Defects & Decay in Trees — An Arboricultural Guide 

Fungal Decay and Wood Condition

Fungal fruiting bodies

• Mushrooms or bracket-like structures on the trunk, branches or root buttresses. These are usually a sign of advanced decay, because by the time a fungus fruits it has generally been established within the wood for some time.
• Species matters enormously. Some fungi cause aggressive decay that seriously compromises strength; others are largely superficial or affect only already-dead wood. Identifying which is present is central to knowing whether the tree is in trouble.

Cavities

• Open cavities indicate the loss of heartwood and a reduction in strength. But a hollow tree is not automatically an unsafe one — trees are structurally efficient hollow cylinders, and what matters is the thickness of the remaining wall and its distribution around the stem, not the presence of a void.
• That is a measurement question, not a visual one.

Bleeding and oozing

• Sap, fluid or staining running from the trunk or a branch can indicate a wood-rotting pathogen or bacterial infection beneath the bark. The extent of the underlying problem is rarely visible from the outside.

Weakened and discoloured wood

• Wood that has been degraded by decay is considerably more susceptible to storm loading than sound wood of the same dimensions. Where decay is suspected but its extent unknown, that is where instruments become necessary.

Structural Defects

Cracks and fractures

• Cracks in the stem or major limbs can indicate internal decay, or that the tree is already failing structurally. A crack that is opening over time is considerably more concerning than one that has stabilised.

Included forks

• A fork where bark has become incorporated into the junction rather than a proper branch collar forming. Included unions are weaker than open unions of the same diameter and are a common failure point, particularly on species prone to forking such as ash, lime and some maples.

Wounds and callus

• Trees do not heal wounds — they compartmentalise them, growing callus tissue to seal the damage off. Small wounds close within a season or two. Large old wounds may have decay established behind apparently sound callus, which is why the visible surface does not tell you the internal position.

Abnormal swellings and bulges

• Localised swelling can indicate the tree responding to internal stress or decay by laying down additional wood — an adaptive response, but one that signals something is happening beneath.

Reaction and compression wood

• Trees respond to loading by growing additional wood where stress is greatest. Visible reaction growth is not itself a defect — it is the tree compensating — but it indicates a load the tree has found significant, and is worth understanding rather than dismissing.

Crown Condition

Dead branches and dead sections

• Deadwood in the crown is normal in mature trees to a degree — it is part of how a tree sheds unproductive growth. Extensive or increasing deadwood, or dead sections of trunk, indicates decline rather than natural shedding. Position matters as much as quantity: deadwood over a footpath is a different proposition from the same deadwood over rough ground.

Dieback and leaf condition

• Discoloration, spotting, undersized foliage, sparse crowns and progressive dieback from the shoot tips all indicate the tree is struggling — through disease, root damage, drought stress or waterlogging. Where ash is involved, see ash dieback.

Loose or detaching bark

• Bark becomes loose when the woody tissue beneath it dies. On a stem, that indicates a substantial area of dead cambium and usually a serious underlying problem.

Roots and Stability

Root plate lifting

• Where the soil on one side of a tree lifts or the root plate visibly moves, roots have broken or decayed and the tree may no longer be adequately anchored. This is among the more serious findings and normally requires prompt assessment.

Soil cracking and heaving

• Cracked or raised soil around the base can indicate root plate movement, root decay, or that the tree is beginning to lean progressively rather than having always grown that way.

Severe root damage

• Trenching, excavation, level changes and compaction within the rooting area all reduce anchorage and stability. The effect is frequently invisible above ground for several years, and then appears as decline or, in the worst cases, as failure in the first serious wind after the roots were cut.

Leaning

• A tree that has always leaned has grown to accommodate it and is usually adapted to the loading. A tree that has recently begun to lean is a different matter entirely — the question is whether the lean is new, and whether it is progressing.

Other Indicators

Insects and invertebrates

• Ants, fungus beetles, millipedes, woodlice and white grubs within the stem or root buttress are generally a secondary indicator rather than a cause. They colonise wood that has already been softened by decay, so their presence suggests decay is present rather than causing it.

When Looking Is Not Enough

Almost everything above is assessed visually, and for most trees a competent visual inspection answers the question.

Where it does not — a cavity of unknown extent, a fungal bracket on a valuable tree, a union that looks wrong but not obviously so — the answer lies inside the tree, and looking harder does not help.

We hold the full diagnostic suite in-house: Arbotom sonic tomography to map internal decay across a cross-section, Rinntech Resistograph R650 resistance drilling to confirm remaining wall thickness, and structural and wind load analysis to establish what capacity actually remains. Where the defect is high in the crown, aerial inspection gets us to it.

That is the difference between concluding a tree is probably unsafe and demonstrating what its condition actually is.

Frequently Asked Questions