Tree Risk Assessment
Tree risk assessment is a systematic process for estimating how likely a tree or tree part is to fail and how severe the consequences would be. Purdue Extension puts it as a combination: risk is the likelihood of a tree failure event together with the severity of its possible consequences.
Sources: Spotting Danger: How to Identify & Assess Hazard Trees (FOR-2175), Alabama Cooperative Extension System ; Tree Risk Management (FNR-475-W), Purdue Extension, Forestry and Natural Resources
What goes into a rating
A University of Kentucky specialist describes three parts. Likelihood of failure is a professional judgment based on the defects present, the load on them and the tree’s age and vigor. Likelihood of impact depends on the expected direction of fall, the type of target, how often it is there and any protection. The consequences of failure are judged separately, mainly by what the target is worth, with people ranking above everything else.
Without a target there is nothing to harm: Purdue defines a target as people, property or activities that a failure could injure, damage or disrupt.
Sources: Trees: Dendrophobia - The Fear of Trees ; Tree Risk Management (FNR-475-W)
Three levels of assessment
Alabama Extension describes three levels: Level 1 is a limited visual inspection to find obvious problems, done by walking, driving or using drones; Level 2 is a detailed evaluation of the tree and its site; Level 3 uses advanced techniques and tools, such as aerial inspection by climbing or crane.
Source: Spotting Danger: How to Identify & Assess Hazard Trees (FOR-2175)
Who does it
When an inspection includes assessing a tree’s risk, Purdue Extension advises contacting an ISA Tree Risk Assessment Qualified (TRAQ) arborist, and a University of Kentucky specialist notes that professionals with that qualification are trained in making tree risk assessments. TRAQ is a separate ISA qualification from the Certified Arborist exam, but the exam’s Tree Risk domain covers the same ideas.
Sources: Tree Defect Identification (FNR-614-W) ; Trees: Dendrophobia - The Fear of Trees
How the ISA arborist exam tests it
The Tree Risk domain asks what risk combines, how each part is rated, and how targets and occupancy change the picture. Try 3 reviewed practice questions:
In tree risk management, what does Purdue Extension say risk combines?
Show answer and explanation
Correct answer: C. The likelihood of a failure and the severity of its consequences
Purdue Extension describes risk as combining how likely a tree failure event is with how severe its possible consequences would be. Alabama Extension likewise treats risk assessment as estimating both the chance of structural failure and how serious its consequences, such as injuries or property damage, would be. A tree's age, its defects and size, or the cost of removal can feed into an assessment, but none of them is what risk combines.
- A: Age and lifespan can inform an assessment, but Purdue defines risk by failure likelihood and the severity of consequences.
- B: Defects and size bear on the likelihood of failure, but risk also weighs what happens if the tree fails.
- D: Costs and tree value matter when choosing mitigation, but they are not what risk combines.
Sources: Tree Risk Management (FNR-475-W), Purdue Extension ; Spotting Danger: How to Identify & Assess Hazard Trees (FOR-2175), Alabama Cooperative Extension System
Report a problem with this question · ID risk-037
In the ISA Tree Risk Assessment Best Management Practices method, which set of categories is used to rate the likelihood of failure?
Show answer and explanation
Correct answer: C. Improbable, possible, probable, and imminent
The ISA BMP method replaced numeric scores with descriptive categories, and likelihood of failure is rated improbable, possible, probable or imminent. The other sets belong to different steps: very low to high rates the likelihood of impacting the target, negligible to severe rates the consequences of failure, and low, moderate, high and extreme are the overall risk ratings that come out of the decision matrices. Mixing up these four scales is a common error.
- A: Very low, low, medium and high rate the likelihood of impacting a target.
- B: Negligible, minor, significant and severe rate the consequences of failure.
- D: Low, moderate, high and extreme are the overall risk ratings.
Source: Tree Risk Assessment Methods: A Comparison of Three Common Evaluation Forms (ENH1226), UF/IFAS Extension (EDIS)
Report a problem with this question · ID risk-016
Two pines have similar dead limbs. One overhangs a campsite tent pad that is occupied overnight; the other overhangs a park road where a car passes about every half hour. Which limb is more likely to strike a target if it fails, and why?
Show answer and explanation
Correct answer: B. The campsite pine, because campers stay in one place for hours
Likelihood of impact depends on whether a target is in the strike zone when the part falls. A USDA Forest Service guide for Alaska notes that a stationary target is more likely to be hit than a moving one, and the Oregon and Washington field guide stresses that the longer the exposure, the greater the potential for injury or damage. Campers stay on the tent pad for hours, while each passing car is under the limb only briefly. Target value affects the consequences, not the likelihood of impact.
- A: Target value affects the consequences of a strike, not how likely a strike is.
- C: Limb size affects consequences; likelihood of impact depends on how long targets are present.
- D: A car every half hour is intermittent, and each car passes through quickly.
Sources: Hazard Trees in Alaska: A Guide to the Identification and Management of Hazard Trees in Alaska (R10-TP-142), USDA Forest Service, Alaska Region (GovInfo) ; Field Guide for Hazard-Tree Identification and Mitigation on Developed Sites in Oregon and Washington Forests (R6-NR-TP-021-2013), USDA Forest Service, Pacific Northwest Region (GovInfo)
Report a problem with this question · ID risk-019
0 of 3 correct
Common mistakes in the field
Relying on drive-by surveys
The USDA Forest Service community guide says drive-by surveys cannot see defects on the far side of a tree from the road; use them only as a preliminary or supplemental tool.
Rating the tree but not the target
A dead limb deep in the woods and one over a bench are not the same risk. Likelihood of impact and consequences depend on what is underneath and how often it is there.
Inspecting once and forgetting
Purdue Extension bases inspection intervals on the risk found at the first inspection and suggests flagging trees with a history of failures or problem structure for more frequent monitoring.
Sources
- Spotting Danger: How to Identify & Assess Hazard Trees (FOR-2175), Alabama Cooperative Extension System
- Tree Risk Management (FNR-475-W), Purdue Extension, Forestry and Natural Resources
- Trees: Dendrophobia - The Fear of Trees, University of Kentucky Urban Forest Initiative
- Tree Defect Identification (FNR-614-W), Purdue Extension, Forestry and Natural Resources
- Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry
Written in our own words from the public sources above, checked October 19, 2026. Arborist Practice Test is not affiliated with ISA.