Included Bark
Included bark develops when two branches meet at a very narrow angle, so bark rolls inward and is trapped as a wedge between them. The USDA Forest Service guide How to Prune Trees says it prevents strong attachment and often causes a crack below the point where the branches meet.
Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry
Where it forms
Narrow, V-shaped unions often form included bark. So do codominant stems: stems of about the same size that arise from the same position. Branches with strong, U-shaped attachments are the ones to keep.
Why it matters
The Forest Service climbing guide describes a weak union as one where branches of similar size grow so close together that bark grows inside the union. The included bark can act as a wedge and force the union apart when it is loaded, so climbers inspect every union at or below a planned tie-in point.
Source: National Tree Climbing Guide (2015 Electronic Edition)
What to do about it
On young trees, act early. Rather than cutting a codominant stem off at the union, Alabama Extension says it is typically better to subordinate it, and the Forest Service guide notes that removing some lateral branches from one codominant stem can slow it enough for the other to become dominant.
On mature trees, a union with included bark is a defect to weigh in a tree risk assessment, especially if cracks or decay are also present.
Sources: Assessing the Structure of Trees in the Landscape ; How to Prune Trees (NA-FR-01-95, revised 2012)
How the ISA arborist exam tests it
Included bark shows up in three domains: tree risk (how likely the union is to fail), safe work (tie-in points) and pruning (how to correct codominant leaders). Try 3 reviewed practice questions:
Which of these branch or stem unions has the highest likelihood of failure?
Show answer and explanation
Correct answer: A. A V-shaped union with included bark and an open crack below it
Narrow V-shaped unions often contain included bark, which significantly weakens the junction. When an open crack, decay or conks are also present, the weakness has progressed toward failure: the USDA Forest Service field guide for Oregon and Washington assigns high failure potential to a V-shaped fork that has embedded bark plus an open crack, decay or conks, and medium failure potential to the same kind of fork without them. A U-shaped union, or a branch much smaller than the trunk, forms a stronger attachment.
- B: Included bark alone is a weakness, but adding an open crack raises the likelihood of failure.
- C: A U-shaped union without included bark is the stronger form.
- D: A branch much smaller than the trunk is more strongly attached than codominant stems.
Sources: Field Guide for Hazard-Tree Identification and Mitigation on Developed Sites in Oregon and Washington Forests (R6-NR-TP-021-2013), USDA Forest Service, Pacific Northwest Region (GovInfo) ; Is My Tree Safe? Recognizing Conditions that Increase the Likelihood of Tree Failure (ENH1246), UF/IFAS Extension
Report a problem with this question · ID risk-022
While choosing a tie-in point, a climber notices two upright stems of similar size with bark pressed into the union between them. How should this union be judged?
Show answer and explanation
Correct answer: D. As weak, because included bark keeps the stems from being well attached
Bark trapped inside the union of two similar-sized upright stems is called included bark. It prevents a solid wood connection and can act as a wedge that forces the union apart when it is loaded, so the union is weak. The Forest Service climbing guide tells climbers to inspect all unions at or below the tie-in point before choosing one, and this union should not be trusted as an anchor.
- A: Included bark does not reinforce a union; it separates the stems and can wedge them apart under load.
- B: Live unions with included bark are a recognized weak point; failure is not limited to dead branches.
- C: Diameter alone does not determine union strength; how well the stems are joined matters, and included bark weakens the joint.
Sources: National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service ; National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service ; National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service
Report a problem with this question · ID safe-005
A young shade tree has two upright codominant leaders of nearly equal size, and removing either one at the union now would leave a large wound. Which approach is generally better?
Show answer and explanation
Correct answer: D. Shorten one leader with a reduction cut so the other becomes dominant
Rather than removing a codominant stem at the union, extension guidance favors subordinating it: a reduction cut shortens and slows one leader so the other can take over as the dominant trunk, without a large wound at the union. Equal thinning keeps the two leaders competing, and a cable does not change which leader dominates. Waiting for a crack lets a known defect become a failure risk.
- A: Thinning both leaders equally leaves them in competition, so the weak union keeps developing.
- B: A cable does not correct the structure; subordination pruning is the tool for that.
- C: Waiting for a crack allows a known defect to become a failure risk.
Sources: Assessing the Structure of Trees in the Landscape, Alabama Cooperative Extension System ; Making the Cut: It Matters When Pruning Trees, Penn State Extension
Report a problem with this question · ID prun-019
0 of 3 correct
Common mistakes in the field
Letting codominant leaders grow on
Two equal leaders on a young tree only get harder to correct. Subordinate one while the wounds are still small.
Tying in at a weak union
A union with bark inside it can split under a climber’s load. Inspect unions at and below the tie-in point first.
Keeping V-shaped branches when thinning
When thinning, the narrow V-shaped attachments are the ones to remove, not the strong U-shaped ones.
Sources
- How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry
- National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service
- Assessing the Structure of Trees in the Landscape, Alabama Cooperative Extension System
Written in our own words from the public sources above, checked October 19, 2026. Arborist Practice Test is not affiliated with ISA.