Tree Risk Assessment Glossary: Hazard Terms Explained

July 20, 2026

Before budgeting for removal with the tree removal cost calculator, an arborist’s assessment report often uses specific terms to explain why a tree is being recommended for removal rather than treatment or monitoring. Here’s what the most common ones mean.

Structural Defect Terms

Codominant Stems — Two or more trunks of roughly equal size growing from the same point, rather than one dominant trunk with smaller branches. This structure is inherently weaker than a single-trunk tree, especially when combined with included bark (below).

Included Bark — Bark that gets trapped inside a narrow, V-shaped union between codominant stems instead of forming a strong, U-shaped joint. Research published in the ISA’s arboriculture journal has found that codominant stems with included bark are significantly weaker than those without it, making this one of the most commonly cited reasons for recommending removal or cabling of a multi-trunk tree.

Decay — Internal wood breakdown from fungal activity, often invisible from the outside until a cavity, fungal conk (mushroom-like growth), or hollow sound develops. The USDA Forest Service’s hazard tree guidance treats decay exceeding a safe proportion of sound wood in the trunk or major limbs as a key red flag in a hazard rating.

Cavity — A hollow opening in the trunk or a major limb, usually a visible sign of significant internal decay. The size and location of a cavity relative to the tree’s overall diameter factors heavily into a hazard rating.

Root Plate Lift / Heaving Soil — Visible lifting of soil or roots on one side of the trunk base, a warning sign that a tree may be in the process of falling over (or already partially uprooted) even if the canopy still looks healthy.

Risk Assessment Process Terms

TRAQ (Tree Risk Assessment Qualification) — A credential offered through the ISA that trains arborists in a standardized method for rating hazard trees, looking at the likelihood of failure, the likely size of the failing part, and what (or who) is in the potential fall zone.

Target — In risk-assessment terms, whatever a failing tree or limb could hit — a house, a parked car, a walkway, or a person. A tree with a high-likelihood defect but no target nearby (open field, no structures) is often rated lower risk than the same defect on a tree overhanging a home, even though the tree itself is equally likely to fail.

Hazard Rating — An overall risk score combining the likelihood of failure, the size of the part that could fail, and the target’s exposure — used to decide whether a tree needs to come down immediately, needs a follow-up inspection, or can be left alone.

Removal-Adjacent Terms

Difficult Access — Site conditions (proximity to a structure, fence, or power line) that prevent felling a tree whole, requiring section-by-section dismantling instead. See the tree removal cost calculator for how this affects pricing.

Crane-Assisted Removal — Removal method used for very large or tightly hemmed-in trees where sections are lifted out by crane rather than lowered by rope, adding significant cost but reducing risk to the structure below.

Using This Glossary

Match these terms against an arborist’s written assessment or verbal explanation before deciding between removal, cabling, or continued monitoring. Once removal is the confirmed plan, use the tree removal cost calculator to budget for the job, and see DIY vs. professional tree removal: a safety-first guide for why most of the defects above are exactly the kind of thing that make DIY removal considerably riskier than it looks. For how these hazard factors translate into a quoted price, see tree removal cost worked examples.