ISA - ISA Certified Arborist Tree Risk Assessment 2 — Questions and Answers
Question 1: An arborist is evaluating a large silver maple that has a visible crack running along the trunk but otherwise appears healthy with a full canopy. The tree overhangs a busy playground used daily. When determining the likelihood of failure for this tree, which factor is the arborist primarily evaluating?
- The probability that the defect will lead to part or all of the tree failing under a defined load condition (Correct answer)
- The number of people who use the playground per day
- The financial cost of removing the tree versus treating it
- The seasonal variation in the tree's canopy density
Correct answer: The probability that the defect will lead to part or all of the tree failing under a defined load condition
In ISA TRAQ methodology, 'likelihood of failure' refers specifically to the probability that a tree or tree part will fail given the presence of a structural defect under a defined loading condition (e.g., normal weather, wind event). Target occupancy and financial factors are separate components of risk assessment.
Question 2: A risk assessor is completing a Level 2 Basic Assessment on a row of oak trees along a school property. After identifying a hazardous tree requiring immediate attention, the assessor recommends a specific mitigation action. According to ISA standards, which of the following is NOT a recognized tree risk mitigation option?
- Pruning to reduce the size or weight of a branch
- Installing structural support systems such as cables or braces
- Increasing irrigation to improve the tree's overall vigor (Correct answer)
- Removing the tree entirely
Correct answer: Increasing irrigation to improve the tree's overall vigor
ISA TRAQ recognizes mitigation options that directly address structural defects or target zones: pruning, cabling/bracing, and removal. While improving tree vigor through irrigation may benefit long-term health, it is not a recognized mitigation technique for an identified structural risk in the ISA framework.
Question 3: During a risk assessment, an arborist notices that a large elm tree has a significant trunk wound on the leeward side relative to the prevailing wind. The tree has a slightly asymmetrical canopy with more mass on the windward side. How does this combination of factors most directly influence the risk assessment?
- It increases the likelihood of failure because wind loading is amplified toward the side with the structural defect (Correct answer)
- It decreases the likelihood of failure because the wound is sheltered from direct wind pressure
- It has no effect because trunk wounds are assessed independently of canopy distribution
- It only affects the consequence of failure, not the likelihood
Correct answer: It increases the likelihood of failure because wind loading is amplified toward the side with the structural defect
When a tree has greater canopy mass on the windward side, it experiences greater sail effect and mechanical loading in that direction. If the structural defect is on the leeward side and the tree leans or has mass distribution toward the wind, the cumulative stress on the defect is increased, raising the likelihood of failure.
Question 4: An ISA TRAQ-qualified arborist completes an assessment and assigns a risk rating of 'High' to a mature cottonwood. The property owner declines the recommended action of removal and asks the arborist to reassess in six months. What is the arborist's most appropriate professional response?
- Document the owner's decision, communicate remaining risk in writing, and note the agreed reassessment date (Correct answer)
- Immediately notify local authorities since the owner has created a public liability
- Lower the risk rating to 'Moderate' to reflect the owner's acceptance of the risk
- Refuse further involvement until the owner agrees to removal
Correct answer: Document the owner's decision, communicate remaining risk in writing, and note the agreed reassessment date
When an owner declines a recommended mitigation, the arborist's professional obligation is to clearly communicate the residual risk in writing, document the owner's decision, and establish a follow-up timeline if the owner chooses to monitor rather than act. Altering the risk rating to match the owner's preference would be professionally and ethically incorrect.
Question 5: When evaluating a tree's root zone during a risk assessment, an arborist discovers that approximately 40% of the root plate on the windward side has been severed by a recent utility trench. Which structural failure mode is now the primary concern for this tree?
- Uprooting or windthrow due to reduced root plate anchorage (Correct answer)
- Stem failure at the base due to internal decay
- Branch failure from end-weight imbalance
- Crown fracture due to structural weakness in scaffold branches
Correct answer: Uprooting or windthrow due to reduced root plate anchorage
Severing roots on the windward side significantly compromises the root plate's ability to anchor the tree against wind loading. The primary resulting failure mode is uprooting (windthrow), where the tree tips over from the base rather than breaking at the trunk or branches.
Question 6: An arborist is rating the 'consequences of failure' component for a tree located adjacent to a single-family home. The tree's crown extends over the roof but the trunk is 15 meters from the structure. Which factor is most critical when estimating the consequence of failure in this scenario?
- The size and weight of the tree part most likely to fail and its proximity to the target (Correct answer)
- The age and market value of the home
- The time of year and expected weather patterns
- The species of the tree and its historical failure rates
Correct answer: The size and weight of the tree part most likely to fail and its proximity to the target
Consequence of failure in ISA TRAQ is primarily determined by the size and energy of the falling tree part combined with the vulnerability of the target it could strike. A large, heavy branch or stem falling on an occupied structure would produce severe consequences regardless of property value, season, or species statistics.
ISA - ISA Certified Arborist Tree Risk Assessment 2
An arborist is evaluating a large silver maple that has a visible crack running along the trunk but otherwise appears healthy with a full canopy.
The tree overhangs a busy playground used daily.
When determining the likelihood of failure for this tree, which factor is the arborist primarily evaluating?
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