Certified Arborist Test Tree Risk Assessment 2 — Questions and Answers

Question 1: In the ISA Tree Risk Assessment Qualification (TRAQ) framework, what are the three components of tree risk?

Probability of failure, target occupancy, and consequence of failure (Correct answer)
Structural defect, site hazards, and occupant vulnerability
Tree health, environmental factors, and human exposure
Failure likelihood, branch size, and distance from target

Correct answer: Probability of failure, target occupancy, and consequence of failure

ISA's TRAQ framework defines risk as the combination of three components: the probability that a part will fail, the likelihood that it will strike a target (target occupancy), and the severity of the consequences if it does (consequence of failure).

The ISA Tree Risk Assessment Qualification (TRAQ) framework provides the standardized methodology used by certified arborists for formal tree risk assessments. At its foundation is the risk equation: Risk = Probability of Failure × Likelihood of Impact × Consequence of Impact. Probability of failure encompasses both the likelihood that a structural defect will fail (fail probability — considering defect type, magnitude, and the tree's ability to compensate) and the probability that a triggering event (wind, ice load, flooding) will occur with sufficient magnitude to cause failure at that defect. A severe defect on a tree that experiences no significant loading events may have a lower probability of failure than a moderate defect on a tree in a high-wind environment. Likelihood of impact (target occupancy) considers whether people, property, or other targets are likely to be in the impact zone when a failure occurs. A large cavity in a tree over an empty field poses negligible risk; the same cavity over a school playground may pose extreme risk during school hours. Target occupancy is evaluated for the specific parts likely to fail and the specific times when that area is occupied. Consequence of failure considers the severity of outcomes should a failure occur and a target be struck. Factors include the size and weight of the potential failure part (a dead 2-inch twig versus a 12-inch dead trunk section), the nature of the target (person, vehicle, occupied structure, utility lines), and the vulnerability of the target. Even moderate defects produce extreme risk ratings when the consequence of failure would likely be life-threatening or cause major property damage. The three-component model allows arborists to explain risk ratings objectively and identify which component(s) drive risk — helping owners make informed decisions about mitigation options that address the most significant contributing factor.

Question 2: What is a 'target' in tree risk assessment terminology, and how does target occupancy affect overall risk rating?

A specific defect in the tree structure that the arborist is trying to locate during the assessment
People, property, or other valued objects that could be struck if a tree part fails (Correct answer)
The geographic point directly beneath the center of mass of a tree being assessed
The intended pruning cut location that the arborist designates as the assessment endpoint

Correct answer: People, property, or other valued objects that could be struck if a tree part fails

In TRAQ, a target is any person, property, or structure that could be struck by a failing tree part. Target occupancy — how frequently and for how long the target is present in the impact zone — directly scales the likelihood component of risk, since failure causes harm only when something is present to be struck.

Target identification and occupancy assessment is a critical and often underemphasized component of tree risk assessment. A target is defined as any person, property, or object that could be struck by a tree or tree part if it falls. The categories include people (by far the most important), structures (occupied buildings, vehicles, utility lines), and other valued assets. Target occupancy is the frequency and duration that a specific target is likely to be present in the impact zone (also called the target zone or strike zone) under normal conditions. A sidewalk under a potentially failing tree might be occupied for 8-12 hours per day in a busy urban area, while a remote hiking trail through forest might have only occasional human presence for an hour or two per week. These scenarios produce dramatically different risk ratings even for identical tree defects. The TRAQ matrix combines probability of failure and consequences (which incorporates impact likelihood and severity) to produce an overall risk rating. Trees over high-occupancy targets with high consequences of impact receive higher risk ratings and warrant more urgent intervention. Trees over very low-occupancy targets (rarely visited parks, remote forest areas) with lower consequences can tolerate higher failure probabilities before reaching the same risk threshold. Determining target occupancy requires careful observation and sometimes investigation beyond what is visible during the assessment visit. School playgrounds are empty on weekends but heavily occupied weekdays; parks may have seasonal use patterns; construction sites have target occupancy determined by work schedules. Arborists should document their target occupancy assumptions in risk assessment reports to clarify the basis for risk ratings, which also affects liability exposure if outcomes are later questioned.

Question 3: What is 'trunk flare inspection' and what does its absence or deformity potentially indicate?

