Damage to Trees

Q: After a storm, some of our clients experience damage to their trees, and they often want to know why their trees were damaged while others were unscathed. How can we explain this random damage?

A: Answering the “whys” of storm damage can be a challenge, but the answer involves a study of tree species, structure, and Dr. Shigo’s CODIT (compartmentalization of decay in trees). Although the storm may be a random event, rarely, is the damage.

Species is important because some trees have less structural strength in their wood. Usually, fast growing genera, such as willow, elm, silver maple and poplar are considered to be “weak” and more prone to damage. Fast growing trees incorporate more air space and less woody tissue in their structure, and air isn’t very strong! While this generalization is usually accurate, structure and CODIT are often more important. For example, our neighborhood has several hundred, mature silver maples lining the streets, and the damage from wind and ice storms has been very minor while the same species on private property often suffered serious damage. Also, supposedly “tough” species, such as oak often suffered serious damage. In most cases, structure and CODIT will provide the answers.

Many of the branches that have been torn out of trees had narrow branch angles, which are inherently weaker than a wide branch angle. The general public usually thinks the opposite is true. For an explanation with clients try the following... as each limb grows larger in diameter there is a constant pressure forcing the limbs apart. This pressure creates a “fault line” in the trunk starting at the bottom of the “v” in the crotch. In some cases, if three or more limbs originate in the same area, this pressure can almost shove an outer limb off the trunk. This type of damage can be reduced by convincing clients to prune trees to eliminate narrow branch angles as the tree is developing. This corrective pruning prevented serious damage to our street trees while private property trees often had competing leaders with a very narrow angle. Another technique to prevent injury is to cable and brace suspicious branches. The load is then distributed to other limbs via the cables.

Dr. Shigo’s CODIT theory can very briefly summarized with the following points:

1)   Trees do not replace damaged tissue. They compartmentalize this tissue within the tree.

2)      Compartmentalization barriers can stop the spread of decay both vertically and horizontal within the wood present at the time the tree was wounded. These barriers are often not very effective. The most effective barrier is between the wood present when the tree was wounded and the new wood produced in subsequent years.

3)   The extent of decay progression within a tree will depend on the wound, species, and vigor.

Perhaps, a couple of examples will illustrate this theory in action.

A tree breaks off during the storm and it is hollow but no signs of this condition were visible. How did the tree become hollow? The diameter of the hollow was the size of the tree when it was wounded. Decay organisms gradually consumed this “tree”, and the extent of the vertical column of decay reflects a barrier in action. The decay, however, did not move into the new wood that was produced in subsequent years. Also, consider this implication. Young and rapidly growing trees quickly bury injured wood in large amounts of new wood. Older or less vigorous trees are producing much less new wood to compartmentalize the decay. If a vigorous tree and an aging or less vigorous tree receive a equal injury, the older tree will more likely become a hazard because decay can consume the bulk of the trunk.  Remember, new wood vs wood present at wounding!

Another tree breaks off and an inspection reveals pockets of decay. A possible cause could be sloppy pruning, years ago, that left stubs that provided long lasting avenues for decay.

A tree that had trunk decay but the decay was cleaned out to healthy wood breaks off. An inspection reveals the decay has continued to expand regardless of the treatments. This “surgery” probably broke the tough barrier between the “old and new” wood and allowed the decay to consume even more of the trunk.

I have attempted to provide only a brief overview of CODIT. For more information, refer to Dr. Shigo’s book “A New Tree Biology” or if they are still available, his excellent forest service publications titled “A Tree Hurts Too!”, “Rx for Wounded Trees”, and “CODIT”. Keep in mind that many of these defects take years to develop. For prevention, prune properly, avoid wounds, and keep trees healthy and vigorous. Most wound “surgery” is of little benefit, and some practices can compound the injury.

A final factor to consider, but sometimes difficult to evaluate, is the relationship between site and plant orientation. For example, wind velocity can be changed drastically by other objects, and a limb pushed into the tree by the wind will not break as easily as a limb pushed away.

Let’s conclude with uprooting as another form of storm damage. Usually, environmental and/or cultural factors that affect the root system can be implicated. For example, sidewalk repair cuts roots and can lead to wind throw. Plastic wraps and even wire baskets can hinder root development and also be a causal factor. Excessive soil moisture is subtle but leads to a very shallow root system that often will not support the tree. The next time you’re near a swamp or wetland look for “downed” trees and check the root development. Usually, you’ll find all the roots in a shallow disk, not greater than 12 inches deep.