Growing Concerns: What causes tree cavities?

March 11, 2019

I frequently get questions from homeowners wondering what causes decay and hollow spots (cavities) in trees. They also frequently inquire if the cavities harm the tree.

Cavities in trees are a result of decay caused by fungi that invade the wood after a tree is wounded. The wood gradually decomposes and shrinks in volume. The soft decayed wood is excavated out by carpenter ants, small mammals or birds. Bark protects the wood from these fungi, so wounding is required to predispose a tree to decay. These wounds can be caused by insects, animals, adverse weather and people.

To understand why decay results in cavities and often results in hollow trees, I need to introduce a concept known as CODIT, which stands for “compartmentalization of disease in trees.” Put simply, this means that when a tree is wounded, it responds by creating decay-resistant physical and chemical barriers that help contain decay to zones in the tree.

Here is a link to a document that illustrates CODIT in much more detail than I can address in my column www.nrs.fs.fed.us/pubs/misc/ne_aib405.pdf. More detail can also be found by doing a web search for “CODIT.”

The extent to which fungi can decay a tree after wounding is influenced by tree species, type of wound, wound size, location and the vigor of the tree.

Unfortunately, decay in urban trees is often initiated by well-intended but ill-informed people who prune improperly. Well-trained arborists can assess trees and prune using standards that limit the potential for extensive decay to advance in the tree. Of course, arborists wound trees on a daily basis in the course of their work, but an understanding of tree physiology enables us to employ pruning that maximize the trees’ ability to respond to these wounds.

In most cases, trees can remain healthy even with extensive decay columns in the center of the tree because their vascular system is in the outer few rings of new wood. Extensive decay can cause structural weakness in trees, so any tree with visible decay, that poses a threat if it fails, should be evaluated for structural integrity.

Many people also ask if painting a wound will stop decay in trees. The answer is no. Painting does prevent decay in dry lumber because it prevents moisture from entering the wood. Moisture is necessary to create an environment conducive to fungi. Living trees move moisture up from their root systems, so painting a wound has little benefit since moisture is available from inside the tree.

The best advice I can give people is to not do any pruning or other activity that wound trees without a thorough understanding of proper pruning techniques and the ability to evaluate a tree’s ability to respond to pruning based on species and vigor.