Arboriculture & Urban Forestry 45(1): January 2019 telling signs of decay in a tree. Jim and Zhang (2013) performed basic visual assessments on 352 heritage trees in Hong Kong—based on characteristics such as tree habits, defects, and disorders—and they were able to identify which species were less likely to fall and which species posed the greatest hazards to the public. More broadly, Hickman et al. (1995) con- ducted visual tree assessments for a range of external defects on 695 oak (Quercus spp.) trees in 1987. Upon returning to the site in 1994, the researchers found that the defects noted in the prior evaluation, particularly tree lean, trunk condition, and decline symptoms, could be used to accurately predict the trees that remained standing 83% of the time and the trees that failed 78% of the time (Hickman et al. 1995). And while some see the often subjective nature of visual assessments as a potential limitation, others have argued that visual assessment can be effective when the user has an understanding of the factors that can cause a tree to fail (Gruber 2008). To date, only a few studies have tested for variation among assessors and assessment approaches (Hickman et al. 1995; Rooney et al. 2005; Koeser et al. 2015; Koeser et al. 2017; Koeser and Smiley 2017). Rooney et al. (2005) demonstrated that even drive-by visual assessments can be effective in detecting severe defects and decline—especially in less treed, urban areas. Koeser et al. (2017) found that the likelihood of failure rat- ings derived from visual assessments could be less variable than assessments informed by advanced assessment technologies, like resistance microdrill- ing and sonic tomography. Koeser and Smiley (2017) found that arborists with industry qualifications and certifications tended to have lower risk ratings and prescribe less severe mitigation measures than their non-credentialed peers. Researchers have criticized the validity of basic visual assessments when attempting to predict tree fail- ure in the extreme winds associated with hurricanes and strong storms (Gruber 2008), or even in the absence of such events (Stewart et al. 2013). In response to the former criticism, Fink (2009) explained that there are no assessment methods that can predict the storm threshold of a sound tree (i.e., one does not know how strong is strong enough in extreme conditions). How- ever, basic visual assessments have been accepted as being an efficient and dependable means of identify- ing compromised trees, as compared to other trees (Kennard et al. 1996; Pokorny 2003; Rooney et al. 31 2005; Fink 2009; Dunster et al. 2013). As noted ear- lier, recent findings from Hurricane Matthew offer some insight into the validity of pre-storm risk assess- ment (Koeser and Smiley, unpublished data). TREE RISK ASSESSMENT METHODS IN THE URBAN FOREST Over the years, several different risk assessment meth- ods have been used by arborists and urban foresters. In addition to changes in systems over time, different regions of the world often adopt different risk assess- ment methods. Examples of the more commonly used methods are: ISA Tree Hazard Evaluation (Matheny and Clark 1994), Visual Tree Assessment (VTA) (Mattheck and Breloer 1994), United States Depart- ment of Agriculture Forest Services Community Tree Risk Evaluation Method (Pokorny 2003), Quantified Tree Risk Assessment (QTRA) (Ellison 2005a), and ISA BMP Method (Smiley et al. 2011). While the above is not a comprehensive list of risk assessment methods, the summary represents some of the more commonly used approaches for assessing risk (Koeser et al. 2016). In 1991, Matheny and Clark released A Photo- graphic Guide to the Evaluation of Hazard Trees in Urban Areas. Later revised in 1994, this publication outlined a systematic method for tree risk assessment and is often cited as the first comprehensive guide for tree risk assessment (Kane et al. 2001; Hayes 2002; Pokorny 2003; Ellison 2005b; Norris 2007; van Was- senaer and Richardson 2009). Matheny and Clark’s system focused on three key components to deter- mine the hazard level of a tree: failure potential, the size of the part likely to fail, and the target rating (Ellison 2005a). Each component was given a numer- ical rating from one to four. The sum of these three served as an overall hazard rating intended to help urban forest managers prioritize their management efforts. Around this time, the VTA system was devel- oped to evaluate tree structure in the presence of stressors (Mattheck and Breloer 1994). Users of the VTA system first look for noticeable defects while examining the overall vitality of the tree. After this initial survey, a more thorough examination of the defects is conducted. Defects are measured and ana- lyzed to assess the general strength of the tree. Later on in 2003, the United States Department of Agricul- ture Forest Service published a risk management guide featuring the Community Tree Risk Evaluation ©2019 International Society of Arboriculture
January 2019
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