Tree Wound Response

Trees have a natural defense response to wounds and pruning cuts. They form four types of walls to compartmentalizing the area thus preventing the spread of decay organisms (Figure 1). The decay or injury remains but is sealed off and does not increase in size if the walls are stronger than the decay organisms. The storage capacity and function of the injured part is lost forever.

Wall 1 is formed by plugging the vertical vascular system vessels following an injury. It is the weakest wall but can slow the vertical spread of decay. Wall 2 is formed at the outer edge of a growth ring. It is a weak barrier but does offer resistance to inward spread of decay. Each growth ring is subdivided into compartments with a radial wall (Wall 3). It is the strongest of the three walls and provides resistance to lateral spread. It presents a maze of physical obstacles as well as a chemical barrier. Wall 4 is formed by cambium growth after an injury. It is the strongest of all the walls. Internally, it separates the wood present at the time of injury from new wood formed as the tree grows. Externally, callus tissue develops around the injury and should eventually cover it by growing over the dead wood.

Some tree species can activate Walls 1, 2, and 3 very rapidly and maintain them so effectively that the amount of decay is limited. When a tree responds slowly or the walls are weak --- infection can damage a large volume of wood.

A tree branch has a branch bark ridge, often referred to as a branch collar, that separates the branch from the tree trunk (Figure 2). The collar is the swelling located at the base of a branch where the branch meets the trunk. The callus that forms the collar is an area of tissue that contains a chemically protective zone. The natural decay of a dead branch stops when it reaches the collar. When pruning a dead branch, do not create a new wound by cutting into the ring that forms around the dead branch.

Consumer Horticulture | Trees | Plant Fact Sheets

Prepared by: Erv Evans, Consumer Horticulturist, NC State University