Branch attachment, Part I | Tree Biology

Terms: branch collar, branch bark ridge, branch protection zone, branch union
Knowing how branches are attached is among the most foundational knowledge for tree workers.
For clarity, stem = parent stem, larger of the 2; branch = smaller one that's attached to the parent stem (once it starts branching, it too becomes a stem)
The branch union is the point on the stem that a branch is attached, starting at the original node. As the branch and stem grow, they put down alternating layers of xylem that embed the branch firmly in the stem. The alternating layers creates a swelling in the base of the branch called the branch collar.
At the top of the attachment, bark from both sides is being pushed up into the branch bark ridge.
Once a branch is damaged or killed, cells within the branch collar will produce chemicals to protect the main stem. This creates a branch protection zone.
Avoid cutting inside the branch bark ridge and branch collar, as those include stem wood. