Tree Defects: What to Look For and What They Actually Mean
Co-dominant stems, cavities, overextended branches, and fungal growth are the most common structural defects homeowners can spot themselves. Seeing a defect does not mean the tree needs to come down. Most defects can be assessed, monitored, and managed by a qualified arborist using diagnostic tools and documented methodology.
Why This Matters
Tree defects are not death sentences. They are conditions that affect structural integrity to varying degrees, depending on the species, the size of the tree, the location of the defect, and what’s underneath the tree if something were to fail. The goal of identifying defects is not to generate alarm. It is to give you enough information to have a useful conversation with a qualified arborist and ask the right questions when someone shows up at your property.
A lot of trees get removed unnecessarily because someone looked at a cavity or a V-shaped union and concluded the tree was dangerous. Visual inspection alone is not sufficient to make that call. There are scientific methodologies, including Resistograph drilling, sonic tomography, and documented risk assessment frameworks, that can tell you what is actually happening inside a tree and what the real level of risk is. Eyes alone are not the right tool for that job.
Co-Dominant Stems and Included Bark
A co-dominant stem is a stem that competes with the main trunk for dominance. Instead of one clear central leader, two or more stems of similar diameter grow upward from the same point. The problem is in what happens at the union between them.
On a normally occurring branch union, the wood fibers twist together and bind as the branch grows off the trunk. The union is mechanically integrated. On a co-dominant stem with included bark, that integration doesn’t happen. Instead of wood fusing, bark gets pinched between the two stems as they grow. Bark does not have the structural strength of wood. The union looks fine from the outside but has significantly less holding power than it should, particularly under dynamic wind loading.
A useful way to picture this: make two fists and press them together at the knuckles, rotating slightly so the fingers interlock. That’s a well-formed union. Now lay your hands flat, palm to palm, fingers straight. That’s included bark. The surface area of contact is there but the mechanical integration is not.
Co-dominant stems with included bark are more prone to failure as the tree gets older and the stems grow larger. The mechanical disadvantage compounds with diameter. That does not mean the tree needs to come down. It means the defect warrants evaluation, and in many cases can be managed with structural pruning or supplemental support systems.
Cavities and Hollow Trees
A cavity is a visible opening in the trunk or a major limb where decay has removed wood tissue. Homeowners often assume that any cavity means the tree is structurally compromised. The research says otherwise.
The old standard in the industry was the “one-third rule” from Visual Tree Assessment (VTA): if the remaining sound shell wall was less than one-third of the trunk radius, the tree was considered unsafe. Research by Frank Rinn, inventor of the Resistograph and a leading authority on tree biomechanics, demonstrated that this rule is not scientifically supportable, particularly for large mature trees. His 2013 paper in Western Arborist showed that as trees stop growing in height but continue adding radial growth annually, the absolute thickness of the remaining shell wall increases even as decay progresses. A large mature tree with a significant cavity may have more actual load-bearing capacity than a smaller tree with the same percentage of decay. What matters is the thickness of the sound outer shell, not the ratio of decay to total diameter.
Source: Rinn, F. (2013). Shell-wall thickness and breaking safety of mature trees. Western Arborist, Fall 2013, 39(3): 14–18.
Cavity location also matters. A decay column running through the center of a trunk behaves differently than an external cavity on one side. The geometry of the remaining wood, not just the percentage affected, determines load-carrying capacity. On large trees, even significant hollowing can leave a shell wall with enough structural strength to carry the tree’s load under normal conditions.
Two additional factors are worth understanding. First, trees respond to wounding. The wood that forms around a cavity as the tree attempts to seal it, called woundwood or callus, is denser and more mechanically resistant than normal wood. Active woundwood formation is a positive sign. Second, the CODIT model (Compartmentalization of Decay in Trees), developed by Dr. Alex Shigo, describes four walls the tree forms to limit decay spread, numbered in order of increasing strength: Wall 1 plugs vertical conducting elements above and below the wound; Wall 2 follows growth ring boundaries; Wall 3 follows the ray cells laterally; Wall 4 is the strongest: it’s the barrier the tree grows separates the wood present at the time of wounding from new wood forming afterward, protecting future growth rings from the existing decay column. When Wall 4 holds, decay is confined to the wood that was there when the wound occurred. Source: Shigo, A.L. (1977). Compartmentalization of Decay in Trees. USDA Forest Service, Agriculture Information Bulletin No. 405.
What this means practically: a cavity alone does not tell you whether a tree is safe. A qualified arborist using a Resistograph can map the extent of internal decay and quantify how much sound wood remains. That data can be input into biomechanical formulas to calculate actual strength loss. That is a very different process from looking at a hole and guessing.
