Journal of Arboriculture 31(2): March 2005 57 QUANTIFIED TREE RISK ASSESSMENT USED IN THE MANAGEMENT OF AMENITY TREES By Michael J. Ellison Abstract. A system of tree risk assessment is proposed that expands concepts developed by others and enables a probability of significant harm to be applied to tree failure risk. By evaluating the components of a tree failure hazard and assigning to them estimates of probability, the proposed system enables the skilled tree inspector to calculate the product of those probabilities to produce a numerical estimate of risk. 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 limit of reasonable or acceptable risk. Key Words. Risk assessment; tree hazards; safety; target; quantified risk; amenity; saproxylic habitat. potential. By first assessing the value or use of targets upon which trees might fail, tree owners and site managers can establish whether and at what degree of rigor tree inspec- tions are required. By assessing the probabilities of the three components and calculating their product, it is possible for the skilled tree assessor to quantify the risk of significant harm from tree failure in a way that enables owners and managers to balance safety with tree values. Tree managers work in a climate of increasing environmen- tal awareness, in which trees are greatly valued and yet potentially hazardous. There is, therefore, a need to reconcile different management objectives, especially on sites where old and perhaps structurally unstable trees are present. As trees age, they increasingly develop features that might compromise their mechanical integrity while provid- ing increasingly diverse wildlife habitats and visual interest. These features include cavities and decaying wood, which together with other niches in the tree, provide habitats for many rare plants and saproxylic (deadwood) animals and fungi (Kirby and Drake 1993). In Britain, a large proportion of such trees occur in rural areas, but there are also many on the streets of towns and in gardens, churchyards, and city parks. If old trees and their younger successors are to be managed responsibly with regard both to safety and their value, methods must be developed for the reliable assess- ment of hazards and valuable features alike. As far as hazards are concerned, the need is to be able to quantify them and any associated risk so that the risk can be kept within acceptable or reasonable limits without implementa- tion of disproportionate risk control measures. This paper introduces quantified tree risk assessment, which is an expansion of concepts proposed by Paine (1971), Helliwell (1990, 1991), and Matheny and Clark (1994). Quantified tree risk assessment provides a frame- work for the assessment of the three components of tree failure risk—target value, probability of failure, and impact DEFINITION OF TERMS Hazard “A hazard is the disposition of a thing, a condition, or a situation to produce injury” (Health and Safety Executive 1995). A tree failure hazard is present when a tree has potential to cause harm to people or property. Probability Statistical probability is a measure of the likelihood of some event happening. There are rules of addition and multiplica- tion in probability theory. In tree failure risk assessment, the probability that the three components will combine in a common outcome is the product of their independent probabilities. Risk Risk is the probability of something adverse happening. “Quantified risk assessment is a risk assessment which incorporates numerical estimates.… There are many forms of risk and therefore of risk assessment. The underlying concept is that of seeking to identify in some quantitative or at least comparative way the connection between some hazardous agency, and actual exposure to harm” (Health and Safety Executive 1995). Acceptable Risk We are constantly exposed to and accept or reject risks of varying degrees. For example, if we desire the convenience of electric lighting, we must accept that, having implemented control measures such as insulation and periodic inspection, there is a low risk of electrocution; this is an everyday risk taken and accepted by millions of people. When evaluating tree failure hazards, two types of risk must be considered. We must consider the person upon whom a risk is imposed, as with the neighbor of a tree ©2005 International Society of Arboriculture
March 2005
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