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
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.