First, lets look how a tree deals with healing a wound caused by a broken or dead lateral branch. In the photo at right, a lateral branch has died from lack of sunlight. Look carefully at the branch and you can see the bark is starting to slough off and the wood of the dead branch is starting to decay. However, what is more important to see in this photo is the prominent branch collar that has developed around the base of the dead branch. The red arrow points to the upper edge of live tissue that will grow over the wound that will be created when the dead limb finally rots away.
When making pruning cuts its always best to work with the tree's natural wound healing strategy. If you prune a limb off at the upper edge of the branch collar, that wound will quickly grow over to seal out wood rotting organisms.
The interesting thing I found below the pruning cut was a very distinctive ridge in the bark that angled down away from the new shoot (red arrow). It looks like the tree is trying to develop a branch collar around the pruning wound.
I took my knife and scratched the bark above and below that bark ridge. Below the ridge the inner bark was green indicating living and growing tissue. Above the ridge was brown and dead tissue. This summer, the living portions of the bark will continue to grow and the pruning wound will eventually be covered over by new wood.
Trees do not heal pruning wounds. They simply grow new wood to cover the wound.
