26 Klein et al: Risk Assessment and Risk Perception of Trees Arboriculture & Urban Forestry 2019. 45(1):26–38 Risk Assessment and Risk Perception of Trees: A Review of Literature Relating to Arboriculture and Urban Forestry Ryan W. Klein, Andrew K. Koeser, Richard J. Hauer, Gail Hansen, and Francisco J. Escobedo Abstract. In the presence of a target, tree failures have the potential to damage property, disrupt services, or threaten public safety. Worldwide, several qualitative methods have been developed to provide a systematic approach for tree risk assessment and management. The consistency and accuracy of these methods, the values placed on the tree in question and its potential targets, and the risk perceptions and levels of accep- tance of the evaluator and tree owner all influence how risk is managed. This review explores the concept of risk, examines and contrasts the most commonly referenced tree risk assessment methods, and summarizes research on public perceptions of trees and the risk of trees and greenspaces in built environments. The review identifies general summarized themes and gaps in the available literature to guide future research. Key Words. Decision Making; Hazard Tree; Mitigation; Public Safety; Qualitative and Quantitative Risk Assessment Methods; Risk; Tree Risk Assessment; Tree Risk Perception; Urban Forest. Trees provide a wide variety of benefits, as well as potential risks (Miller et al. 2015). While a significant body of research exists for the former consideration (Dwyer et al. 1992; Clark et al. 1997; Lohr et al. 2004; McPherson et al. 2005; Tyrväinen et al. 2005; Nowak and Dwyer 2007; Roy et al. 2012), less is known about the costs and risks posed by trees (Hauer et al. 2015; Vogt et al. 2015). What is now known is that it is not uncommon for a city to face litigation for tree-related injuries or property damage. In a survey of urban forestry programs in the United States, Koeser et al. (2016) found that 52% of responding communities acknowledged that past claims had been filed against them. This litigation appears to be a sig- nificant motivator for risk management. Communi- ties involved in a past tree-related claim were twice as likely to have active risk assessment and manage- ment programs (Koeser et al. 2016). The history of tree risk assessment as documented in the literature is relatively recent. Most sources cite the publication by Wagener (1963) for recreational ©2019 International Society of Arboriculture sites in California, U.S. as being the first to explore the idea of trees being hazards to both people and property (Kane et al. 2001; Pokorny 2003; Norris 2007). Paine (1971) provided further guidance to assess hazardous trees in recreational areas (Pokorny 2003; Ellison 2005a; Norris 2007). Hazard tree assessment guides remained focused on recreation areas during the 1970s and for much of the 1980s (Johnson and James 1978; Johnson 1981; Mills and Russell 1981). While the terms “hazard” and “haz- ardous tree” were common vernacular during that period, the term “tree risk assessment” is used more commonly today (Pokorny 2003). Interest surrounding tree risk has continued to grow in recent years. Several international research summits have focused on tree risk assessment as a whole, as well as the costs associated with not main- taining trees, and the biomechanics of trees as these relate to tree failure potential (Koeser 2009; NTSG 2011; Dahle et al. 2014; Koeser et al. 2016). The lat- ter area of research has arguably been the most active,
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