How tree risk assessment works: terms and levels in the guidance

This page compares how published sources define the terms used in tree risk assessment. It gives no verdict on any tree and is not an assessment. The sources come from the United Kingdom and the United States.

Terms in the National Tree Safety Group guidance

The 2024 guidance has a glossary defining hazard, harm, risk and target, and describes risk as made up of the likelihood of failure and the consequence. Its section 6.3 defines hazard, harm, risk, defect and obvious defect. Section 6.4 describes zoning, which sorts land by how it is used, and section 6.5 describes informal procedures and formal procedures, with the formal ones running from drive-by and walk-over checks to detailed inspections.

Other systems side by side

Source	What it says
University of Florida IFAS Extension	Frames the question as likelihood of failure and the consequences of failure
US Forest Service brochure	Describes a hazard tree as a defect together with a target
QTRA	Says it moves away from safe and unsafe labels, starts from the target, and uses advisory thresholds
ISA Tree Risk Assessment Qualification	Describes a two-day course; the third edition of the ISA Best Management Practices on tree risk assessment (2025) covers concepts, levels, targets, categorising risk, mitigation and reporting

Where the sources differ

The sources use the word risk in different ways. The National Tree Safety Group glossary defines risk as the likelihood of harm arising from a particular hazard and hazard as any source of potential harm. Both it and the Florida publication build risk from likelihood and consequence. The Forest Service brochure uses the older term hazard tree. QTRA gives its own method. This page does not rank them. Each source uses its own wording for the same ideas. Prices and course fees are left out because they change.

Why the terms matter

A reader comparing a report from one system with a report from another can find the same word meaning different things. Hazard in the Forest Service brochure joins a defect and a target. Hazard in the National Tree Safety Group glossary is a separate entry from risk, harm and target. Checking which source a report follows is the way to read its words.

Who looks at this

For the methods and the credentials, see tree risk assessment methods and consulting arborist credentials. For conditions the sources list, see structural defects in trees, and for the legal wording see tree inspection and occupier duty.