ISA Tree Risk Assessment 3 — Questions and Answers

Question 1: In the ISA Tree Risk Assessment Qualification (TRAQ) methodology, what are the three components used to determine the overall risk rating?

Tree species, age, and location
Likelihood of failure, likelihood of impacting a target, and consequences of failure (Correct answer)
Trunk diameter, lean angle, and crown density
Root depth, soil type, and wind exposure

Correct answer: Likelihood of failure, likelihood of impacting a target, and consequences of failure

The TRAQ risk assessment matrix evaluates three components: the likelihood that the tree or part will fail, the likelihood that the failure will impact a target (person or property), and the severity of consequences if impact occurs.

The ISA TRAQ methodology uses a systematic approach to evaluate tree risk. Likelihood of failure considers structural defects, species failure characteristics, load history, and environmental conditions. Likelihood of impacting a target evaluates the spatial relationship between potential failure zone and targets, considering target occupancy rate (how often people or vehicles are in the impact zone). Consequences range from negligible to severe based on the target type — a failed limb over an empty field has different consequences than one over a playground. These three factors are combined using a risk matrix to generate an overall risk rating of low, moderate, high, or extreme. This structured approach provides consistency and defensibility in risk management decisions.

Question 2: What does 'target zone' mean in tree risk assessment?

The area where herbicide should be applied around the tree
The area within striking distance of the tree or its parts where people, property, or activities could be impacted by a failure (Correct answer)
The area where the tree's roots extend underground
The specific location where a tree should be planted

Correct answer: The area within striking distance of the tree or its parts where people, property, or activities could be impacted by a failure

The target zone is the area that could be struck by the tree or its parts if failure occurs. Anything within this zone — people, structures, vehicles, utilities — is considered a potential target.

The target zone is a fundamental concept in tree risk assessment. It is the ground area (and airspace above it) that could be impacted if the tree or a specific structural component fails. The size and shape of the target zone depends on the tree's height, lean, branch spread, and the specific defect being assessed. A tree leaning north has a primary target zone to the north. Within the target zone, the assessor identifies specific targets: pedestrians, buildings, parked vehicles, playground equipment, power lines, or other valuable property. Target occupancy — how frequently the target is present in the zone — is a critical factor. A rarely used trail has lower occupancy than a busy parking lot, significantly affecting the overall risk rating.

Question 3: Which of the following is considered a 'Level 2' assessment in the TRAQ system?

A limited visual assessment from a distance during a walk-by survey
A basic assessment involving a detailed visual inspection of the tree from the ground, walking 360 degrees around it (Correct answer)
An advanced assessment using tools such as resistance drilling, sonic tomography, or aerial inspection
A desktop review of the tree's maintenance history and species characteristics

Correct answer: A basic assessment involving a detailed visual inspection of the tree from the ground, walking 360 degrees around it

A Level 2 (basic) assessment involves a thorough 360-degree ground-level visual inspection of the tree, examining roots, trunk, scaffold branches, and crown for structural defects, signs of decay, and other risk indicators.

The TRAQ system defines three assessment levels. Level 1 (limited visual) is a walk-by or drive-by survey used to screen large tree populations for obvious hazards — it does not involve close inspection of individual trees. Level 2 (basic) is the standard assessment for individual trees: the assessor walks completely around the tree, examines the root zone for heaving, severed roots, or fungal fruiting bodies, inspects the trunk for cavities, cracks, cankers, included bark, and lean, evaluates the scaffold branches and crown for deadwood, broken limbs, and structural defects, and considers the site and targets. Level 3 (advanced) is triggered when Level 2 identifies defects requiring further investigation, using tools like resistance drills, sonic tomographs, or aerial inspections.

Question 4: What structural defect is indicated when a tree develops a pronounced 'bulge' or swelling on one side of the trunk at the base?

A graft union from nursery propagation
A potential response growth indicating internal decay or structural stress on the opposite side of the bulge (Correct answer)
Normal buttress root flare development
A site where the tree was previously cabled

Correct answer: A potential response growth indicating internal decay or structural stress on the opposite side of the bulge

Asymmetric bulging or swelling on the trunk often indicates reaction (response) wood formation in response to internal decay or structural weakness. The tree adds wood on the stressed side to compensate for lost strength.

Trees are remarkable at detecting and responding to structural stress through a process called adaptive growth (or response growth). When internal decay, a crack, or other defect weakens one side of the trunk, the tree detects the increased mechanical strain on the remaining sound wood and adds extra xylem (wood) on the stressed side to compensate. This creates visible asymmetric bulging. An experienced risk assessor recognizes this bulging as a red flag indicating a hidden internal defect, often on the opposite side or at the core. The bulge itself represents the tree's attempt to maintain adequate structural strength — a phenomenon described as Mattheck's axiom of uniform stress. This finding typically warrants a Level 3 advanced assessment with tools like a resistance drill or sonic tomograph to map the extent of internal decay.

Question 5: When assessing a tree with codominant stems, what visual feature indicates the highest probability of failure at the union?

Bark ridges pushed upward at the union forming a visible collar
A visible crack or seam at the union combined with included bark between the stems (Correct answer)
Lichen growth covering the union area
Both stems being exactly the same diameter

Correct answer: A visible crack or seam at the union combined with included bark between the stems

A visible crack at a codominant union with included bark indicates the highest failure probability. The crack shows active separation, while included bark means the union lacks the interlocking wood grain that provides strong attachment.

Codominant stems with included bark are one of the most common structural defects leading to catastrophic tree failure. Included bark prevents the formation of overlapping wood fibers that normally strengthen branch or stem unions. Instead, the two stems are essentially pushing against each other as they grow, separated by a wedge of bark. When a visible crack appears at such a union, it indicates that mechanical forces (wind, ice, or the weight of the stems) have begun to physically separate the attachment point. This combination — included bark plus active cracking — represents an advanced state of failure progression. Additional risk indicators include oozing sap from the crack, asymmetric crown loading, and previous storm damage history. This finding typically warrants immediate mitigation such as cabling, bracing, weight reduction, or removal.

Question 6: What is 'occupancy rate' in tree risk assessment and how does it affect the risk rating?

The percentage of the tree's crown that is alive and has foliage
A measure of how frequently people or moveable property are present within the tree's target zone, directly affecting the likelihood of a failure impacting a target (Correct answer)
The ratio of trees to open space in the assessment area
The number of birds or animals nesting in the tree

Correct answer: A measure of how frequently people or moveable property are present within the tree's target zone, directly affecting the likelihood of a failure impacting a target

Occupancy rate estimates how often targets (people, vehicles, etc.) are present in the potential failure zone. A tree over a busy sidewalk has high occupancy; one over a rarely visited forest trail has low occupancy. Higher occupancy increases the overall risk rating.

Occupancy rate is a critical component of the target assessment in the TRAQ methodology. Even a tree with severe structural defects poses lower risk if no one is ever in the target zone. Occupancy is typically categorized as: constant (nearly always occupied, such as a building or parking structure), frequent (regularly occupied, such as a playground during operating hours), occasional (intermittently used, such as a trail on weekday afternoons), and rare (seldom occupied, such as a remote forest area). The assessor considers both fixed targets (buildings, utilities) that are always present and moveable targets (people, vehicles) whose presence varies. Occupancy rate directly modifies the likelihood of impact: a tree that is equally likely to fail has a higher overall risk rating over a busy playground (constant occupancy) than over a rarely used service road (rare occupancy).

ISA Tree Risk Assessment 3

In the ISA Tree Risk Assessment Qualification (TRAQ) methodology, what are the three components used to determine the overall risk rating?