Arboriculture & Urban Forestry 46(6): November 2020 governments insist that members have a formalised risk management system (Hewett et al. 2003; State of Victoria 2017; VAGO 2018). Local government to a degree transfers its responsibility for risk by purchasing insurance. The insuring bodies require a risk manage- ment plan to minimise their exposure and therefore transfer some of the expense back to local govern- ment. It is assumed that if tree risk assessments are to be undertaken, then the context of the possible inter- action of the tree, people, and property has been set, and only the level of risk an organisation is willing to accept needs to be defined. This means that risk assessment is done on a specified tree with potential elements of risk identified. A ranking system based on an accepted risk level will allow for variation of risk acceptability (Koeser et al. 2016). Some widely acknowledged tree biology underpins tree risk assessment and the identification of hazards (Table 1). While the risks to property and persons posed by urban trees are small (Helliwell 1990; Lonsdale 1999, 2007; Ball and Watt 2013; Hartley and Chalk 2019), the issues and liability arising from foresee- ability and the resultant duty of care are significant. The risks associated with trees are so low that an exaggerated perception of these risks is likely to be the greater concern (Slovic 2000), but the insurers of local governments in Australia dedicate significant resources to the liability mitigation aspects of public tree management (Hewett et al. 2003; VAGO 2018). A good tree risk assessment methodology should be complete, robust, valid, repeatable, available, usable, 403 and credible (Lowrance 1976). Any person using a risk assessment process should understand the limitations of the decision theory underpinning the particular meth- odology, industry heuristics, and their own cognitive processes and biases. Since risk = likelihood × conse- quences (Standards Australia 2004a), risk assessment must be undertaken in a climate of uncertainty, as both components are estimates and predictive. An assessor is in the business of uncertainty, and tree risk assessment systems utilise professional experience and judgement (Coder 1996, 2000). Risk assessment is a logical-step–driven decision-making process (Figure 1) that assists with processing data into intel- ligence that provides the information required to achieve competent decisions (Haimes 1998; Tomao et al. 2015). A primary rationale in using a method is to improve the validity and repeatability of assess- ments by reducing the potential for errors and biases by using defined parameters, terms, and variables. In general risk assessment, once the consequence of a risk has been established, credibility will be affected by the technical community’s best estimate of magnitude, how good the technical community believes the estimate to be, and how far the technical community’s judgement can be trusted (Fischhoff 1994). In dealing with tree risk assessment, the tech- nical community is made up of arborists who have the level of professional competence to undertake risk assessments. The establishment of credibility becomes the responsibility of the arboriculture pro- fession. While tree risk assessment potentially aligns Table 1. Elements of tree biology considered in tree risk assessment. Aspect of tree biology Every tree will eventually fail. Trees are biological entities. Trees compartmentalise. Most trees have some defects. Trees are slow to react. Trees are regenerating systems. Trees are shedding/biological organisms. Healthy trees can drop limbs or break. Many defects are not visible. Limited knowledge of tree biology and physiology may be a limitation to risk assessment. Considerations in risk assessment Unless the tree is removed first. They are variable genetically and in responding to their environment. They seal off and grow over damaged tissues. The extent of the defects varies. Tree growth is “adaptive,” therefore “visualisation” within the context of a longer tree time is required to understand tree actions and reactions. Trees do not heal and must develop new material to continue growing. They create litter (leaves, bark, twigs, branches). They may do so naturally or during certain weather conditions. Technology may assist, but judgement is important. Experience and judgement may be important in risk assessment, particularly in predicting failure or development of hazards with a greater degree of certainty. ©2020 International Society of Arboriculture
November 2020
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