Tree Risk Management and Risk Assessment Basics

Domain Review: This topic is one of the [10 ISA exam domains](/blog/isa-arborist-exam-domains-explained) you must master.

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)

1. Likelihood of Failure: How probable is it that the branch will snap? (Improbable, Possible, Probable, Imminent).
2. Likelihood of Impact: If it falls, will it hit the target? (Very Low, Low, Medium, High).
3. 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.

1. Remove the Target: Move the park bench. (Often the cheapest option).
2. Mitigate the Defect: Prune the dead branch. Cable the weak union.
3. 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.