How a Tree Cavity Forms: Decay, the Tree's Own Walls, and the Wildlife That Moves In
In gardens and on wooded slopes, we often come across a large tree with a dark opening in its trunk. Many owners ask us the same thing: it is rotten inside, so should it come down?
We are a small tree care and landscape business in Mie Prefecture, central Japan. A hollow is not always a sign that a tree is dying. It is the visible result of a long contest between decay fungi and the tree's own defences, and it is also a home that many animals cannot find anywhere else. This page explains how a cavity forms, who lives in it, and what we look at before deciding whether a hollow tree can stay.
It starts with a wound
Every cavity begins with an opening in the bark. A large branch breaks in a storm, lightning splits the trunk, insects bore in, or a pruning cut is made in the wrong place. Through that opening, wood-decay fungi enter.
Two broad groups do most of the work. White rot fungi break down lignin, the substance that makes wood rigid, and leave the wood pale and stringy. Brown rot fungi break down cellulose and leave the wood dark, dry and cracked into small blocks. Either way, the fungi slowly turn solid wood into something soft enough to fall away.
The tree builds walls
A tree cannot repair wood that has already been damaged. What it can do is wall it off. In 1977, the American forest pathologist Alex Shigo and his colleague Harold Marx described this in a US Forest Service bulletin, and named it CODIT: Compartmentalization of Decay in Trees.
They described four walls. Wall 1 resists spread up and down the trunk, by plugging the vessels that carry water, and it is the weakest. Wall 2 resists spread inward, across the growth rings. Wall 3 resists spread around the trunk, along sheets of ray cells, and it is fairly strong. Wall 4, the strongest, is new: after the wound, the cambium, the thin living layer under the bark, lays down a barrier zone that separates the damaged wood from all the wood the tree grows afterwards.
This is where a hollow comes from. In one of the bulletin's photographs, a red maple with three severe wounds at its base has lost all the inner walls, and only the barrier zone remains. The caption says it plainly: this is how hollows develop. The decay is confined to the wood that existed when the tree was wounded, and the tree goes on living in the new wood outside it.
A cavity large enough for an owl can take decades to form, and in old trees more than a century. From the outside the tree may still carry a full crown of leaves.
Hollow does not always mean weak
Think of a tree trunk as a tube rather than a solid post. Much of the strength lies in the outer wood, which is also where the tree is alive. If that outer shell is thick and sound, a hollow tree can stand for a very long time.
In Japan there is a famous example. The Great Camphor Tree of Takeo, the sacred tree of Takeo Shrine in Saga Prefecture, is said to be more than 3,000 years old. Its trunk is about 20 metres around, and at its base the tree opens into a hollow roughly the size of a twelve-mat room. Inside, a small shrine to Tenjin, a Shinto deity, has been set. In Japan a large old tree at a shrine is often treated as a sacred tree, and its hollow is part of what people come to see, not a defect to be removed.
Who lives inside
The inside of a cavity changes temperature more slowly than the open air, and it keeps out rain and wind. For animals that cannot dig their own hole, that is a rare thing.
In Japan, owls such as the Ural owl, small birds such as the Japanese tit, the varied tit and the white-cheeked starling, and scops owls all nest in tree holes. The Japanese giant flying squirrel, a species found only in Japan, also nests in the hollows of large trees. Because it glides from tree to tree, it needs tall trunks to launch from, and in the lowlands the old trees of shrine forests are often the places that still meet that need. Stag beetle larvae feed on the softened wood inside.
As old trees are removed from towns and farmland, these animals have fewer places to raise their young. We make nest boxes from logs of trees we have felled, and offer to put them up in the gardens and woods we look after. At one site in Suzuka, Japanese tits moved into one of our boxes and laid eggs. A nest box helps, but it is not the same as a natural cavity, which is deeper, older and more varied inside.
What we look at before deciding
When a customer finds a hollow in a garden tree, we do not start with the hole. We start with what is still alive around it.
First, how thick is the sound outer wood compared with the size of the hollow? Second, how full and healthy is the crown? A tree that keeps putting on new growth is still building new wood around the cavity every year. Third, are there cracks, bulges, or fungal fruiting bodies on the outside of the trunk, and is the tree leaning or exposed to strong wind? Fourth, what is underneath: a path, a car park, a house?
If the outer wood is thick and the crown is vigorous, there is often no need to rush to fell the tree. If the shell is thin, cracks are spreading, or people and buildings are within reach, we recommend a closer assessment by a specialist before deciding.
For a hollow tree that stays, two things help it last. We adjust the branches so that rainwater does not run into and collect in the opening, and we keep the roots in good condition by adding organic matter to the soil around them rather than compacting it.
A small note on words
In Japanese, a tree hollow is called a juudou, or more simply an uro. When we stand in front of one, we try to remember that we are looking at something the tree spent a hundred years making, and that the decision to cut or to keep is only as good as how long and how carefully we have watched that tree.
