Tree Risk Management
Core concepts
Risk combines likelihood of failure, likelihood of impact (the target and its occupancy), and consequences. Assessments follow ANSI A300 Part 9 and the ISA BMP: limited visual, basic, advanced, and specialized (e.g., air excavation, decay detection) levels. Identify the target and its occupancy rate, categorize likelihood of failure (improbable, possible, probable, imminent), and record residual risk after mitigation. Risk assessment is not the same as hazard identification - hazards are the agents (defects, decay, poor architecture); risk is the probability-weighted outcome, and it is never zero. Document assumptions, limitations, the assessment level, and recommendations with timeframes; propose mitigation (prune, cable or brace, support, move the target, remove, monitor) proportionate to the client's risk tolerance.
Traps
- Equating 'defect' with 'hazard' - a defect with no target is a different risk.
- Forgetting the target: a dead tree in closed woodland may be low risk.
- Presenting a single number as certainty; qualitative ratings need defined criteria.
- Omitting the assessment level and its limits from the report.
- Failing to specify a re-inspection interval.
Rule of thumb
Use the triad - failure × impact × consequences. If any factor is missing you cannot rate high risk. Apply the mitigation hierarchy: avoid, reduce, transfer, accept, then monitor on a defined cycle.