Understanding Tree Risk Assessment in Utility Vegetation Management
A dead, damaged, or diseased tree may immediately draw attention in the field. But poor tree health alone does not necessarily make a tree a risk.
In ACRT’s latest webinar, Introduction to Tree Risk Assessment with a Colorado Focus, Autumn Quinteros, Senior Consulting Utility Forester 1, ACRT, explores how tree risk assessment principles can help utility vegetation management (UVM) professionals look beyond the condition of an individual tree and consider the larger picture.
How likely is the tree or a portion of it to fail? If it fails, how likely is it to impact a target? And what could happen if it did?
These questions form the foundation of tree risk assessment and are particularly important around utility infrastructure, where trees interact with power lines, equipment, roads, homes, and people.
Tree Health and Tree Risk Are Not the Same
One of the fundamental concepts Quinteros emphasizes is that an unhealthy tree does not automatically equal a high-risk tree.
A dead or declining tree with nothing within striking distance may present little risk. Conversely, a tree that appears relatively healthy could warrant attention because of its location, growth pattern, environmental conditions, or proximity to critical infrastructure.
Tree risk assessment considers three primary factors: likelihood of failure, likelihood of impact, and consequences of failure. Together, these factors help vegetation management professionals evaluate the level of risk associated with a tree and make more informed management decisions.
Choosing the Appropriate Level of Assessment
Tree risk assessments can be conducted at different levels depending on the situation and information needed.
- A Level 1 limited visual assessment is conducted from a specified perspective, such as on foot, from a vehicle, or during an aerial patrol. In UVM, this can provide an efficient way to evaluate trees along a circuit and identify obvious concerns.
- A Level 2 basic assessment takes a closer look at the tree and surrounding site, including visible aboveground roots, the trunk, branches, and potential defects.
- A Level 3 advanced assessment provides more detailed information and may require specialized equipment, data collection or expertise.
The goal is not necessarily to conduct the most detailed assessment on every tree. It is to recognize which trees warrant a closer look and respond appropriately.
Consider the Target and Its Surroundings
Risk cannot be evaluated without considering what a tree could affect.
For UVM professionals, power lines are static targets that are constantly present. But the line itself should not be the only consideration.
If a tree fails into utility infrastructure, is the line above a busy roadway or bus stop? Could an outage affect a significant number of customers? Is the tree in an area with elevated wildfire risk? How accessible is the location for emergency response?
Looking beyond the immediate target provides a more complete understanding of potential consequences and can influence how the tree’s overall risk is evaluated.
Read the Tree and Its Environment
Trees do not exist independently of their surroundings. Their growth can provide clues about the stresses they have experienced over time.
Response growth and reaction wood can indicate how a tree has adapted to wind, snow, erosion, previous damage, and other loads. Assessors can also look at tension roots, branch attachments, and how wounds have responded over time.
The surrounding site can provide additional clues. Are multiple trees leaning in the same direction? Has a new utility corridor changed the site’s exposure to wind? Is the soil saturated? Is the tree growing on a steep slope?
Changes to a site can be particularly important. Trees that once grew within a protected forest interior may suddenly become exposed when neighboring trees are removed or a corridor is established. Unlike trees that developed along the forest edge, these newly exposed trees may not have developed the same response growth to environmental loads, potentially increasing their likelihood of failure.
Account for Colorado’s Conditions
Tree risk assessment also requires an understanding of regional conditions. In Colorado, wind, elevation, slopes, snow loading, and significant temperature fluctuations can all influence how trees grow and potentially fail.
High winds can contribute to windthrow, particularly among shallow-rooted species or trees that have recently become exposed. Snow and ice can quickly add significant weight to branches, while slope direction can influence snow accumulation, soil moisture, sunscald, and frost cracking.
Human activity can create additional stressors. Damage from off-road vehicle recovery winches, girdling, snowplows, and vehicle collisions may leave wounds that warrant further evaluation. Assessors can look at how the tree is responding, including whether wounds are closing properly and whether there are signs of decay or insect activity.
These factors reinforce an important principle: assessors need to consider the environment in which a tree is growing, not just the tree itself.
Turn Observations Into a Risk Rating
Once the tree and site have been evaluated, the information comes back to three components.
Likelihood of failure considers the probability that the tree or tree part will fail during a specified period. Factors can include response growth, site conditions, tree health, species, load and observed defects.
Likelihood of impact considers the chance that the failed tree or tree part will strike the target. Target occupancy, location, surrounding protection and anticipated direction of failure can influence the assessment. In utility vegetation management, this can also include the likelihood of vegetation growing into the right of way and contributing to arcing, sparking or equipment failure.
Consequences of failure consider potential personal injury, property damage or disruption. For utilities, those consequences can vary depending on the infrastructure involved, surrounding land use, wildfire potential and accessibility.
Combining these three factors provides an overall tree risk rating and a framework for determining an appropriate response.
The same thinking can be applied to right-of-way clearance. Professionals can consider whether vegetation is likely to grow into the minimum approach distance during a maintenance cycle, how likely that interaction is to contribute to an outage, fire or equipment failure and what the consequences could be.
Rather than simply asking, “Is this tree a problem today?” this approach considers how the tree and its environment may change before the next inspection or maintenance cycle.
Risk Doesn’t Necessarily End With Mitigation
Mitigation can reduce risk, but it does not necessarily eliminate it.
Residual risk is the risk remaining after mitigation. Depending on their role, vegetation management professionals may be responsible for evaluating whether that remaining level of risk is acceptable or whether additional action is warranted.
Tree risk assessment provides a structured way to evaluate trees, targets, environmental conditions and potential consequences so utilities can make more informed vegetation management decisions.
Take a Closer Look at Tree Risk Assessment
Want to explore these concepts further and see how they apply to real-world scenarios?
Watch Introduction to Tree Risk Assessment with a Colorado Focus, presented by Autumn Quinteros, Senior Consulting Utility Forester 1, ACRT. The webinar provides a practical introduction to tree risk assessment, explores Colorado-specific considerations and walks through examples of determining tree risk ratings.
Featured Webinar
A Practical Introduction to Tree Risk Assessment (TRAQ)
Autumn Quinteros covers tree risk assessment fundamentals and how real-world and Colorado-specific conditions influence field decisions.
This webinar and article are intended for educational purposes and are not replacements for the ISA Tree Risk Assessment Qualification (TRAQ) program.
