Journal of Arboriculture 31(2): March 2005 63 example, if the highway in example 2 (below) had been accurately surveyed using an electronic traffic counter, and it had been established that the usage was on average seven vehicles per day, it could be stated with confidence that the probability of target occupation was 1/5,000. The risk would then be calculated: impact potential 4 × probability of failure 1 × target value 1 = risk of harm 1/82. ∴ risk of harm × measured target value 1/5,000 = revised risk of harm 1/410,000. Example 2 (Before Remedial Action) The tree is a mature beech (Fagus sylvatica) overhanging a minor road with good visibility. The crown of the tree contains long, unstable, dead branches up to 100 mm (4 in.) diameter. The most significant part likely to strike the target area is deadwood up to 100 mm diameter. Target Impact value Probability ratio 1/100 Probability Risk of potential of failure of harm × 1/82 × 1/1 = 1/8,200 To reduce the risk to a broadly acceptable level, an overall probability of 1/10,000 must be achieved. Removal of all deadwood is unnecessary. Removal of dead branches greater than 50 mm (2 in.) diameter overhanging the target should reduce the risk to an acceptable level. (After Remedial Action) Target Impact Probability Risk value Probability ratio 1/100 × 1/1 potential of failure of harm × 1/450 interactions among a diverse range of tree taxa, wood- digesting organisms, and environment are of such complex- ity that precise quantification of potential for tree failure is unlikely to be achievable. However, with training and the application of a systematic approach, reasonable estimates of probability of tree failure can be achieved. Evaluation of the targets on which trees might fail might require input from property managers, arborists, and others. Reasonable estimates of target value can be achieved by assessing monetary value and the frequency of target occupation. Stem or branch weight is probably the most realistic measure of impact potential to apply in the quantification of tree failure risk, and the relationship between diameter and the mass of the stem or branch provides a readily measur- able estimate of this component of the risk. Weather conditions greatly influence tree failure. A walk through woodland and other recreational areas after a moderate storm will often reveal paths and tracks littered with dead and recently living branches. The same weather conditions might result in reduced pedestrian access to recreational areas, substantially reducing the risk of harm from tree failure. Conversely, the risk of branch failure in trees susceptible to summer branch drop (Lonsdale 1999) increases during periods of hot, dry weather when pedestri- ans might seek shade beneath trees. The influence of weather on tree failure and patterns of pedestrian, eques- trian, and cyclist access requires further consideration and research. = 1/45,000 The predefined ranges used on the calculator are designed to simplify field operation of the system. If a high-value tree is identified as requiring remedial action that will significantly reduce its value, a more detailed evaluation of the target value and probability of failure, establishing probabilities rather than a probability ranges, will provide a more accurate quantification of risk. DISCUSSION Property owners and managers have a duty (under English law) to ensure, insofar as reasonably practicable, that people and property are not exposed to unreasonable levels of risk from the mechanical failure of trees in their control. To achieve this, prudent owners and managers employ arborists to advise on the health, mechanical integrity, and management of trees. While our knowledge of tree structure, tree defects, and host–pathogen interactions is ever increasing, the dynamic The concept of reasonable practicability is a central tenet of English law, which is evident throughout the English Health and Safety legislation and guidance (e.g., Health and Safety at Work Act 1974) and in judgments of the higher courts in relation to tree failure. In regard to trees, this concept should be embraced through the implementation of reasonably practicable tree safety management. If absolute safety from tree failure were achievable, society would almost certainly find the cost in terms of tree losses unacceptable. In this regard, Paine (1971) suggests that “it is high time we admit that we cannot achieve complete safety—and still provide a desirable product—any more than industry can.” The use of quantification in the assessment of tree hazards enables property owners and managers to operate, as far as is reasonably practicable, to a predetermined level of acceptable risk. Application in both urban and rural situations over a period of 8 years indicates that when using the proposed system, risk reduction measures required in high-value target areas are broadly comparable with or below the level of remedial action that might be considered appropriate without the system. In low-value target areas, the risks associated with tree failure hazards are frequently considerably lower than assumed or perceived prior to applying the system. ©2005 International Society of Arboriculture
March 2005
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