Codominant stems and included bark: what the standards and agencies describe

This page describes what published sources say about one structural condition 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. The parent page, structural defects in trees, compares the conditions that have pages here.

What the sources describe

The US Forest Service brochure lists weak branch unions among its defect types. It describes bark that grows between two similar upright branches, which it says acts as a wedge, and notes that this bark is called included bark. The Illinois Extension article says the same of codominant stems: where stems of similar size grow from one point, the included bark inside the union acts as a wedge.

The University of Florida extension publication says V-shaped unions are more likely to fail than U-shaped unions, and that the likelihood rises as the stems approach the same diameter. The ISA Trees Are Good leaflet lists the condition as one item in its list of conditions to look for.

Species the sources name

Illinois Extension names Callery pear, maple, ash and elm. The US Forest Service brochure says elm and maple often form them. The University of Kentucky leaflet names maples, elms and pears that have multiple branches from one point.

Topping and weak unions

The US Forest Service brochure says topping makes weak unions. That is the brochure's statement about how such unions form.

Where the sources add a caution

The National Tree Safety Group guidance says that not every unusual form is a defect. A defect there is a condition that predisposes a tree or part of it to structural failure. So the examples above are descriptions of a condition, not a finding about a particular tree.

Who looks at this

For the methods and the credentials, see tree risk assessment methods and consulting arborist credentials.