Tree work words: tree risk and decay
Targets, defects, loading, decay types and wood failure words, each defined from the QTRA glossary, Penn State and Colorado State extension pages and OSHA.
Checked against the sources at the bottom of this page on October 8, 2026. Rules, fees and pay change: the source has the last word.
Tree risk reports and decay talk use words that sound plain but have set meanings in the glossaries. This page lists those meanings as the named sources give them. It does not say what any feature means in a particular tree, and it does not say whether a tree is safe or unsafe.
Every meaning is written in our own words from the page named in its row. A word that none of those pages defines is not listed. The wider A to Z list is the tree work glossary.
Words and meanings
| Term | Meaning | Source |
|---|---|---|
| Tree risk assessment | The QTRA glossary describes weighing up the risks, and where relevant the values, tied to a tree or group of trees. Its main concern is falling trees. It adds that damage to infrastructure, interrupted services and building subsidence may also be considered. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Walkover, drive-by, individual | The QTRA glossary lists three scales of assessment. A walkover is a general look at the trees of an area against the neighbouring land use. A drive-by is the same look from a moving vehicle. An individual assessment covers one tree in its setting. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Target | In the QTRA glossary, the people, property or other valued things that a tree failure or a falling object could hurt. The glossary notes this stretches the usual meaning of the word. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Probability | In the QTRA glossary, a statistical measure of how likely a particular event is. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Defect | In the QTRA glossary, used for tree hazards: any feature that spoils the even spread of mechanical stress or leaves the tree poorly suited to where it stands. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Danger tree | In OSHA's logging standard, a standing tree that is a hazard to workers, for reasons such as damaged or deteriorating roots, trunk, stem or limbs, and the way the tree leans. | OSHA logging standard 1910.266, definitions and felling rules |
| Incipient failure | In the QTRA glossary, a failure in wood tissue that shows up only as deformation or cracking, without the affected part falling or detaching. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Loading | In the QTRA glossary, the force a structure carries from one source, such as its own weight or wind pressure. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Lever arm | In the QTRA glossary, how long the lever is that a structure forms when it can move freely at one end, as a tree or branch does. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Buckling | In the QTRA glossary, permanent deformation of a structure under bending load. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Wind exposure | In the QTRA glossary, how much wind a tree or other object meets, in both duration and speed. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Windthrow | In the QTRA glossary, a tree blown over at its roots. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Included bark (ingrown bark) | In the QTRA glossary, bark from neighbouring parts of a tree, usually forks or sharply joined branches, pressed face to face. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Compression fork | In the QTRA glossary, a sharp-angled fork shaped by the growth pressure its stems put on each other. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Hazard beam | In the QTRA glossary, an upward-curving part of a tree where strong internal stress can build up that adaptive growth does not relieve, and which tends to split along its length. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Delamination | In the QTRA glossary, wood layers coming apart along their length, which shows as splitting. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Bottle-butt | A stem base and buttresses broader than normal. The QTRA glossary says this can sometimes mark a growth response to weakness there, especially from decay of the selective delignification kind. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Rib | In the QTRA glossary, a ridge of wood that usually forms where mechanical loading has locally increased, often linked to internal cracking. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Lions tailing | In the QTRA glossary, a branch bare of side branches except at its tip, which it says can snap under that end load. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Deadwood | In the QTRA glossary, branch or stem wood with no living tissue. It says keeping it gives habitat, and that removing it can open sound tissue to decay. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Minor deadwood | In the QTRA glossary, deadwood under 25 millimetres across, or too small to do real harm or damage if it hit a target below the tree. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Snag | In the UK glossary, the part of a cut or broken stem, branch or root that sticks out past a growing point or dormant bud. In the US logging standard it means any standing dead tree or part of one. | QTRA glossary of tree terms, PDF with 1999 extracts; OSHA logging standard 1910.266, definitions and felling rules |
