Trees cannot regenerate tissue over a wound as humans do. They seal off the injury and encapsulate it, hopefully stopping pathogen entry or progression in the process. But the wound is always there and visible on the inside if not also from the outside.
This is why responsible arborists try to take as much care as possible to minimize wounding caused by pruning. Because, technically, all pruning cuts are wounding the tree. But then so is the swing set attached to a limb, the bump from the car, the bicycle leaned against it, the dog tethered around the base, the lawn mower hitting it every time you mow, and don’t even get me started on weed whacker injuries!
Every poster, sign, birdhouse, decoration affixed to a tree is injuring it to one degree or another. Cats mark the trunks with their claws, birds peck holes as they go for larva under the bark or accessing sap exudations, tapping to produce maple syrup, the list simply goes on and on. In fact, I often say, trees live in spite of us, not because of us.
When a tree sustains an injury, an air embolism is created. The tree responds by trying to contain the damage this will cause by initiating chemical responses of various strengths, containing it from progressing up and down the trunk (axially), into the center of the tree (radially) and sideways (laterally). There is a more powerful reaction produced as a barrier which inhibits disease or dysfunction from extending out into new wood formed after the initial injury.
Arborists will know this sequence by the acronym CODIT, Compartmentalization of Decay in Trees, as initially described by Dr. Alex Shigo, with the boundary zones, Walls 1, 2 and 3 and the barrier zone of Wall 4.
Dr. Dirk Dujesfieken has described this process further and CODIT is now being referred to as Compartmentalization of Damage in Trees. Discussion of the CODIT Model and the CODIT Principle are lengthy subjects and topics for future blogs.
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