Urban and suburban trees are more likely to have wounds and decay than trees in native stands because people cause most wounds. These wounds are usually unintentional, such as automobiles, construction equipment, or lawn mowers bumping the tree trunk or surface roots, or improper pruning. Naturally occurring events, such as storms, fires, or damage by birds or other animals, may also cause wounds.
Trunk wounds that penetrate the bark will damage the cambium layer, a thin layer of vascular tissue, which is vital to movement of water and nutrients. If less than 25% of the bark around the trunk has been damaged, the tree will probably recover. When fresh wounds occur on the trunk, the injured bark should be removed carefully, leaving healthy bark that is sound and tight to the wood. A wound dressing (tree paint) is not necessary. You will be able to observe the wound closing from the edges each year as the tree grows. When an older wound is discovered, remove the dried and loose bark back to the area where the new wood can be seen along the edges of the wound. Trunk wounds that are not addressed could potentially be a hazard in the future.
Once a wound occurs, decay-causing fungi can enter the heartwood and the decay process begins. Trees have a unique defense. The wood around the wound begins to produce special compounds in the wood cells that set up a wall or barrier to isolate the infected area. This is called compartmentalization. In a vigorous tree, new growth continues to form and add to the sound wood Once compartmentalized, discoloration and decay will spread no further unless one of the barriers is broken. Cleaning decayed wood from cavities is not recommended since the compartment wall might be breached and further decay of the trunk could result. Storm-damaged branches should be properly pruned to expedite the healing process. Avoid pruning directly against the trunk since flush cuts can lead to extensive decay. Prune hazardous branches immediately.