406 Norris and Moore: How Tree Risk Assessment Methods Work Table 2. Examples of tree risk assessment data collection (Hickman et al. 1989; Matheny and Clark 1994; Forbes-Laird 2003; Hewett et al. 2003; Pokorny et al. 2003; Ellison 2005a; van Wassenaer and Richardson 2009; State of Victoria 2017). Category Assessment details Location data Site data Tree data Tree health data Defects Target damage (consequence) Target occupancy Risk rating/ranking/value Treatment/controls/management Residual risk level Typical data collected Date, name of assessor, scope of works, authority to undertake work Identify site and tree within site; could include tree inventory (what species of tree and its location), tree record number, owner Area use type, soil type and issues, site restrictions (utilities, paving, buildings), site history, exposure to weather Identification, history, age, physical data canopy and trunk (height, spread, DBH), physical data root system (root crown exposed/damaged roots) Health, vigour (or vitality), pests, or diseases Defects (decay, cracks, lean, poor architecture, deadwood, mechanical damage), defect symptoms (dieback, included bark, fungal fruiting bodies), site environment defects (low branches, raised path), severity and location of defects Degree of damage possible, injury severity, “value” of target (children) Amount of time the area is occupied and sometimes “value” factors, influence of weather Rating for various likelihood and consequence factors. Methods tend to rate size of part, likelihood of failure, and target ratings, and combine these to form a risk score Options to mitigate risk, including accepting risk, often includes tree work recommendations The level of risk remaining after the assessment’s recommendations have been undertaken (this includes the “take no action” assessment) of public risk, but not in relation to trees, and owners of tree risk should develop defendable approaches to risk in line with national risk standards. The level of acceptable risk depends upon site management objectives and management’s perceptions and expectations of tree performance (Coder 1996). The Australian risk standard AS/NSZ 4360:2004 states that it is the function of management to set the context and evaluation criteria against which risk assessments are to be evaluated. Helliwell (1990) suggested that a 1:10,000 (0.0001) risk per annum would be a suitable figure for trees based on everyday risk fatality rates. Lonsdale (1999) used a United Kingdom Health and Safety Executive report of the Interdepartmental Liaison Group on Risk Assessment (HSE 1996) to suggest that the 1:10,000 was perhaps appropriate, and Ellison (2005a) supported 1:10,000 based on the papers and reports cited by Helliwell (1990) and Lonsdale (1999). These reports explain that whilst 1:10,000 is the point where risk is totally unacceptable, and that an individual risk of death of 1:1 million is a very low level of risk and should be considered as broadly acceptable, the area between is the tolerability region and subject to context and other considerations. ©2020 International Society of Arboriculture The QTRA method definitively sets acceptable/ unacceptable criteria (1:10,000) but tempers this by stating that risk owners might adopt the 1:10,000 limit of acceptable risk or choose to operate to a higher or lower level (Ellison 2005a). Matheny and Clark (1994) state that ratings have a relative meaning, but that the greater the hazard rating, the greater the risk associ- ated with a tree. Many methods use qualitative terms to describe the risk, such as Low, Slight, and Critical. Table 3 summarises a range of terms from different methods. The Australian fatality risk level for trees is less than 1:5 million per annum (Hartley and Chalk 2019). To defend due diligence, an authority needs to inspect trees, but at what frequency and at what standard? Most tree assessment methods suggest a frequency of between 0.5 to 3 years, with 9 of the 16 methods reviewed suggesting annual general inspections (Har- ris 1983; Grey and Deneke 1986; Robbins 1986; Rushforth 1987; Matheny and Clark 1994; Mattheck and Breloer 1994; Albers et al. 1996; Fraedrich 1999; Lonsdale 2000; Dockter 2001; Hayes 2001; Kane et al. 2001; Dunster 2003; Forbes-Laird 2003; Wildlife Tree Committee 2003; Ellison 2005b). Such annual inspections are not a full risk assessment, but may lead to such an assessment for a subset of selected
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