Tree Risk Management and Risk Assessment Basics
Understand the basics of Tree Risk Assessment (TRA). Learn the 3 levels of assessment and the formula: Risk = Likelihood + Consequences.
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Tree Risk Assessment (TRA) is about predicting the future. Since we don't have a crystal ball, we have a system.
Hazard vs. Risk
- Hazard: The tree defect itself (e.g., a dead branch).
- Risk: The combination of the hazard AND a target.
- Key Concept: A dead tree in the middle of a dense forest with no trails has Zero Risk because there is no target.
The Risk Formula
Risk = (Likelihood of Failure) x (Likelihood of Impact) x (Consequences)
- Likelihood of Failure: How probable is it that the branch will snap? (Improbable, Possible, Probable, Imminent).
- Likelihood of Impact: If it falls, will it hit the target? (Very Low, Low, Medium, High).
- Consequences: If it hits, how bad will it be? (Minor, Significant, Severe).
Levels of Assessment
You don't inspect every tree with a drone. You match the effort to the need.
- Level 1: Limited Visual Assessment. A drive-by or walk-by "scan" to catch obvious defects.
- Level 2: Basic Assessment. 360-degree walk-around. Mallet sounding. Looking with binoculars. (This is the standard).
- Level 3: Advanced Assessment. Aerial inspection, resistance drilling (Resistograph), sonic tomography.
Mitigation Options
Once you find a risk, you have to do something.
- Remove the Target: Move the park bench. (Often the cheapest option).
- Mitigate the Defect: Prune the dead branch. Cable the weak union.
- Remove the Tree: The last resort.
Duty of Care
Arborists have a "Duty of Care" to inspect trees reasonably. You are not expected to see invisible internal decay, but you are expected to see a giant fungal conk on the trunk.