Tree Cavities

People are always concerned about the health and welfare of their shade trees. And they should be concerned about the health and welfare of shade trees because such trees are a valuable asset to the landscape. What really gets people worrying is a hole in their precious tree and then, what should they do about it.

First of all, let's examine how the hole or tree cavity began. The "dead" wood in the tree center is encircled by living wood, that is surrounded by living bark. Outside this area is dead bark that protects the living tissues from injuries and diseases, and from drying out. Fungi that rot the central column of "dead" wood cannot invade through the outer bark or the living tissues just beneath the bark. Instead, they enter through wounds.

Decay fungi may soften and/or discolor the wood. Sometimes a black line is formed at the edge of each season's growth. They may also attack living sapwood. After several years of food gathering in the rotten wood, decay fungi produce characteristic fruiting structures ("conks", "brackets", "toadstools", "mushrooms", etc.) on the surface of the tree, often where bark was broken. These fruiting bodies release millions of tiny, dry spores. A few spores happen to land on, and infect wounds in other trees.

How fast decay spreads in a tree depends on the species of tree, the fungus, and the differences between trees and between fungi, and on what combinations of fungi and bacteria enter the wound. One cannot say how long a tree will live after it begins to decay. The trees will suddenly break in a windstorm or slowly die back, branch by branch, as rot begins to block the supply of water. Or, it may last for decades. In the long run, however, it is decay that dooms all trees that escape other diseases and injuries.

Compartmentalization is the process in which a tree lays down barriers that keep wood decay inside the wood that existed prior to wounding. The nature of this barrier is not yet fully understood but apparently is related to toxic phenols.

Sometimes, the wood that is present when the wound occurs will eventually rot away so that the tree gets a "hollow trunk". At other times only a wedge of that former wood rots, sometimes only a silver of that wedge rots and sometimes the decay is limited from much longitudinal spread. New wounds may allow decay to enter more recent tissues, but heart rots from old wounds apparently cannot enter new tissues.

How should trees with cavities be treated? If a cavity is in a branch, the branch can be cut off, flush with the next larger branch or trunk. Many attempts have been made to treat cavities in tree trunks. Rotten portions have been gouged out back to the sound wood, braces installed in weakened trees, the inner surfaces of cleaned cavities painted with antiseptic tree wound paints. Sometimes cavities are filled with solid, preferably somewhat flexible, materials. Foam?type plastics, covered with a layer of more impervious plastic, have been tried.

Whether cavity treatment prolongs tree life is not clear. The last vestiges of decay can seldom be removed. Decay often progresses after treatment. If there is a large opening in the trunk and the wood is unsound, the decay is probably established permanently. Bracing or filling a cavity may, for a time, give additional support to the tree trunk, but there is a danger that cavity?cleaning will actually make the situation worse by creating internal wounds, thus allowing decay from the center of the tree to enter younger tissues.

Therefore, serious doubts are cast on the value of any cavity treatments at all. Natural "compartmentalization" may be the best life insurance for a tree that is partly decayed. All cavities should be made to drain water that could accumulate and aggravate the rotting process.