A hazard rating system is essential to ensure that appropriate and reasonably consistent criteria are used to evaluate the relative hazard of a tree. But hazard rating systems are like belly buttons – everybody has one. 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 prize 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 very limited. In some cases we just have to go with educated guesses. These will naturally vary among the pathologists or arborists setting up the rating system.
Rating system components
All systems incorporate some measure of the degree of tree defect and a measure of risk: likelihood and value of target loss if the tree fails. These two values are then added or multiplied to get a hazard rating.