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Decay development

lighter spot on tree stump caused by microorganisms

Click for the USDA Wood Decay in Living and Dead Trees: A Pictorial Overview (PDF).

Injuries result from inappropriate pruning cuts, storms, ice, snow, animals, wind, excess weight, temperature extremes, trunk wounds, disease, chemicals, and other stresses. A tree reacts to injury by creating boundaries around it. The boundary-setting process that resists loss of normal wood function and resists the spread of discoloration and decay has been referred to as compartmentalization.

Discoloration is the orderly response of the tree to microorganisms resulting in darkened wood but no strength loss. Decay is the orderly breakdown of tissue resulting in strength loss. The rate of the discoloration and decay process depends on the severity of the wound, position in the tree, size of the wound, time of year, species, tree age, and the types of infecting microorganisms.

Compartmentalization may be described using the CODIT (Compartmentalization Of Decay In Trees) model. Four different boundaries (also called walls) in trees have been presented in CODIT: wall 1, wall 2, wall 3, and wall 4. Each forms in a different manner and protects the tree in unique ways.

The walls are numbered in increasing order of their ability to retard movement of decay organisms. For example, wall 3 is stronger than wall 1. Wall 1 may or may not be present at the time of wounding. Walls 2 and 3 are present in the tree at all times. Wall 4 forms in response to injury.

Wall 1: Xylem vessels immediately above and below an injury plug with chemicals when a tree is injured. This plugging response forms wall 1. Some plugging occurs normally without injury. Plugging forms a weak boundary in some trees such a hackberry (Celtis) and poplars (Populus), but is stronger in others such as many of the oaks (Quercus). Because wall 1 is weak, decay in some trees can advance rapidly up and down the trunk from the injury resulting in long columns of decay and hollow branches and trunks.

Wall 2: The growth rings make up wall 2. The transition from one growth ring to the next retards advancing decay organisms. Decay organisms often have a tougher time moving across growth rings (wall 2) than they do up and down the stem (wall 1). The functioning of wall 2 can be demonstrated when you view a cross section of a trunk or branch that was injured previously. A darkened region often appears to stop its advancement inward toward the pith at the boundary of one growth ring with the next. This is wall 2 working.

Wall 3: The rays make up wall 3. They have plenty of decay fighting capability because they are rich in starch. Discoloration and decay have a tougher time moving across wall 3 that walls 1 and 2. The strength of wall 3 can be demonstrated by viewing a cross section of a trunk or branch injured several years ago. Notice that there is a clear demarcation between darkened tissue and normal light colored wood. If walls 2 and 3 fail and decay organisms break through, the affected trunk or branch can become hollow. Wall 4 forms the outside edge of the hollow.

Wall 4: This is the strongest boundary that retards spread of discoloration and decay in trees. This reaction zone forms from the cambium along the edge of the outer-most growth ring present at the time the tree was injured. It begins at the point where the tree was injured and it may extend all or part way around the tree. Wall 4 stays in the same position in the tree but may extend further around or up and down the trunk with time. It does not move out with the new cambium. There may be numerous wall 4s in a tree, depending on its wounding history. Wall 4 forms the edge of a hollow.

Wall 4 develops in response to many different types of injuries. It can take several years for wall 4 to reach the other side of the trunk - or it may never reach that far. Wall 4 extends above and below the injury essentially in the shape of a pipe. It may develop a few inches or many feet above and below the injury.

Wall 4 prevents discoloration and decay organisms from moving into the wood produced after the injury occurred. This means it is extremely difficult for discoloration or decay to move from inside wall 4 to the outside of wall 4. Although this task appears simple, it is vital to the longevity of trees. Imagine if decay organisms could spread into wood formed after injury - trees could not live to become old majestic masters.

The obvious advantage of wall 4 is that it retards decay; but there are two very important disadvantages. The first disadvantage is that sugars cannot move across wall 4, i.e. sugars have a more difficult time moving in or out of the portion of the bank (xylem) surrounded by wall 4. As a result, some stored starch can get trapped in the rays and xylem located inside of wall 4. However, the starch is available to decay organisms. The tree may have wasted the effort required to produce the sugar and store it as starch. It earned the money (made the sugars), deposited the money in the bank (stored it as starch), then could not withdraw some of the money (starched was trapped inside).

You can imagine how much stress this causes the tree by imagining your stress after you earned money, deposited it in a bank, and could not get some of it back. Creation of wall 4 also makes the energy bank smaller so less starch can be stored. This occurs because wall 4 essentially shuts off new deposits of sugars into the walled off portion of the xylem.

The second disadvantage is that a crack can form along wall 4. This separation or delamination is called a ring crack and it may follow wall 4 all or part way around the trunk. One or more secondary cracks, called radial cracks, can form from the ring crack along a ray.

Another serious crack is the closure crack that occurs as the callus and woundwood attempt to grow over and close the wound. This crack often extends from the point of injury out to the current location of the bark. Sometimes this crack never closes. Even if it closes, the crack remains along with its associated weakness. Cracks in trees cause weakness that can make them susceptible to breaking. In fact, cracks are probably of more concern than the decay that results from injury.

Trees vary in their ability to form walls 1, 2, and 3. These walls are weak in trees that are poor compartmentalizers of decay (See: list of species by compartmentalization ability). Hollow trunks result from weak walls. These three walls are stronger in trees that are good compartmentalizers. Following injury to a poor compartmentalizer, wall 4 may reach the opposite side of the trunk quickly, within a few years. Because walls 2 and 3 are stronger in a good compartmentalizer, wall 4 may only need to be produced around part of the trunk.