Tree Risk Assessment and Management: Targets, Ratings, and Mitigation
Tree Risk Assessment and Management tests a repeatable process: define the target, assess likelihood of failure and impact, rate consequences, and choose mitigation proportionate to risk.
Core rules and concepts
- Risk combines likelihood of failure, likelihood of impact, and consequence. A defect without a target is not a risk; a large tree far from people can be lower risk than a small tree over a bench.
- Inspection levels are limited visual (walk or drive-by), basic (ground-level 360-degree inspection), and advanced (aerial inspection, resistograph, or other tools). Match the level to the site and objectives; ISA Tree Risk Assessment BMP and TRAQ methodology provide structure.
- Likelihood of failure considers species, defect type and location, load, site, and history. Decay fungi, cavities, codominant unions, root-plate problems, and recent grade change are key indicators.
- Mitigation hierarchy: avoid the target (move a bench or path), reduce load (prune), support (cables or bracing), monitor, or remove. Document residual risk and who accepted it.
- Record the date, inspector, target, defect, ratings, photos, actions, and re-inspection interval.
Worked example
A silver maple in a picnic area has a 12-inch opening at the base and a soil crack on one side; a picnic table sits within the fall zone on summer weekends. Likelihood of failure is assessed as probable because of the basal defect and crack; likelihood of impact is medium because occupancy is occasional; consequence is significant because of tree size and occupancy. The risk is high enough to act now.
Reasoning: the target drives the rating. Moving the table reduces impact immediately; fencing the zone and obtaining an advanced assessment are proportionate because the defect is basal and the target is easy to move. Removal is justified if advanced assessment confirms extensive decay.
Traps that cost marks
- Treating “hazard tree” and “risk” as synonyms.
- Rating by species alone.
- Ignoring the target.
- Recommending indefinite monitoring without a decision point.
- Failing to document the rating and mitigation.