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. The exceptions are fungus at the base or on the roots, a new lean or cracking trunk, and dead or hollow wood above a house, a road or a place people sit.

Most of a tree is dead wood, by design

The calls come to Blue Whale Trees every fall, when the shelf fungi fruit, and in most of the photographs that arrive with them the tree still carries a full crown.

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. The outer edge of that sheath carries a green layer that does photosynthesis without leaves. 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. What the tree cannot do is undo the wound.

“Poor pruning can cause damage the tree must grow over causing the wound to stay within the tree forever.” (International Society of Arboriculture, Pruning Trees)

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. The reason is that 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.

The root and butt rots are also the ones that travel, which is why a neighboring tree of the same species is worth a look.

“Armillaria mellea and Ganoderma spp. commonly infect woody roots and can spread to nearby trees through root grafting.” (UC Statewide IPM Program, Wood Decay Fungi in Landscape Trees)

Call an arborist, or leave it alone

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. A photograph of the base of the trunk and one of the whole crown, sent to us, usually settle which case it is.

Common questions

Does a mushroom on my tree mean the tree is dying?

Usually not, because most of the wood in a big trunk was already dead before the fungus arrived. A tree can be hollow for its whole length and still put on a full crown every spring.

Is a hollow tree safe?

A hollow trunk keeps most of its bending strength while the sound outer shell is at least about a third of the radius. The question worth asking is therefore what stands underneath the tree, not whether it is hollow.

Should I fill the hollow or clean out the rot?

No: do not fill it, do not paint the wound and do not clean it out, because all of that does more harm than good.