Included bark is a structural defect that occurs at the junction of two branches or, more commonly, at the fork between two competing stems, known as codominant stems. This condition is a significant concern in arboriculture because it creates a point of structural vulnerability, increasing the risk of tree failure. Included bark describes an unnatural growth pattern where the tree’s outer bark layers are physically trapped within the union as the stems grow in diameter. While a healthy union is robust, included bark makes the tree susceptible to splitting, especially during adverse weather conditions.
Defining Included Bark
Included bark is defined by the physical presence of bark tissue within the attachment of two woody parts, typically where a branch meets the trunk or where two main trunks meet. This defect arises when two stems grow in close proximity, forming a narrow, often V-shaped, crotch angle. When a tree forms a healthy union, the wood fibers overlap and interlock, creating a strong, fused joint, often visible as a U- or Y-shape. Included bark involves two layers of non-structural bark pressed tightly against each other inside the union, separating the wood tissue of the two growing parts.
The physical result is an unattached seam instead of a continuous, integrated wood structure. Bark serves a protective, not a structural, function, lacking the tensile strength of wood fibers. Because bark is trapped, the two stems are not structurally united by interlocking growth rings. The union is held together only by the friction and compression of the surrounding wood, resulting in a fundamentally weaker connection.
The Mechanism of Formation
Included bark is a direct result of competing growth in a confined space, due to the formation of a tight union angle. Trees increase their girth through the vascular cambium, a thin layer of cells that annually produces new wood (xylem) inward and inner bark (phloem) outward. In a healthy, wide-angled junction, the cambial layers meet and merge seamlessly, allowing new wood fibers to grow over and around the attachment, forming a strong branch collar.
When two stems are too close, the cambial layers are forced to grow against each other, pushing the outer bark inward. As the diameter of both stems increases, the layers of new wood formation compress the bark between them. This prevents the cambium from producing continuous wood across the junction. The bark, which is not designed to bear load, becomes buried and incorporated into the center of the union as the stems expand, leading to a weak, non-fused joint.
Structural Weakness and Failure Potential
Included bark creates a profound mechanical disadvantage for the tree, leading to a high potential for failure. Unlike a proper union where wood fibers interlace, the included bark acts as a shear plane, a point where the two stems are separate and can easily slide past each other. Attachments with included bark are significantly weaker than those with normal, fused unions. The bark layers cannot withstand the forces exerted on the tree, making the union the weakest point in the structure.
This structural vulnerability is most evident during periods of high mechanical stress, such as strong winds, heavy snow loads, or thick ice accumulation. The added weight and leverage from these environmental factors place a splitting force on the union. Since the two stems are not truly connected by wood, this force causes the union to tear apart, often resulting in one or both stems failing catastrophically. Failure frequently originates at the plane of the included bark, leading to a complete, sudden split down the center of the V-crotch.
Identification and Mitigation Strategies
Identifying included bark often starts with observing the angle of the attachment. Unions that form a sharp V-shape are highly likely to contain included bark, while those with a wider U- or Y-shape are stronger. Visual cues include a ridge or seam of bark that appears pinched or compressed down the center of the crotch, sometimes accompanied by a slight bulge around the union point. The lack of a visible branch collar—the swollen ring of wood at the base of a healthy branch—is another reliable indicator of this defect.
For young trees, the most effective mitigation is proactive structural pruning. This involves reducing one of the competing stems to favor the other, allowing the remaining stem to develop into a stronger leader. In larger, mature trees, corrective pruning can reduce the weight and wind sail on the affected stems, lowering the stress on the weak union. For high-risk, mature trees, arborists may recommend installing supplemental support systems. These systems, such as non-invasive cabling or bracing, physically limit the movement of the stems and prevent splitting.