Under the ISA's TRAQ framework, tree risk is assessed across three components — likelihood of failure, likelihood of impact, and consequences of impact — at three levels of assessment. Understanding this structure helps property managers and institutions evaluate what an assessment delivers and what it does not.
The three components of risk
Tree risk is distinct from tree health. Risk under TRAQ has three components: likelihood of failure (the probability that part of the tree fails within a defined time period); likelihood of impact (given a failure, the probability that the failed part reaches a target); and consequences of impact (the severity of the outcome if impact occurs). None are observable directly — each is estimated by a trained assessor against defined matrices.
Three levels of assessment
Level 1 is a limited visual assessment from a distance, suited to large tree populations. Level 2 is a basic 360-degree inspection from ground level using simple tools — the standard inspection level for managed tree populations on a defined cycle. Level 3 is an advanced assessment bringing in instruments such as sonic tomography, resistance drilling, or root crown excavation, reserved for high-value or high-target trees where additional certainty supports the decision. A structured programme uses the three levels in combination.
What a Level 2 assessment examines
A Level 2 assessment is structured around observable conditions by tree part. The root system and root collar are examined for lifted soil, fungal fruiting bodies, girdling roots, cavities, and changes in trunk lean. The trunk is checked for visible decay, cavities, large wounds, included bark, splits, cracks, and embedded objects. Scaffold branches and canopy are assessed for deadwood, weak attachments, codominant stems, cracks, and canopy asymmetry. Site context — recent construction, soil compaction, drainage changes, utility trenching — is also part of the assessment.
What an assessment establishes
A Level 2 assessment identifies observable conditions, rates likelihood of failure within a defined time period, identifies targets at risk, produces a risk rating, and recommends mitigation. It does not predict the timing of a specific failure. Internal decay can be present without external symptoms, which is part of why TRAQ specifies inspection on cycles. An assessment also serves as the documentary record that the duty of care was discharged.
Mitigation options
For trees rated above the action threshold, mitigation options in increasing order of intervention are: monitoring (re-inspection at a defined interval); pruning (removal of deadwood, reduction of end-weight, treatment of weak attachments); cabling and bracing (mechanical support for weak unions); target modification (relocating a parking spot, redirecting a footpath); and removal where risk cannot be reduced to acceptable levels through other means.
Inspection cycles
The output of a single assessment ages with the tree and the site. A defined inspection cycle — annually for high-target trees, every two to three years for moderate-target trees, every five years for low-target trees, with re-inspection triggered by major weather events — keeps the risk picture current over time.