A visual inspection of the trunk for fire damage that may have compromised the structural wood
Examination of the base of the trunk to detect buried root flare, girdling roots, or basal decay that indicates structural or root system problems (Correct answer)
An assessment of surface root exposure to evaluate erosion risk and tripping hazard
A measurement of trunk diameter at the widest point to calculate structural capacity

Correct answer: Examination of the base of the trunk to detect buried root flare, girdling roots, or basal decay that indicates structural or root system problems

Trunk flare inspection examines the tree base for the natural widening where trunk transitions to roots. Absence of visible flare (buried or compressed) can indicate deep planting, fill soil, girdling roots, or basal decay — all potentially serious structural or health issues.

The root flare (also called the root collar or stem flare) is the natural, visually distinctive widening of the trunk as it transitions into the structural root system at or slightly below the soil surface. Examining this zone is one of the most diagnostically valuable components of any tree inspection, yet it is frequently neglected because it requires getting close to the base and sometimes moving mulch or debris. A healthy, properly planted tree should show a clear flare — the trunk should become visibly wider as it approaches the ground before transitioning to individual structural roots. If the trunk descends into the soil with no visible flare (appearing like a telephone pole going straight into the ground), several problems could be responsible: the tree may have been planted too deeply (common with container-grown stock where the root ball is fully buried), fill soil or excessive mulch may have buried a proper flare, girdling roots may be compressing and obscuring the flare, or basal decay may have destroyed the natural flare anatomy. When no visible flare is present, arborists should probe or carefully excavate the base to determine the actual flare depth and to look for girdling roots, signs of decay (soft wood, fungal fruiting bodies, discoloration), or other abnormalities. Air spades allow non-destructive excavation without cutting existing roots, making them the preferred tool for thorough root flare investigation. Deformities of the flare — one-sided flare, buttressing only on specific sides, asymmetric basal swelling — can indicate lean compensation, prevailing wind loading history, or localized structural root problems. In risk assessment, any anomaly at the trunk base is significant because structural roots originate at the flare and their integrity is fundamental to the tree's stability and wind-firmness.

Question 4: What does 'failure mode' refer to in tree risk assessment, and what are the primary structural failure modes assessed?

The specific pathogen responsible for wood decay causing structural compromise in the tree
The mechanism by which a tree or tree part fails — including stem failure, branch failure, root failure, and whole tree failure (Correct answer)
The sequence of events from initial defect discovery to eventual complete tree failure
The response of the target to the impact of a failing tree part (e.g., property damage vs. personal injury)

Correct answer: The mechanism by which a tree or tree part fails — including stem failure, branch failure, root failure, and whole tree failure

Failure mode describes how a tree or part fails mechanically — the primary modes are branch failure, stem failure, root failure (overturning), and whole tree failure. Each mode has different defect indicators, risk assessment methods, and mitigation strategies.

Understanding failure modes is fundamental to accurate tree risk assessment because different failure modes have different causal defects, different detection methods, different failure thresholds, and different mitigation options. The ISA TRAQ system categorizes structural failures into four primary modes. Branch failure is the most common tree failure type and involves the fracture or separation of a branch from the tree. Contributing factors include attachment defects (included bark, co-dominant unions), decay within the branch, excessive end weight (from extended growth or ice loading), dead wood, and weakening from borer damage. Branch failures range from nuisance small-branch drops to catastrophic failure of major scaffold branches. Stem failure involves fracture of the trunk or main stem, typically at a point of structural weakness — most commonly a zone of significant internal decay, canker, large wound, or a basal crack. Stems fail in bending (lateral force from wind) or in compression (vertical loading from ice or snow). Stem failure is typically the most consequential failure mode in terms of potential damage. Root failure (overturning or root plate failure) occurs when the root system fails to anchor the tree against lateral forces, causing the entire tree to topple with the root plate rotating out of the ground. Contributing factors include root decay, girdling roots, soil failure (saturated soils), construction damage, and inadequate root volume in restricted urban sites. Overturning often has little visual warning — the crown may appear healthy while the root anchorage system has been significantly compromised. Whole tree failure includes uprooting and stem failure that result in complete tree toppling. Assessment requires evaluating multiple systems simultaneously. Arborists must identify which failure mode is most probable for a given tree, as this determines which structural assessment methods (aerial inspection for branch defects vs. root collar excavation for basal defects vs. soil probing for root zone conditions) are most critical.

Question 5: What is the purpose of resistance drilling in tree risk assessment and what does the instrument measure?

