64 Ellison: Quantified Tree Risk Assessment in Management of Amenity Trees Using the proposed system, the assessment of the same tree by different inspectors not trained to a unifying standard has produced variable results, comparison of which indicates that a common standard of training in tree inspection is required. Matheny and Clark (1994) asserted that “training of personnel in field assessment is absolutely essential” and “perhaps the most important aspect of training is to develop consistent evaluation procedures, among individuals and over time.” Experience of the proposed system reinforces the view that training of personnel involved in inspection and assessment of trees is in need of standardization. Also lacking standardization is the vocabulary used by tree inspectors. In the United Kingdom, the terms stem, trunk, bole and butt are all used to describe the same part of the tree; this example is by no means isolated. Inconsistent use of terminology can lead to misinterpretation of data by subsequent inspectors, prop- erty owners and managers, and by contractors employed to implement remedial measures. CONCLUSIONS Tree safety management should not seek to minimize the risk of harm resulting from tree failure but should balance the benefits of risk reduction with the associated costs in terms of both lost tree value and financial expenditure. By allocating quantifiable values to the probability of failure and impact potential of trees, and to targets on which trees might fail, the arborist can, with training, assess tree failure hazards with sufficient accuracy that property owners and managers are able to consider the risk of significant harm from tree failure against a level of reason- able or acceptable risk. Using the proposed system, it is possible, not only to identify unacceptable risks, but also to identify the elements of the risk, which, when adjusted, will effectively reduce the overall risk of harm in the most cost efficient or appropriate manner. The proposed system not only significantly reduces the influence of assessor subjectivity on the outcome of the risk assessment, but it also applies structure to the assessment procedure, requiring detailed assessment of the tree only where there is a significant likelihood of unacceptable risk. By first evaluating and mapping both the general nature of the tree population within an administrative area and the range of targets upon which they could fail, the manager of a large tree population can identify the interface between trees and targets, thus enabling prioritization of risk assessments. A post-mature tree population adjacent to a busy urban thoroughfare might require biannual assess- ment, whereas the same tree population in a remote wilderness might never be assessed in detail. Between these extremes is a range of inspection frequency, which should be applied as appropriate to the situation. Use of the system without training leads to misapplica- tion of the data. To ensure, insofar as practicable, that value of the system is maintained through consistent application, the author intends to provide training and ongoing develop- ment through a licensing program in the United Kingdom and elsewhere. LITERATURE CITED Henderson, M. 1987. Living with Risk. The British Medical Association Guide. John Wiley and Sons, Chichester, UK. Health and Safety at Work Act. 1974. HMSO, London, UK. Health and Safety Executive. 1995. Generic Terms and Concepts in the Assessment and Regulation of Industrial Risks. Discussion Document. HSE Books, Sudbury, Suffolk, UK. 43 pp. ———. 1996. Use of Risk Assessment Within Government Departments. Report prepared by the Interdepartmental Liaison Group on Risk Assessment. HSE Books, Sudbury, Suffolk, UK. 48 pp. Helliwell, D.R. 1990. Acceptable level of risk associated with trees. Arboric. J. 14(2):159–162. ———. 1991. Letters to the editor, Arboric. J. 15(2):179. Highway Code. 1989. Revised edition. Fifth impression. HMSO, London, UK. Kirby, K.J., and C.M. Drake. 1993. Dead Wood Matters: The Ecology and Conservation of Saproxylic Invertebrates in Britain. Proceedings of a British Ecological Society Meeting held at Dunham Massey Park on 24 April 1992. English Nature, Peterborough, UK. 105 pp. Lonsdale, D. 1999. Principles of Tree Hazard Assessment. HMSO, London, UK. 388 pp. Mattheck, C., and H. Breloer. 1994. The Body Language of Trees. HMSO, London, UK. 241 pp. Matheny, N.P., and J.R. Clark. 1994. A Photographic Guide to the Evaluation of Hazard Trees in Urban Areas (2nd ed.). International Society of Arboriculture, Champaign, IL. 85 pp. Paine, L.A. 1971. Accident Hazard Evaluation and Control Decisions on Forested Recreation Sites. USDA Forest Service Research Paper PSW 68. Pacific Southwest Forest and Range Experiment Station, Berkeley, CA. 10 pp. Transport Statistics Great Britain. 1997. HMSO, London, UK. Tritton, L.M., and J.W. Hornbeck. 1982. Biomass Equations for Major Tree Species. USDA Forest Service General Technical Report NE69. Northeastern Forest Experiment Station, Broomall, PA. 46 pp. ©2005 International Society of Arboriculture
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