Hazard Tree Evaluation Systems

An evaluation system is essential to ensure that appropriate and reasonably consistent criteria are used to evaluate the relative risk of a tree. But evaluation systems are like belly buttons – everybody has one, and they’re all different. At first it may seem surprising that a single officially sanctioned, scientifically tested rating system is not used everywhere. Here’s the problem:

  • Purposes and applications of inspections differ. Consider an arborist inspecting a heritage tree adjacent to a kindergarten, a recreation forester inspecting a thousand trees in a seasonal campground before opening, or a utility company inspecting trees that could hit power lines. The issues and objectives are different enough that a uniform system may not be desirable.
  • Tree species and their diseases and defects differ regionally. A certain disease may be unimportant in failure in some areas but could represent severe hazard in another. Thus it may be reasonable to rate the disease differently.
  • Our knowledge of defects and the extent to which they contribute to failure potential is limited. In some cases we just have to go with educated guesses. These will naturally vary among the pathologists or arborists setting up the evaluation system.

Evaluation system components

All systems incorporate:

  • the severity of tree defect, associated with the likelihood or imminence of failure
  • the target, of which there are two components often rolled into one:
    • likelihood of a target being hit if failure occurs, including the frequency and duration of target presence (i.e., if the target is people, how often people are around the tree)
    • value of the target, people obviously being the most valuable.

Especially if the tree part likely to fail is a branch, many systems increase the severity of defect based on its size.

Most systems assign numbers to the components and generate a numeric rating, but it is important to recognize that they are not quantitative measures.  They are merely numbers assigned to categories.  This is useful when dealing with many trees; for example, they can be easily sorted by rating.  The International Society of Arboriculture has moved away from numbers to avoid the implication that the outcome is a quantitative measure of risk, as its members often deal with one tree at a time.