A tree cavity is a hole in a trunk or branch of a tree. A cavity is caused by damage to the bark or woody tissue of the tree. The wound can result from a pruning cut, mechanical injury caused by machinery, storm damage, dead limbs that decay into the trunk, or any injury that damages the tree. When a tree loses the bark it opens up the tree to infection or decay similar to a wound to our skin or damage to the enamel on our teeth.
Trees cannot heal the way people do but rely on compartmentalization to keep the decay from spreading, (Alex Shigo, 1986). Trees use chemical barriers to stop decay by forming a compartment to prevent its movement. This is why cutting boards made of wood have been found to be more effective against eliminating bacteria. A study in California found that families using wooden cutting boards were half as likely to develop salmonella exposure as families using plastic cutting boards. (P.K. Kass, 1992).
Previously, it was believed that cavities should be filled with cement or other fill material. For the past few decades this practice has been discouraged. Filling cavities has been found to offer no benefits to reducing decay or providing structural support for trees with cavities. The only benefit to filling a tree cavity is cosmetic.
One instance where filling a cavity may be advisable is to prevent honey bees from making a nest in a trunk or limb of a tree. The preferred method to filling a cavity is to use expandable foam formed just below the wound or callus roll. The callus roll is the tissue that trees develop to close over the wound. Once the cavity is filled with foam a layer of bondo or automobile body filler should be applied to preserve the foam. Finally a coating of spray paint should be applied to protect the body filler from exposure to the elements. Remember filling cavities is a cosmetic measure at best.