Tree Wounds and How They Heal
When a tree is wounded, bark is usually broken, cracked, split, or peeled away. When we prune a tree, we create a wound that crosses several annual growth rings. Likewise, if we drill into a tree, we are crossing several annual growth rings and potentially into heartwood. There are many ways in which a tree is wounded, including wounds to the tree’s root system, which are all entry points for decay causing organisms.
When a wound occurs, the tree responds immediately to the wound walling off the wood present at the time of wounding. We call this process Compartmentalization. (Trees do not heal like we humans do!)
CODIT is the acronym for Compartmentalization of Damage in Trees. It is used to explain how trees defend themselves when wounded and protect themselves from the spread of decay. Slow growing hardwoods have a strong and effective defense mechanism. Fast growing soft wood species much less so.
A barrier zone is established immediately upon wounding which separates the wood at the time of wounding from the new wood or annual growth ring to be added later. Trees basically grow a new tree over the old tree each year. Reaction zones are also created in the wound vicinity. It is here that starches are converted to compounds that resist the spread of decay. This also includes plugging of the vessels that could enhance the spread of decay. Vessels in the sapwood are a large and easy pathway for disease causing microorganisms to follow.
There are cells within a thin layer just beneath the bark, called the cambium, where new growth takes place. Cambial cells differentiate to form a new annual growth ring on the inside of the cambium and new bark on the outside. When the bark is wounded, it usually leads to death of the cambium. As a result, no new growth takes place in the area specific to the size of the wound. The wounded bark soon falls off.
As the tree continues to grow, it adds healthy new wood. Over time, small wounds may be eventually closed by the wound wood that subsequently forms. When the wound is completely closed, decay is halted. This is the final stage of compartmentalization called encapsulation. It is the tree’s ultimate survival strategy.
Trees also develop a Branch Protection Zone, where branches attach to the parent limb or stem. This protection zone resists the spread of decay from a dead or cut branch stub further into the tree creating a hollow. When pruning, look for the Branch Bark Ridge (the raised bark area at the branch attachment) and a slightly swollen area called the branch or trunk collar. Make your cut just outside of the branch bark ridge at an angle down and away from the trunk or parent stem. If a collar is visible, cut close to it without cutting into it. By keeping the pruning wound small (4 inches or less) you give the tree its best chance to close the wound completely. Large wounds may never close, leading to eventual decay and hollows.
Conifers have the ability to produce resin, which accumulates at a wound, resisting the entry of insects and decay.