From the text... 'Decay is a natural recycling process, but it is also a constant problem in wood preservation, said Shortle. Decay is also the biggest disease of living trees. It represents a hazard to people and to property. Decay begins with a wound or break in the bark. There are two basic types of wounds: stub and scar. They can occur on the roots as well as the stem. Wounds can result from animals, insects, fire, machinery and man's activities. Wounding triggers the tree*s defense system. compartmentalization. Only wood present when the wound occurs can become discolored and decayed. "If a tree is wounded when it is six inches in diameter and it grows to 24 inches, the biggest column of decay you can have is six inches,” Shortle said. "The decay won*t spread into the wood laid down after wounding, unless it is wounded again.” After wounding, a tree lays down a new type of wood, which Shortle called a barrier zone. Decay organisms cannot cross it. Wood discoloration within the barrier zone varies with species.'