Large-Aspect-Ratio Branches
A branch that grows to a diameter close to or exceeding roughly half the diameter of the trunk at the point of attachment starts to behave more like a co-dominant stem than a subordinate branch. The aspect ratio, meaning the relationship between branch diameter and trunk diameter at the attachment point, is one of the factors arborists evaluate when assessing overextended limbs.
As a branch grows larger relative to the trunk, the mechanical forces at the attachment point increase. The branch becomes heavier, the moment arm grows longer, and the load on the union rises. On species with strong wood and good compartmentalization; live oak and bald cypress are the best examples in Pinellas County. These branches can persist for a long time without failure. On species with less resilient wood or weaker compartmentalization, like red maple, the risk profile is different.
Large-aspect-ratio branches do not require removal as a default response. Structural pruning to reduce end weight, crown reduction, cabling and bracing, or propping can all extend the useful life of a branch that would otherwise become a liability. The earlier these interventions happen, the more options are available. Large cuts on older trees invite decay and reduce the tree’s ability to respond.
Fungal Indicators: Conks and Mushrooms
Fungal fruiting bodies, including conks growing from the trunk or root flare and mushrooms at the base or emerging from roots, indicate that decay is present. They do not tell you how extensive the decay is, where it is located, or whether the tree retains adequate structural integrity.
A Ganoderma conk at the base of a tree is one of the most significant indicators. Ganoderma is a white rot fungus that degrades both lignin and cellulose, removing both the rigidity and the tensile strength from wood. The conk is the external fruiting body; the decay column has been developing internally for some time before it appears. That said, a conk does not automatically mean removal. It means a qualified assessment is warranted.
Mushrooms growing from the root zone or lower trunk indicate decay in the root system or butt wood. The concern there is not just structural strength but root plate integrity — a tree’s ability to remain anchored under wind load. This is a higher-consequence situation than trunk decay alone and warrants prompt evaluation.
What You Can Do With This Information
The purpose of recognizing these defects is not to diagnose your trees. It is to know when to call someone and what to tell them. If you see a V-shaped union with bark pinched between stems, a visible cavity, mushrooms at the base of a tree, or a very large branch growing over a structure, those are reasonable triggers for a professional evaluation.
When a qualified arborist shows up, these are useful questions: Is this defect something we can monitor or manage, or does it require intervention now? What diagnostic tools would give us a clearer picture of what’s happening inside the tree? What are the options besides removal, and what are the tradeoffs?
A tree with defects is not a tree that needs to come down. It is a tree that deserves a closer look by someone with the tools and training to give you an accurate answer.
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Frequently Asked Questions
Most of the time NO! A V-shaped union with included bark is a structural defect that warrants evaluation, not automatic removal. The severity depends on the size of the stems, the species, the extent of inclusion, and what’s in the failure zone beneath the tree. In many cases the defect can be managed with structural pruning or supplemental support systems like cabling or bracing. A TRAQ-qualified arborist can assess the actual risk level and give you documented options.
Not automatically. Research by biomechanist Frank Rinn has shown that large mature trees can carry significant internal hollowing and remain structurally sound. What matters is the thickness of the remaining sound outer shell, not the percentage of the cross-section that is decayed. A mature tree with a large cavity may have more load-carrying capacity than a smaller tree with the same percentage of decay. Visual inspection alone cannot answer that question. A Resistograph or sonic tomography assessment can.
Mushrooms at the base of a tree indicate decay in the root system or lower trunk. This warrants a professional evaluation, particularly if the tree is large or positioned near a structure. The concern is not just internal wood strength but root plate integrity — the tree’s ability to stay anchored. Get it assessed before storm season if possible.
Included bark occurs when bark gets embedded between two co-dominant stems as they grow. Unlike a normal branch union where wood fibers integrate and bind together, an included bark union has bark, not wood, at the point of contact. Bark has far less structural strength than wood. As the stems grow larger, the mechanical disadvantage increases. It is one of the most common defects associated with branch or stem failure in mature trees.
Often yes. Structural pruning to reduce end weight, crown reduction, cabling, bracing, and propping are all tools for managing branches that would otherwise become a liability. Species matters: live oak and bald cypress tolerate these interventions well. Laurel oak and other species with weaker compartmentalization carry more risk with large cuts. The earlier the intervention, the more options are available and the less damage the tree sustains in the process.
Look for an ISA Certified Arborist with TRAQ (Tree Risk Assessment Qualification). TRAQ is the specific credential for structured risk assessment and is what qualifies someone to conduct a Level 2 assessment and produce a defensible written report. For complex situations involving high-value trees, significant target exposure, or legal and insurance implications, a Board Certified Master Arborist brings additional diagnostic depth. Verify credentials at treesaregood.org before you schedule a visit.
