Structural defects in trees: what the standards and agencies describe

This page compares what published sources say about structural defects in trees. It gives no verdict on any tree and is not an assessment. The sources come from the United States and the United Kingdom.

How the sources define a defect

The National Tree Safety Group guidance defines a defect as a condition that predisposes a tree or part of it to structural failure, and also uses the term obvious defect with a list of signs. The US Forest Service brochure lists seven defect types and says a defect matters where a target is within striking distance. The University of Florida extension publication also lists seven conditions. The ISA Trees Are Good leaflet gives a list lettered A to H.

The conditions with pages here

Poor taper appears only in the ISA leaflet, as item A of its list: branches or stems with their weight concentrated near the end are more prone to failure. No second source read for this page describes it, so it has no page of its own.

About the sources

The US Forest Service brochure (NA-FR-01-96) is a federal publication from the Northeastern Area, read here through an archived copy. The ISA leaflet is the 2021 edition and the National Tree Safety Group guidance is the 2024 edition. The sources were written for different readers and do not use one shared list, so the same condition can sit under different headings. Each condition page names the sources that cover it and keeps their wording attributed. A page here describes a condition; it does not tell you whether a tree has it.

Who looks at this

For the methods and the credentials, see tree risk assessment methods and consulting arborist credentials. Decay is covered on wood decay fungi on trees.