. Debris eligible for FEMA reimbursement is that from the public right-of-way with some private debris collected when brought and dumped on the public right-of-way (FEMA 2007). Models from this study were developed with this in mind. Ice thickness was a significant variable when explaining tree debris volumes following ice storms in this study. This result is not surprising and is consistent with several past research results that documented a positive relationship between tree damage and ice accumulation (Jones 1996; Van Dyke 1999; Hauer et al. 2006; Liu et al. 2008). Lafon (2004) developed a model showing a strong relationship (R2 = 0.83) between the proportion of trees damaged in forests and ice thickness, y = -0.0696 + 0.0154x, where y = Proportion of Trees Damaged and x = Ice Thickness (mm). Most ice storms result in a mean of 1 cm of ice thickness (Changnon 2003). A typical ice storm event thus only coincides with 10% or fewer trees damaged in a forest. In contrast, from this study, approximately half of the trees would be damaged across all study sites based on a mean ice thickness of 36 mm, assuming rural forest structure and urban forest structure were equivalent. Mean debris volumes from this study fell within ranges re- ported in urban and rural forests following ice storms. Bloniarz et al. (2001) reported that debris estimates per street distance were between 0 and 301 m3 very high tree density (243 to 322 trees per km) for an average ice storm event. These estimates range from 0 to 552 m3 severe (≥75%) canopy loss. A mean of 180 m3 /km of debris oc- curred during storms at the study sites and were within this range. Ninety percent of reported debris volumes from locations in this study fell within the ranges reported by Bloniarz et al. (2001). Within forest stands, debris volumes of 5.1, 19.4, 33.6, and /ha have been reported (Bruederle and Stearns 1985; Rebertus et al. 1997; Hopper et al. 2001; Ryall and Smith 2005). Normalizing these ice storm events by debris volume, land area, and ice thickness, debris amounts were 2.04, 1.52, 3.57, and 22.3 m3 134 m3 /ha/cm, respectively. The 134 m3 /ha debris estimate is nearly a magnitude different, and when excluded, the remaining three reports averaged 2.38 m3 study sites had a mean debris volume of 11.72 m3 m3 /ha/cm. Current /ha or 3.26 /ha/cm and are reasonable with reports from forest stands. Wind alone was not a significant variable in estimating de- bris from this study. This is contrary to other studies that sug- gest wind direction, exposure, and increased drag exacerbates tree damage (Harshberger 1904; Semonin 1978; Bruederle and Stearns 1985; Millward and Kraft 2004; Greene et al. 2007; Houston and Changnon 2007). There may be several reasons for this result. First, wind and tree response is dynamic and not nec- essarily a simple relationship between wind speed and damage (James et al. 2006). Second, wind speed data only reflected the time period during the ice storm. Average wind speeds during ice storms are often reported as calm to moderate with only 4% of wind speeds greater than 32 km/h (Houston and Changnon 2007). Third, the maximum wind speeds at the 40 study locations averaged 29 km/h, which is consistent with moderate breezes on the Beaufort scale (Cullen 2002). Luley et al. (2002a) found few significant branch failures with wind gusts below 64.4 km/h. They expect more frequent branch failures as wind gusts exceed 80.5 km/h during the leafy period. Little tree damage below 50 km/hr occurred during Hurricane Hugo (Francis and Gillespie ©2011 International Society of Arboriculture /km for streets with no (0 trees per km) to /km for no to 1993). Only two locations (both 64 km/h) in the current study approached the lower threshold for wind damage during the leafy period from these examples. Further, the deciduous trees were during their leafless period during the icing events. Luley et al. (2002a) found no consistent relationship between wind and tree damage in the leafless period and stated ice, sleet, or snow loads are probably more important with branch failures in the leafless period than wind alone. Finally, Kane (2008) reported the main cause of tree failure is extreme wind speed >30 m/s (108 km/h), which exceeded the wind speeds at the authors’ study locations. Even though this study did not find significant relationships be- tween tree debris and wind during ice storms, wind plays a role in ice accumulation on surfaces. Rogers’ (1922) early observations found greater accumulation of ice on the windward side of forests in a study site. More recently, a significant two times more macro- litter from an ice storm was recorded on the windward side 23.9 m3 /ha than the leeward side 11.0 m3 /ha of a forest (Bruederle and Stearns 1985; De Steven et al. 1991). As wind speed increases ice accumulation increases (Yip 1995; Jones 1996; Jones 1998; La- fon et al. 1999; Greene et al. 2007; Houston and Changnon 2007). Thus, wind may play a role in enhancing damage with severe wind speeds; however, its role with increased ice accumulation is pos- sibly a better explanation for damage. An explanation for the neg- ative interaction of ice thickness and wind speed from this study was not discerned. The effect of wind gusts and wind duration after the ice storm ended could represent forces resulting in tree damage and debris, but such data were not collected in this study. Canopy cover is commonly used to describe the urban land area covered by tree and shrub canopies (Walton et al. 2008). Its relationship has been explored for ecological perspectives, demo- graphics, urban forest structure, geographic features, and property values (Zipperer et al. 1997; Heynen and Lindsey 2003; Sander et al. 2010). Bloniarz et al. (2001) used canopy loss as a means of es- timating debris from an ice storm. Escobedo et al. (2009) reported canopy cover was related to debris volumes from hurricane winds. The relationship was complex and no significant interactions be- tween wind speed and canopy cover, or among canopy cover, wind speed, and developed urban cover, were found. No relation- ship between percentage canopy cover and debris was found in this study, contrary to the hypothesized positive relationship. Nor were any interactions between canopy cover and other study vari- ables found. One explanation was that more than 70% of study sites had less than 20% canopy cover and possibly a uniform tree structure masked a putative relationship between canopy cover and tree debris that was primarily from the public right-of-way. The study authors’ initial approach to use community sup- plied estimates of canopy cover produced poor results with only 15% providing data of low quality, so the 2001 NLCD canopy cover was used to estimate community canopy cover. These NLCD values had a tendency to underestimate actual canopy coverage, which may have influenced the results (Walton 2008; Walton et al. 2008). Estimating canopy cover from other means should be done, but was beyond the scope of this study. Some limitations of the models presented in this paper ex- ist. First, incorporating an ice storm susceptibility index of tree species into the study was attempted (Hauer et al. 2006). The index ranks the potential susceptibility of the urban forest to an ice storm based on the existing tree structure. Communities were unable to provide reliable estimates of their street tree or community-wide urban forest structure. More than 60% did pro-
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