It measures the electrical conductance of trunk wood to detect moisture content changes indicating decay
It measures the drilling resistance of a small probe as it penetrates the wood, creating a profile that reveals density variations indicating decay, cracks, or voids (Correct answer)
It uses sound waves to image the interior of the trunk and produce a tomographic cross-section
It extracts a small core sample from the trunk that is then laboratory-analyzed for pathogen identification

Correct answer: It measures the drilling resistance of a small probe as it penetrates the wood, creating a profile that reveals density variations indicating decay, cracks, or voids

Resistance drilling (e.g., IML Resistograph) inserts a thin needle into the wood and continuously records the torque required to drill through successive layers — healthy dense wood produces high resistance, while decayed wood, voids, and cracks produce characteristic low-resistance profiles.

Resistance drilling is a minimally invasive diagnostic tool used to detect internal decay, voids, and structural defects in standing trees that cannot be detected by visual inspection or basic probing. The most widely used instrument is the IML Resistograph, which inserts a thin (typically 1.5-3mm diameter) high-speed rotating needle into the wood and continuously records the amplitude of resistance to penetration through an electronic or paper trace. As the needle passes through wood of varying density and condition, the resistance trace reflects the material it encounters. Sound, dense heartwood and sapwood produce high-amplitude resistance traces. Decay-softened wood produces reduced amplitude. Voids (hollow centers or cavities) produce zero resistance. Cracks appear as brief drops to zero. The resulting trace, when read by a trained practitioner, provides a qualitative cross-section profile of the wood from bark to bark along the drilling path. The technique has important limitations that arborists must understand. Resistance drilling detects density differences but cannot distinguish between sound low-density heartwood (some species naturally have soft, low-density heartwood) and incipient decay — biological knowledge of the species is required to interpret results. A single drill path provides only a linear sample — eccentric or non-uniform decay may not be detected in all cases, and multiple readings at different heights and angles improve assessment accuracy. Resistance drilling is typically used in conjunction with visual inspection, probing, and possibly sonic tomography for complex assessments. The drill holes created are small (1.5-3mm) and compartmentalize readily in healthy trees — they do not cause significant harm. However, drilling into active decay can potentially serve as inoculation pathways for other pathogens in rare circumstances, and the frequency of drilling should be considered in relation to information gained.

Question 6: What is the minimum training requirement for an arborist to perform a Level 2 Tree Risk Assessment under ISA TRAQ standards?

Any ISA Certified Arborist with at least 5 years of field experience in tree care
Completion of the ISA TRAQ training and passing the associated written examination, with ongoing ISA certification maintenance (Correct answer)
A state-issued tree hazard assessment license plus ISA Certified Arborist credentials
A master's degree in arboriculture or urban forestry plus ISA Board Certified Master Arborist credential

Correct answer: Completion of the ISA TRAQ training and passing the associated written examination, with ongoing ISA certification maintenance

ISA TRAQ (Tree Risk Assessment Qualification) requires completing an ISA-approved TRAQ training course and passing the written examination. Maintaining TRAQ qualification also requires maintaining ISA Certified Arborist credentials and earning continuing education units.

The ISA Tree Risk Assessment Qualification (TRAQ) program was developed to standardize the credentials and methodology used by tree risk assessors. Prior to TRAQ's establishment, there was significant variability in how 'hazard tree assessments' were conducted and what credentials assessors needed to hold, creating challenges for liability determination, communication with clients, and legal defensibility. TRAQ training covers the ISA's standardized risk assessment methodology based on the publication 'Tree Risk Assessment Manual.' The training includes instruction on the risk matrix framework, identification of defects and structural conditions, assessment level definitions (Level 1 = walk-by visual inspection, Level 2 = detailed visual inspection, Level 3 = advanced methods including diagnostic tools), documentation requirements, risk rating categories, and mitigation recommendation standards. Passing the TRAQ examination certifies that the holder has demonstrated foundational knowledge of the TRAQ methodology. Maintaining TRAQ qualification requires maintaining the underlying ISA Certified Arborist credential (which requires ongoing CEU accumulation) and is typically listed separately on an arborist's ISA credential record. It's important to note that TRAQ qualification does not guarantee competence in all tree species, all defect types, or all complex assessment situations. TRAQ is the minimum qualification baseline — complex high-value assessments often involve arborists with additional experience, Board Certified Master Arborist credentials, or specialized training in specific diagnostic methods (sonic tomography, ground penetrating radar). Expert witnesses in tree failure litigation are typically required to demonstrate credentials beyond basic TRAQ qualification.

Certified Arborist Test Tree Risk Assessment 2

In the ISA Tree Risk Assessment Qualification (TRAQ) framework, what are the three components of tree risk?