Quick answer: Decay in Trees — Trees on this coast live with fog, salt, wind and thin soil, and the work that keeps them healthy is decided by those conditions rather than by a calendar written inland. Structure, clearance and the root zone are what a tree here needs looked at, and small work done early is what keeps a tree out of the emergency list.
Field note. What we see on this coast is that the trees people worry about are usually not the ones in trouble: the pine with a clean crown and a lean that predates the house is often sounder than the tidy tree that has been quietly drowning in its own root zone.
On the other hand, there is no schedule that suits every tree — the same species on two sides of one street can need opposite care, which is why we look before we recommend.
A tree does not rot evenly. Decay follows the anatomy of the wood it is in, and the pattern it produces tells you which part of the tree has lost strength and which part is still carrying load.
Pattern one: heart rot
Decay in the central column, the dead heartwood, leaving a shell of sapwood around it. This is the pattern that produces the hollow tree, and structurally it is the least alarming of the three, because a tree’s strength is concentrated in its outer fibres.
A hollow trunk with a thick shell can be stronger than a solid trunk of smaller diameter. The question is the shell thickness relative to the diameter, and the size and orientation of any opening. The mechanics are set out in hollow trees.
Pattern two: butt rot
Decay entering through the roots and moving up the base. This is the most serious pattern, because the base is where the whole tree’s load is transferred to the soil and there is no redundant structure.
The signs are at ground level: a softening at the flare, a hollow sound when the base is struck, conks or fungal fruiting bodies emerging from the soil at the root collar, and in advanced cases a visible sink around the base.
Butt rot cannot be seen from the crown, which is why a crown-based assessment can miss the failure that actually happens.
Pattern three: top rot and branch decay
Decay in the upper stem and the major limbs, usually entering through an old wound or a failed branch. This produces the failure people see most often — a large limb letting go — because the decay is in the wood that carries the limb’s load.
The signs are a cavity at the base of a limb, a seam of callus tissue with a soft centre, or a conk on the upper side of a branch.
What compartmentalisation does, and doesn’t
A tree walls off decay with chemical and physical barriers, which is why a wound does not automatically become a hollow. The barrier is strongest in species that compartmentalise decisively, and it is weak in others — which is why the same cavity in an oak and in a pine are two different amounts of remaining life.
What defeats the barrier is a wound that is too large, or repeated wounding in the same place. A tree can seal a four-inch wound; it cannot seal a two-foot one, and it will spend the rest of its life trying.
The visible signs, in order of reliability
Fungal fruiting bodies are the most reliable, because they are produced by a fungus that is already established in the wood. A conk on the trunk means decay inside the trunk, and the species of the fungus narrows down which decay it is.
A cavity with a defined rim of callus is next: it tells you the tree has been trying to seal it and how long it has been trying.
Then the sound: a dead, hollow thud rather than a solid note when the trunk is struck. It is a crude test and it only detects advanced decay.
Then cracks running vertically from a wound, and seams of bark that have split.
What to do with a finding
Treat any fruiting body on the lower trunk as the finding that warrants a professional look. Treat a cavity open at the base, or a base that softens under pressure, as urgent.
A written hazard evaluation measures the shell, the opening and the crown balance, and produces a sentence that can be acted on. Anything vaguer — “it looks a bit hollow” — is not a finding.