All trees mature, decline and die. Trees undergo many physical changes during their lifespan. Some changes can result in physical defects reducing structural integrity such as branch loss, cavity development, wood decay, and declining health. Defects in trees or tree parts can be caused by natural events such as wind or ice storms or result from other effects such as construction damage, grade change and improper pruning. Trees affected by structural defects growing in the urban and rural landscape can become a risk to people and property. No tree is safe and any tree can fail at any time. Trees with recognized potential to cause injury or damage as well as actions to mitigate
associated risks can be identified and prioritized through a technical process called Tree Risk Assessment.
Goals of Tree Risk Assessment
1. Identify moderate, high or extreme risk trees or tree parts before personal injury or property damage occurs.
2. Prioritize recommended action to reduce or remove the risk caused by trees or tree parts.
3. Assure acceptable structural integrity and reasonable site retention of a tree or tree part as an acceptable risk.
Tree Risk Assessment is a systematic evaluation of how a site is used by people, the buildings or structures located
on a site and the external, internal or below ground conditions of each tree of concern. The evaluation yields a
listing of risk factors that are ranked, combined and compared against a predetermined threshold of risk to
determine if a tree or tree part should be considered a low, moderate, high or extreme risk.
Three types of risk factors considered in a Tree Risk Assessment are: 1) Targets – People or property within striking
distance of a whole tree or its component parts; 2) Size of a tree or tree part of concern; and 3) Potential of Failure of
each defected tree or tree part.
A high risk or extreme risk tree or tree part exists when the combined risk factors equal or exceed the threshold and
thus present an unacceptable risk for retention in its current condition. Mitigation actions to reduce or remove the
risk associated with moderate, high or extreme risk trees or tree parts are determined and prioritized based on the
Potential of Failure for each defect identified. Mitigation efforts to reduce tree risk can include: retention with
regular monitoring; re-routing traffic; supplemental tree support systems; pruning to reduce tree canopy weight;
fencing off high risk areas or removing the tree.
Because trees and their environments are dynamic, findings of a Tree Risk Assessment are valid only the day the
assessment is performed. Subsequent monitoring is commonly required for retained trees. The assessment is
finalized when the overall site review, target factors, visual and/or detailed tree factors, and prioritized risk
mitigation activities, if needed, are documented in a report.
Qualifications of Assessing Tree Risk
Assessment of tree risks is a specialized area of expertise beyond the scope of other arborist or urban forester training and accreditation. Tree Risk Assessment requires judgment, experience and skill relating to tree biology, tree mechanics, risk assessment and management, recording and report writing, as well as professional responsibilities, standards, obligations and liability. The current standard of care for individuals performing Tree Risk Assessments is completion and passing of the Tree Risk Assessment in Urban Areas and the Urban/Rural Interface Course thus attaining the level of PNW-ISA Certified Tree Risk Assessor.