Arboriculture & Urban Forestry 46(6): November 2020 Table 3. Examples of terms used in tree risk assessment methods to describe levels of risk. Method Acceptable risk category Matheny and Clark (1994) Threats (Forbes-Laird 2006) Bartlett (Smiley et al. 2002) United States Department of Agriculture Forestry Service (Pokorny et al. 2003) QTRA (Ellison 2005a; 2005b) Colorado Tree Hazard Tree System (Colorado Tree Coalition 2004) None; authors suggest use “Hazard Rating” for ranking only, but provide examples. Qualitative method that uses 7 broad categories from insignificant to extreme. Action is taken above the “slight” rating. Qualitative method that is not specific, but uses terms such as Low, Moderate, High, and Critical Risk. Several methods developed by the USDAFS are qualitative and don’t define acceptable risk levels. Most use a numerical score similar to Matheny and Clark (1994). A quantitative probabilistic method that develops a probability-based output value. Probabilities < 1/10,000 per annum are unacceptable. A 2-tiered qualitative system. Once the initial risk score exceeds 36/60, or certain categories reach a threshold, a second level inspection is triggered which prioritises. Fundamentally output values < 36 are deemed acceptable risk. trees. In inspecting large numbers of trees in public areas, time becomes a significant cost and resource limitation. In Australia, current street-tree inspections are being conducted at a rate of about 400 trees per day (50 trees per hour) with an average inspection time of 72 seconds, including travel time between trees. Longer inspection times increase costs signifi- cantly, and few public authorities have such resources. Frequency of inspections is typically based on the need to identify defects, but the frequency of use by a risk target is probably a better approach. Many Victorian councils manage about 50,000 (range 15,500 to 106,000) street trees (Beer et al. 2001). At an inspection rate of 400 trees per day, an annual inspec- tion would require 125 labour days per annum, with an inspection time of around 1 minute per tree. Several authors discuss the use of zoning districts, generally designated by usage levels, to prioritise areas for inspection (Lonsdale 2000, 2007; Pokorny et al. 2003; Ellison 2005a). It would also be possible to designate zones by tree factors, such as age, species, or size. Ellison (2005a) suggests that in low-use sites, the risk could be so low that inspections would be unneces- sary. Zoning fits well with the variable fatality risk criteria that have higher requirements to provide a safe environment for particular land use types (Department of Planning NSW 1992) and with the “target” rating used in risk assessment methods such as Threats (Forbes-Laird 2006) and QTRA (Ellison 2005b). If zoning is to be used, it may change the weighting on any assessment system that uses target rating. For example, the Matheny and Clark (1994) method allows a target rating of 1 to 4 depending on frequency of use. If zoning was used, it probably should pre-set the target value rating for the assessment or be removed from the assessment model. The purpose of this research was to investigate a range of tree risk assessment methods using sensitivity analyses to determine the underlying model factors and their influence on the output value of risk. It was hypothesised that methods would differ in their treat- ment of likelihood and consequences, their outputs, the availability of scores in their output ranges, and their tendency to rate the level of risk. Sensitivity analyses provide an indication for each method of the relative influence that the input variables exert on the final risk value, which makes users of a method aware of its strengths and weaknesses. The analyses can also assist in the choice of method and in determining which method might be most appropriate to a partic- ular situation. Fourteen of the sixteen methods of tree risk assessment analysed in this research existed before the study commenced, and two that conformed to the Australian Standard on Risk Assessment AS/NZS 4360:2004 were developed as part of the research (Standards Australia 2004a, 2004b). While a number of tree risk assessment methods exist, many are mod- ifications of a few established methods. MATERIALS AND METHODS Twenty-six tree risk methods were gathered from sources such as the literature, methods created for in-house use by local governments, and from various arboricultural consultants in Australia. Sixteen meth- ods were chosen for further analysis and comparison (Table 4) using the selection criteria that each had to ©2020 International Society of Arboriculture 407
November 2020
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