Arboriculture & Urban Forestry 39(4): July 2013 Arboriculture & Urban Forestry 2013. 39(4): 165–172 165 Review of QTRA and Risk-based Cost-benefit Assessment of Tree Management Mark G. Stewart, Dealga O’Callaghan, and Mark Hartley Abstract. Quantified Risk Assessment (QRA) has been in wide use in risk management since the 1960s for systems ranging from aviation, nuclear power, and offshore platforms to medical treatment and pharmaceuticals. The Quantified Tree Risk Assess- ment (QTRA) system is examined considering the principles of QRA. A case study of 14 fig trees in Newcastle, Australia, illustrates some limitations of the QTRA process, and extrapolating risks for a single tree to a group of trees. There is a need for any risk man- agement process involving trees, not only to assess the risk, but to weigh the benefits provided by trees by a risk-based cost-benefit analysis. Tree risk assessors should rely on benchmarks to ensure that their assessment is not outside of the realms of reality or scientific rigor. Key Words. Australia; Cost-benefit Analysis; Quantified Tree Risk Assessment; Risk; Risk Management; Trees. An important aspect of tree management is to assess the like- lihood of a tree causing harm (e.g., injury, death, property damage, disruption of activities). In recent years, the poten- tial for trees to cause harm has been highlighted by elevated media coverage of tree caused injuries or fatalities. The real- ity, however, is that the risk of being killed or injured by a tree is extremely low. For example, about three people per year in the United Kingdom are killed by trees in public places, and the fatality risk per tree is 1 in 150 million for all trees in the UK, or 1 in 10 million for trees in or adjacent to areas of high public use (HSE 2007; NTSG 2011). In the United States, there were 407 deaths from wind-related tree failures in the 13 year period 1995–2007 (Schmidlin 2009) or 31 fatalities per year (note that the U.S. population is five times that of the UK). There has been much debate about the risk trees pose to human populations. In the UK, the debate was heightened following a high profile case, Poll v. Bartholomew (2006), which involved a motorcyclist being hurt by a tree that failed. It was never clarified as to whether the tree fell on to Mr. Poll or he came around the corner on his bike and collided with the fallen tree. In any event, he sustained personal injuries. In this case, the court decided that a large private estate did not have an adequate system for the inspection and management of trees, which was an annual drive/ walk by assessment. The court concluded that inspections should be undertaken by qualified and competent inspectors. Speculation that this judgment placed an excessive burden of tree inspections on landowners created an air of anxiety. This resulted in much uncertainty and many trees being unnecessarily felled for fear of litigation (e.g., NTSG 2011). This understandably made decision makers more risk averse than they otherwise might have been. Systems have been developed to try to assess the likelihood of trees failing, and the harm that might be caused in the event of failure. Matheny and Clark (1994) developed a hazard evaluation system that assigns a numerical value between 1 and 4, for (i) fail- ure potential, (ii) size of part, and (iii) target rating, which are then totaled to produce a hazard rating between 3 and 12. Other tree risk assessment systems include: THREATS & THREATS-NR (www. flac.uk.com), The Bartlett Tree Expert Company’s Tree Risk Man- agement 2nd Edition (Smiley et al. 2007), and the ISA’s Best Man- agement Practice for Tree Risk Assessment (Smiley et al. 2011). Ellison (2005) took a probabilistic approach to risk and de- veloped the Quantified Tree Risk Assessment (QTRA) system. This system attempts to provide a quantitative framework for the assessment of the three components of tree risk: (i) target value, (ii) probability of failure, and (iii) impact potential. The system assesses the probabilities of the three components of risk and calculates their product, which allows skilled tree in- spectors to quantify the risk of harm from tree failure, which in turn facilitates the balance between tree safety and tree value. Quantified Risk Assessment (QRA) has been in wide use in risk management since the 1960s. QRA assesses hazard likeli- hood and system vulnerability using sound physical modeling of failure processes and recognition of uncertainty and vari- ability of influencing variables. This is absent from the QTRA system, which seems to rely more on subjective assessments, particularly for probability of tree failure. Moreover, the QTRA system does not consider benefits in the risk assessment com- ponent where costs and benefits are compared, but rather rel- egates this to information contained in the practice notes. In principle, QTRA is a welcome development be- cause a quantified risk-based approach to tree management is based on sound decision-support principles. This paper explains the principles of QRA, and identifies some weak- nesses in the QTRA process that could be rectified by the incorporation of additional QRA principles in any revision of QTRA. The paper will also show how a cost-benefit analysis ©2013 International Society of Arboriculture
July 2013
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