Co-Dominant Stems and Included Bark: The Structural Defect That Splits Trees Apart

A healthy tree usually has one dominant central leader, a single main stem that the rest of the canopy organizes itself around. Sometimes, though, two stems of roughly equal size rise side by side and compete for that lead position. Arborists call these co-dominant stems, and on their own they are not automatically dangerous. The problem is what often forms in the crotch where those two stems meet. Instead of a strong, open U-shaped union, you get a tight V, and inside that V the bark of each stem grows toward the other and gets pinched in between. That trapped bark is called included bark, and it is one of the most common structural defects we are called to evaluate on mature trees.

To understand why included bark matters, it helps to know how a normal branch union gets its strength. As wood lays down each year, the fibers of the two stems should interlock and fuse across the joint, the way two ropes braided together are stronger than either alone. A wide-angled union gives the wood room to do this. A tight, narrow union does not. When bark is trapped in the seam, the two stems never truly fuse. They simply press against each other with a thin, weak layer of dead bark in the middle. The union looks solid from the outside, but mechanically it is closer to two trees leaning on one another than one tree branching cleanly.

The danger is that this defect stays hidden until the loads get high. On a calm day the union holds fine. Then a heavy snow load, a saturated canopy after days of rain, a sudden wind gust, or just decades of accumulated weight finds the weak seam, and the tree splits straight down the middle of the trunk. These failures are sudden and often catastrophic because so much of the canopy is attached to each stem. A split co-dominant trunk can drop half a mature tree onto a roof, a driveway, or a fence line with very little warning, and the torn wood that remains is usually not salvageable.

Learning to spot the defect from the ground is straightforward once you know what you are looking at. Stand back and find the point where the trunk divides into two large, similarly sized stems. Look at the angle between them. A wide, rounded, open angle is reassuring. A narrow, sharp V where the two stems run nearly parallel is a warning sign. Then look closely at the seam itself. A normal union has a raised ridge of bark, called the branch bark ridge, pushing up and out of the crotch. With included bark you often see the opposite: a seam that turns inward, sometimes with a dark crack or a vertical line where moisture, leaves, or debris collect. Cracks, bulges, or a section of dead or peeling bark right at the union all raise the concern level.

What can be done depends entirely on the size and age of the tree. The best fix is prevention through early structural pruning. On a young tree, an arborist can subordinate one of the two competing stems, cutting it back so the other clearly becomes the leader, and the tree develops a single strong trunk over the following seasons. This is cheap, low-risk, and far more effective than anything available later. On a mature tree where two large stems are already established, removal is no longer practical, so the options shift to risk reduction: reducing the weight and length of one or both stems to lower the leverage on the union, or installing a cable or brace between the stems to limit how far they can move apart in a storm.

Cabling and bracing are real tools, but they are not magic. A support system reduces the chance of failure and can buy a valuable tree many more years, yet it does not repair the defect, and it has to be inspected and maintained on a schedule. Hardware can loosen, the tree continues to grow around it, and a neglected cable can do more harm than good. That is why the decision to install support should follow a proper risk assessment that weighs the value of the tree, the size of the defect, and what sits underneath it, rather than being a reflex applied to every forked tree.

The practical takeaway for any property owner is to walk your larger trees and simply look at how they fork. A tight V with an inward seam, especially on a big tree over a target like a house or a play area, is worth a professional look. Caught early on a young tree, a co-dominant stem is a five-minute pruning decision. Left alone for thirty years, it becomes the kind of failure that takes out half a canopy in a single storm. A trained eye can tell you which category yours is in and what, if anything, needs to happen this season.