Tree Risk Assessment and Management
The risk model
Risk combines three factors: likelihood of failure, likelihood of impact on a target, and consequences of that impact. A target is a person or property that would be struck. A defect with no target carries little risk regardless of how alarming it looks. Keep tree health, tree value, and tree risk as separate judgments.
Assessment levels
Expect three levels: a limited visual assessment for a population, a basic assessment with a 360-degree walk-around and simple tools, and an advanced assessment using resistograph, sonic tomography, or pulling tests. Each produces a mitigation recommendation, a residual risk, and a re-inspection interval.
Defects to recognize
Codominant stems with included bark, cracks and splits, decay detected by conks or cavities, root plate and root severance issues, soil heave, lean that has increased, dead or broken limbs, cankers, and poor taper.
Mitigation
Reduce the target whenever possible - move a bench, reroute a path, restrict parking. Then consider pruning, cabling or bracing, monitoring, or removal. Removing the tree is not the only answer and is not automatically the safest one.
Traps
Risk is not binary and does not equal hazard. A large, healthy tree may be low risk; a hollow tree with a thick sound shell may also be low risk. Do not confuse a defect's visual severity with the probability of impact.
Worked example
A cracked limb over an empty lawn versus the same limb over a playground: the defect is identical, but the risk differs because the target differs. Fix the target first.