| Residual wall | In the QTRA glossary, the wall of wood left undecayed after the inside of a stem, branch or root has rotted. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Decay | Penn State Extension describes decay-causing fungi invading the wood at wounds, and says trees can only defend by walling off damaged wood and giving up those cells. | Penn State Extension, spread of decay in trees |
| White rot | A wood decay in which lignin, usually with cellulose and other constituents, is broken down, in the QTRA glossary. Penn State Extension says the remaining wood is light-coloured, soft and spongy or stringy. | QTRA glossary of tree terms, PDF with 1999 extracts; Penn State Extension, spread of decay in trees |
| Brown rot | A wood decay in which cellulose is broken down while lignin is only modified, in the QTRA glossary. Penn State Extension says the remaining wood is brown, dry, fragile and crumbles into cubes. | QTRA glossary of tree terms, PDF with 1999 extracts; Penn State Extension, spread of decay in trees |
| Soft rot | A wood decay in which a fungus breaks down cellulose within the cell walls, in the QTRA glossary. Penn State Extension says bacteria and fungi cause it and that fungi grow slower than in white or brown rots. | QTRA glossary of tree terms, PDF with 1999 extracts; Penn State Extension, spread of decay in trees |
| Selective delignification | In the QTRA glossary, white-rot decay in which lignin is broken down faster than cellulose. It names stringy white-rot as the result. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Simultaneous white-rot | In the QTRA glossary, a wood decay in which lignin and cellulose break down together at about the same pace. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Target canker | In the QTRA glossary, a lasting canker with concentric rings of dead occluding tissue. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Cavity | Penn State Extension lists cavities and nesting holes in a trunk among its signs that decay is expanding in a tree, alongside conks and mushrooms. It says filling cavities with concrete or foam gives no structural benefit. | Penn State Extension, spread of decay in trees |
| Reaction zone | In Colorado State Extension's publication, the area of chemical change around damaged tissue that seals it off. In most species it shows as darker wood. | Colorado State University Extension, tree growth and decay |
| CODIT (walling off decay) | Colorado State Extension says the spread of decay is controlled by four walls built from annual growth rings and ray cells, and names the concept CODIT. Penn State Extension describes the same four walls and says wall 4 is the strongest. | Colorado State University Extension, tree growth and decay; Penn State Extension, spread of decay in trees |
| Occlusion, occluding tissues | In the QTRA glossary, occlusion is a wound being closed over bit by bit by new wood and bark, and occluding tissues are the roll of wood, cambium and bark that builds up around the wound. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Ram's-horn | In the QTRA glossary, used for tree wounds: a roll of occluding tissue that shows a spiral in cross-section. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Increment borer | In the QTRA glossary, a hollow auger that takes wood cores, which can be used to count or measure growth increments or to inspect the condition of the wood. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Microdrill | In the QTRA glossary, an electronic rotating steel probe that, pushed into woody tissue, gives a reading of tissue density. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Root-collar examination | In the QTRA glossary, digging away surfacing and soil around the root collar to judge the structural soundness of the roots or stem. | QTRA glossary of tree terms, PDF with 1999 extracts |
| Veteran tree | In the QTRA glossary, a tree that by recognised criteria has biological, cultural or aesthetic value from surviving beyond the usual age range for its species, with features such as large girth, crown retrenchment and a hollowing stem. | QTRA glossary of tree terms, PDF with 1999 extracts |
Reading these sources
Most entries come from a glossary hosted by QTRA, which incorporates extracts from a 1999 UK government publication on tree hazard assessment. It is old, and its words are British, so some terms such as snag differ from the OSHA meaning in the US logging standard. Penn State Extension and Colorado State University Extension supply the decay and compartmentalization entries. The decay-type words have a shared meaning across the extension pages, and the rows say where a page gives details.
The glossaries define the words only. How a tree risk assessment is carried out is covered by the credential and method pages: see tree risk assessment by country. Words for living wood, roots and branch unions are on tree work words: tree biology and plant health.
Sources
- QTRA, Glossary of Arboricultural Terms (incorporating extracts from Lonsdale 1999, HMSO), PDF uploaded June 2023, read October 8, 2026
- OSHA, 29 CFR 1910.266 Logging operations, definitions and felling rules, read October 8, 2026
- Penn State Extension, Understanding the spread of decay in trees, updated November 10, 2024, read October 8, 2026
- Colorado State University Extension, Tree growth and decay, read October 8, 2026