Fungus on a tree is not a death sentence

Someone finds a shelf fungus on an old oak, or looks into a hollow where a limb tore out, and calls to ask whether the tree should come down. Usually the answer is no. Trees are not animals; they do not have organs that fail, and most of the wood in a big trunk was already dead before the fungus arrived.

Most of a tree is dead wood, by design

A living tree is a thin sheath of active tissue, the cambium, phloem and outer sapwood, wrapped around a column of heartwood that is chemically defended but no longer alive. Heartwood gives the tree stiffness; it does not conduct water or store food. When decay fungi colonize heartwood through an old wound, a pruning cut or a torn branch socket, they are eating tissue the tree has already written off.

Trees wall off damage instead of healing it

Wounded wood does not regenerate the way skin does. Instead the tree builds chemical and structural barriers around the damaged zone, a process the plant pathologist Alex Shigo described as compartmentalization. Decay is confined behind those walls while new wood grows over the outside. That is why you see a ring of healthy growth closing over an old cavity, and why a tree can be hollow for its entire length and still put on a full crown every spring.

Structurally, a hollow trunk keeps most of its bending strength as long as the sound outer shell is at least about a third of the radius, because the outer wood does most of the work in bending. That is why old hollow trees stand for decades: the shell that carries the load is the part that is still alive and still growing.

Retrenchment is a life stage, not a death

Old trees shed. Leaders snap out, big limbs tear away, the crown lowers and thickens toward the center, and the trunk hollows. Ancient-tree specialists in Britain call this retrenchment, and they have learned that trees managed through it, rather than removed at the first sign, live for centuries in that state. Oaks, black gums, sycamores, beeches and bald cypresses in the eastern United States do the same. A tree that has lost half its crown and hollowed its trunk may be in the middle of its life, not the end; the oldest trees ever dated in the East look exactly like that.

Fungus on dead wood, then, tells you almost nothing about the tree’s future. Fruiting bodies on a torn branch stub, on the dead face of a pruning wound, or on the wood inside an old hollow are decomposing material that was already lost. Do not fill the hollow, do not paint the wound, and do not “clean out” the rot; all of that does more harm than good.

When to pay attention

The exceptions involve the parts that hold the tree up, and the things underneath it.

• Fruiting bodies at the base of the trunk or on major buttress roots, especially in a ring around the tree. Root and butt rot fungi such as Armillaria, Ganoderma and Kretzschmaria attack the living roots and the structural base, and those are the failures that drop a whole tree.
• A sudden lean, cracked or heaving soil on one side, or roots that have lifted.
• Cracks in the trunk that open and close, or that run through a hollow to the outside.
• Large dead limbs or a hollowed section directly above a house, a road, a play area or a place people sit.

Where a tree with these signs stands over a target, get a qualified arborist to assess it; a resistance drill or sonic tomograph can measure how much sound wood is left. Where it stands in a woodlot or a back corner with nothing beneath it, leave it alone. A hollow tree is the richest habitat in the forest: owls, flying squirrels, bats, bees and dozens of other creatures live in exactly the cavities that make people nervous.

The short version

Fungus on dead wood is normal decomposition and not a threat. Fungus on living roots or the base of the trunk, combined with a target below, is worth a professional look. Everything in between is a tree getting old the way trees have always gotten old, and it has probably been doing it longer than anyone alive has been watching.