ISA TRAQ flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Definition of tree risk

Tree risk is the combination of the likelihood of failure, the likelihood of impact on a target, and the consequences of that impact. All three must be assessed together.

What is a target

A target is a person, property, or activity that could be struck by a failing tree or tree part. Without a target, there is no risk even if a defect is severe.

Occupancy rate

The proportion of time a target is present in the fall zone. A target occupied 8 hours a day, 5 days a week has roughly a 24 percent occupancy rate, lowering impact likelihood.

Likelihood of failure scale

A five-level qualitative scale: improbable, possible, likely, probable, and imminent. It reflects the tree's condition, species, and site factors, not just the presence of a defect.

Likelihood of impact scale

A four-level scale: very low, low, medium, and high. It is derived from the occupancy rate and the value and use of the target within the fall zone.

Consequence categories

Consequences are rated negligible, minor, significant, or severe. Ratings consider potential injury, property damage cost, and disruption of the target's use or access.

Level 2 basic assessment

A 360-degree visual inspection of the tree from the ground, often with binoculars, recording defects, targets, and site conditions. It is the standard assessment level for most risk evaluations.

Level 3 advanced assessment

An inspection conducted with climbing, aerial lift, or other means to examine parts of the tree not visible from the ground, such as upper crown unions and cavities.

CODIT four walls

Compartmentalization of decay in trees uses four walls: wall 1 plugs vessels, wall 2 toughens latewood, wall 3 resists via ray cells, and wall 4 forms new wood.

Tension versus compression wood

Tension wood forms on the upper side of leaning stems in angiosperms. Compression wood forms on the lower side in gymnosperms. Both are corrective responses to lean or load.

Sound shell thickness ratio

The ratio of sound wood thickness to stem radius (t/R). A value below about 0.3 is commonly used as a threshold that warrants advanced assessment of the tree.

Included bark union

Bark trapped between two codominant stems creates a weak attachment. Indicators include a V-shaped union, a bark ridge turning inward, and swelling at the union.

Shear versus radial cracks

Shear cracks run longitudinally and indicate torsional failure. Radial cracks run across the stem. Separation of crack faces signals active, progressing failure.

Root plate failure signs

Soil mounding or cracking on the tension side of a lean, exposed roots, and a recent increase in lean indicate root plate movement and possible imminent failure.

Brown rot versus white rot

Brown rot leaves brittle, cubical residue and mainly removes cellulose. White rot leaves spongy or fibrous residue and removes lignin as well as cellulose.

Codominant stem risk

Two stems of similar diameter sharing a union are codominant. The union is weaker than a branch union, especially when included bark is present, and failure risk rises with stem size.

Resistograph function

A resistograph measures drilling resistance through wood. Density changes from decay or cavities produce a resistance profile, but the output is relative, not an absolute strength measure.

Sonic tomography principle

Sonic and ultrasonic tomography measures stress wave velocity through wood. Lower velocity indicates decay, cavities, or cracks because sound travels slower through less dense or discontinuous material.

Tomography output limits

Tomography produces a two-dimensional cross-sectional image of relative velocity. Results are qualitative and should be confirmed with visual assessment and other diagnostic methods.

Increment borer use

An increment borer extracts a wood core to verify decay extent, ring width, and growth history. It is invasive and leaves a wound that can admit pathogens.

Static load test

A controlled pulling force is applied to the stem while trunk deflection and root plate movement are measured. It evaluates anchorage and stem stiffness under known load.

Visual tree assessment first

Visual tree assessment identifies swelling, ribs, seams, and hollows that indicate internal defects. It is the first-line method and precedes any instrument-based testing.

Dynamic stem testing

An elastometer or similar device measures stem deflection under a known load to estimate modulus of elasticity. It is useful for comparing trees and tracking change over time.

Diagnostic tool limitation

No single tool measures wood strength directly. Advanced diagnostics estimate the presence and extent of defects and must be interpreted with visual findings and species biology.

Risk rating matrix

The matrix combines likelihood of failure with likelihood of impact to produce a risk category from negligible to extreme. It is qualitative and ordinal, not a numeric probability.

Risk category actions

High and extreme risk typically justify immediate action. Moderate risk warrants mitigation or monitoring. Low and negligible risk generally require only periodic re-inspection.

Mitigation hierarchy

Options include target removal or relocation, pruning, cabling and bracing, root collar excavation, soil management, and tree removal. The least intrusive effective option is preferred.

Residual risk

Residual risk is the risk remaining after mitigation. The assessor documents it and communicates that mitigation reduces but does not eliminate the risk of failure.

Who sets risk tolerance

The tree owner or manager sets risk tolerance, not the assessor. The assessor provides the rating and mitigation options, and the owner decides whether to accept residual risk.

Report contents

A report includes tree location and species, inspection level and date, observed defects, targets, likelihood ratings, consequences, risk category, and recommended mitigation with a timeframe.

Re-inspection intervals

Intervals are set from risk category and growth rate. High-risk trees often need 6-12 month intervals, while low-risk trees may be re-inspected every several years.

Health versus risk

Tree health and tree risk are different. A healthy tree can present high risk if a defect and valuable target exist, and a declining tree may present low risk if no target is present.