Arboriculture & Urban Forestry 42(4): July 2016 Arboriculture & Urban Forestry 2016. 42(4): 217–226 217 Frequency, Severity, and Detectability of Internal Trunk Decay of Street Tree Quercus spp. in Tampa, Florida, U.S. Andrew K. Koeser, Drew C. McLean, Gitta Hasing, and R. Bruce Allison Abstract. Wood decay is a factor considered in all commonly accepted tree risk assessment methods; however, few studies have attempted to assess its presence in the urban forest or its predictability given visual cues and site factors. A random sampling of trees situated on hurricane evacuation routes was inventoried and assessed for risk in the city of Tampa, Florida, U.S. In addi- tion to a basic visual assessment, a sampling of larger diameter (>30.5 cm) live oak (Quercus virginiana) and all large diameter laurel oak (Quercus laurifolia) trees were tested with a resistance-recording drill to determine the amount of decay present (look- ing specifically at the sound-wood-to-stem-diameter ratio). Overall, 56.9% of the trees tested had some level of decay, though the incidence of decay was higher in laurel oak (67.4%) than in the live oak (29.4%). Additionally, tree species (P < 0.01), diam- eter (P < 0.01), and the presence or absence of visual decay indicators (P = 0.03) were all significant predictors of decay presence. Key Words. Advanced Assessment; Basic Assessment; Decay Incidence; Decay Severity; Florida; Laurel Oak; Live Oak; Quercus lauri- folia; Quercus virginiana; Risk Assessment; Tampa; Visual Risk Assessment. Wood decay is a key defect that factors into the final ratings of most commonly accepted tree assessment methods (Matheny and Clark 1994; Pokorny et al. 2003; Ellison 2005; Johnstone et al. 2010a; Smiley 2011). As wood decays, the strength of the wood is reduced, and the likelihood of failure is increased. This is especially true if the decay is present at the outer edges of the trunk or stem, as the newest layers of wood experience the greatest torsional, compres- sive, and tension stress (Dunster et al. 2013; Ciſtci et al. 2014). Visual assessments of advanced decay can be performed by noting cavities or changes in wood color and texture; however, the incipient stages of decay are not detectable visually or with simple tools (e.g., sounding hammer, increment borer). During these early stages of decay, the wood may appear normal, with little color or textural changes, yet the strength of the wood may still be significantly re- duced (Wilcox 1978). To add to the problem, decay is sometimes completely internalized, with few out- side signs or symptoms present to indicate the true impacts to potential tree risk (Kennard et al. 1996). More precise tools have been developed to help quantify the extent of internal wood decay and help estimate the associated strength loss. In particular, resistance-recording drills and sonic tomography have been tested by numerous researchers (Rinn et al. 1996; Costello et al. 1999; Gilbert and Smiley 2004; Johnstone et al. 2007; Wang and Allison 2008; Johnstone et al. 2010a; Johnstone et al. 2010b; Arciniegas et al. 2014). While shown to be effective, these tools are often beyond the financial means of practitioners and require additional user expertise. Decay-detection devices also require calibration for the tree species of interest to help develop typical species/wood type profiles before results can be interpreted accurately (Mattheck et al. 1997). This process may involve an adjustment of equipment settings, or it may simply be a visual training process to recognize patterns in the output. Even if suffi- ciently equipped and trained, an arborist should consider the value of a given tree to a homeowner or community before recommending a more ©2016 International Society of Arboriculture
July 2016
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