DECAY
Structural weakening of wood due to decay is a major cause of tree failure.
Many species of fungi cause decay and each produces a characteristic type of
reproductive structure (Figure 1). Some, such as conks, are woody and perennial
while others, such as mushrooms, are soft and deteriorate after only a few
weeks. Their presence on the outside of a tree indicates that there is advanced
decay inside the tree. |
Figure 1 - Conks, such as these of
Phellinus tremulae on aspen, indicate extensive internal
decay. |
Decay often exists
without conks. The presence of cracks, seams, butt swell, hollow branch stubs,
and large old wounds suggest internal decay. Decay fungi generally require
openings in the bark to enter a tree, and these openings indicate that a
problem may exist. Reliance on external indicators alone or soundings with an
ax to detect decay are unreliable methods, especially when inexperienced
inspectors are involved. Sometimes an increment borer must be used to confirm
the existence of suspected decay or to determine how much solid wood surrounds
a column of decay (Figure 2). This technique should be used only when necessary
because the wound created by a borer may provide an entryway for harmful fungi
and insects. |
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Figure 2 - Thickness of sound wood in
outer shell required to maintain 66% of original strength in trees with center
rot. If the amount of sound wood exceeds that established by the line on the
graph, the tree can be considered relatively safe from failure Johnson 1981).
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