Arboriculture & Urban Forestry 46(6): November 2020 409 Table 4. Brief descriptions of the 16 tree risk assessment methods analysed (full explanations are available by request from the corresponding author). Name of method (Reference) (Country) (Coding) Bartlett Tree Experts (Smiley et al. 2002) (USA) (Bartlett) Brief description and derivation of method Bartlett was a 2-assessment–category summation model (“Failure Potential/Defect Severity” and “Con- sequence of Failure”) using ordinal ranked values. The assessment criteria varied in range and number of values. The risk rating scores ranged from 2 to 15. Reasonably strong descriptors for each criterion were provided that relied on the assessor being suitably qualified and experienced. The final risk rating score was quantified with scores being aligned to 4 categories: < 7 = Low Risk, 7 to 9 = Moderate Risk, 10 to 12 = High Risk, and 13 to 15 = Critical Risk. Bartlett withdrew the method after Tree Risk Assessment Qualification (TRAQ) was endorsed by the International Society of Arboriculture (ISA). Colorado Tree Coalition (Colorado Tree Coalition 2004) (USA) (CTC) A qualitative method from the USA with 2 tiers of assessment and several base criteria that can trigger a second tier inspection. For this research, only the initial risk assessment was analysed. Three fields, each with different ranges (“Tree Species,” “Potential Target,” and “Defects Present”), were multiplied to create a risk score. The output scores ranged from 1 to 60. A unique element was the “Tree Species Index” category, where species were allocated to one of 5 groups ranging from Very Low (1) to Very High (5) Hazard. The authors suggested such groupings be developed from local consensus. The total rating (output score) was grouped into 3 predefined ratings: Low (1 to 14), Medium (15 to 35), and High (36 to 60). Hume City Council (Hume City Council, nd) (Australia) (HCC) A version of the Matheny and Clark (1994) method, highly modified by a Melbourne (Australia) local government authority and 2 consulting arborists. It was a complex model with 5 assessment criteria and a final risk score created by a combination of addition and multiplication. The assessment criteria were “Failure Potential,” “Failure Size,” “Target Usage,” “Target Value,” and “Damage Probability.” The first 4 were evenly scored (5 values from 0.5 to 4), whilst the “Damage Probability” criterion was scored in 5 values from 0.2 to 1. Risk output values ranged from 3 to 64. No definitions or quantifica- tion of the output score values were provided. Matheny and Clark (Matheny and Clark 1994) (USA) (MandC) This 3-category summation model is one of the older published urban tree risk methods. “Failure Potential,” “Size of Part,” and “Target Rating” were each scored from 1 to 4 and summed to derive a “Hazard Rating.” The method was superseded by the TRAQ training program endorsed by the ISA (Matheny and Clark 2007, 2009; Dunster et al. 2017). However, it is probably the most widely used method in Australasia and in North America by those not affiliated with the ISA who have not undertaken the fee-for-service TRAQ program. The output scores ranged from 3 to 12. No definition or quantification of the output score values was provided. Kenyon 1993 (Kenyon 1993) (Australia) (Kenyon) Private 1 (no reference) (Australia) (Private 1) An early quantitative method that attempted to quantify risk rather than the more common approach of ordinal ranked values: at the time a novel tree risk assessment. It multiplied 4 fields, the “Probability of Failure,” “Target Value” (in dollars), “Target Risk Time,” and a “Damage Factor” to give a dollar value. Two of the fields with predefined ranges and outputs were expressed financially ranging from $0 to more than AUD $4.5 million. No definition or quantification of the output score values was provided. A qualitative method developed by an arboricultural business in Melbourne, Australia. The method was provided confidentially and used 3 assessment criteria. The final risk score was created by (Likelihood of Failure × Likelihood of Impact)/2 × Consequences. The assessment criteria were discontinuous ordinal ranked numbers, and the Likelihood of Failure criterion had a different scale (1, 2, 4, 6, 8, and 10) to the others (1, 4, 6, 8, and 10). The final risk output values ranged from 1 to 500. The levels of risk were defined: 1 to 125 points = Very Low Risk, 125 to 250 points = Low Risk, 250 to 375 points = Medium Risk, 375 to 500 points = High Risk. Private 2 (no reference) (Australia) (Private 2) A modification of the Matheny and Clark method. The method was provided confidentially and had 6 assessment fields; each field was summed to create a “Hazard Rating” of 6 to 24. This method’s novelty was the inclusion of 3 categories: “Wind Alignment,” “Defect Height,” and “Remediation.” The “Hazard Rating” scores are quantified: 5 to 10 = Low Risk, 10 to 16 = Moderate Risk, 16 to 20 = High Risk, 20 to 24 = Extreme Risk. (Table 4 continued on next page) ©2020 International Society of Arboriculture
November 2020
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