Arboriculture & Urban Forestry 45(1): January 2019 with knowledge gained from biomechanics research often being used to develop guidelines, techniques, and technologies for gauging tree failure potential. Biomechanics researchers have developed or assessed devices and procedures for testing the presence of decay (Rinn et al. 1996; Costello and Quarles 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), designed proto- cols for measuring the strength of different branch attachments or tree leans (Lilly and Sydnor 1995; Kane and Clouston 2008), modeled wind load dynamics (James et al. 2006; James and Kane 2008), and conducted comparative assessments of the mechanical stability of root systems (Smiley 2008; Bartens et al. 2010; Gilman and Masters 2010; Ow et al. 2010; Gilman and Grabosky 2011; Gilman and Wiese 2012; Gilman et al. 2013). Given this level of research activity, several researchers have conducted literature reviews specifically about tree biomechan- ics (Dahle et al. 2017; James et al. 2014). In contrast, the aim of this literature review is to move beyond factors that influence likelihood of failure. While tree-related factors are important, research indicates that the arborist assessing a tree can have more influ- ence over the final risk determination than the actual tree assessed (Norris 2007; Koeser and Smiley 2017). Until recently, little attention has been given to the role that professional and public perception plays in influencing tree risk assessment and management in landscape settings (Norris 2007; Koeser et al. 2015; Klein et al. 2016; Koeser and Smiley 2017). Attempts to independently assess the effectiveness of tree risk assessment methods are also limited (Matheny and Clark 1994; Mattheck and Breloer 1994; Hickman et al. 1995; Pokorny 2003; Matheny and Clark 2009; Smiley et al. 2011; Dunster et al. 2013). Research has shown that there is significant variability in the final determinations made by risk assessors (Ball and Watt 2013a) and by arborists using common tree risk assessment methods (Norris 2007; Stewart et al. 2013; Koeser and Smiley 2017). This indicates that there are inconsistencies in risk assessment that have yet to be addressed through existing best manage- ment practices and training programs. Personal biases likely account for some of the variability seen (Norris 2007), but more research is needed to determine if there are interactions between the method of assess- ment and the risk perceived by those conducting the 27 assessment. This review covers the current state of research and knowledge on tree risk assessment meth- ods and risk perception as it relates to arboriculture and urban forestry. The summarization and synthesis that follows depicts what is currently known, given the limited research at hand, and offers suggestions for future research intended to improve the effectiveness of tree risk assessment methods. LITERATURE REVIEW METHODS Several review strategies were used in combination to identify and assess relevant publications in this lit- erature review. First, the authors reviewed the schol- arly journals Arboricultural Journal, Arboriculture & Urban Forestry (formerly Journal of Arboriculture), and Urban Forestry & Urban Greening in their entirety (from the initial volumes until the drafting of this manuscript in 2017) for articles related to tree risk and the perceived risk associated with trees and greenspaces. A past citation list compiled by Matheny and Clark (2009) was cross-referenced to identify omitted, yet relevant publications. Additionally, researchers performed keyword searches in several electronic databases, including Google Scholar, JSTOR, Web of Science, Science Direct, and the University of Florida George A. Sma- thers Library collections database. The following English language terms were used to conduct the search: urban forest risk assessment; tree risk assess- ment; tree failure; risk perception; perception of trees; perception of natural spaces; and environmental psy- chology. Articles in the search were not limited to any particular time frame. Articles were first assessed by their title, filtering nonrelevant papers. After this ini- tial screening of approximately 1,000 articles, researchers read the abstract of each article, and again eliminated those articles outside the scope of the review, leaving 150 relevant articles. The remaining articles were read and qualitatively analyzed for inclusion in the literature review. RESULTS & DISCUSSION Definitions of Risk, Risk Assessment, and Risk Perception Ball (2007) defined risk as the probability of some specified adverse event occurring within a specified time interval. In their tree risk assessment guidebook, Dunster et al. (2013) defined risk as the likelihood of ©2019 International Society of Arboriculture
January 2019
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