In the truest sense of the word, trees do not “heal.” They do not repair and regenerate tissue like animals do; rather they compartmentalize damaged tissue with specialized cells, and grow new healthy wood around the damage. The damage itself is never repaired; it is simply separated by a physical barrier of cells to keep any infection from spreading to the rest of the tree. These barrier cells are called woundwood or callus tissue.

The Branch Defense Zone

The term for this process is “CODIT,” or Compartmentalization Of Decay In Trees, coined by renowned arboricultural scientist Dr. Alex Shigo in 1986. While trees can create woundwood anywhere the damage occurs, the process is fast and effective when it is triggered in the collar (the swollen ring where the branch meets another branch or the trunk). The collar contains specialized cells called the branch defense zone, which activate the growth of woundwood. When making pruning cuts, it is essential that the collar never be damaged in the process. If the branch defense zone is damaged, it will prevent the proper growth of woundwood around the cut. This can lead to improper compartmentalization, decay and even death of the tree. The branch defense zone is the tree’s natural emergency plan to save the rest of its “body” if a limb is severely damaged: it will seal off that limb from the rest of the tree by creating a layer of barrier cells at the limb’s junction. Once sealed off, the rest of the limb will not receive any more nutrient flow and will eventually die. Should woodpeckers, insects, fungus, or other creatures create further damage in this dead wood, it will no longer affect the rest of the tree, and can be pruned off or will eventually fall off and be recycled as organic matter in the soil.

Sapwood versus Heartwood

However, not all branch damage results in the compartmentalization and decay of entire limbs. This is in part due to the differences in the wood composition. Sapwood, the outer region through which nutrients and water flow, is highly resistant to decay and is thus less likely to be compartmentalized than heartwood, which has no resistance to decay. Branches that are less than 2 inches in diameter are typically composed entirely of sapwood. At around 4 inches, the high percentage of heartwood in the branch makes it highly susceptible to decay. This science can be applied when making decisions on where to make a pruning cut. Since small branches heal easiest, and with the least risk of infection and decay, it is best whenever possible to make pruning cuts that are less than 2 inches diameter. You should train your young tree to its final shape in the first few years of its life, while the branches are still small. If you do need to make a larger pruning cut, you can help your tree to heal as best possible by fertilizing properly, providing adequate water, aerating the soil if it is compacted, and treating any diseases and pests as needed. By keeping your tree in good overall health and reducing stress, it will have a better chance of surviving a potentially dangerous large cut. For more information on pruning and caring for your trees, check out the videos and articles at Fruit Tree Central. Photo of Compartmentalization of Decay from USDA Forest Service - Northeastern Area Archive, USDA Forest Service, Bugwood.org Photo of the branch collar by Joseph O'Brien, USDA Forest Service, Bugwood.org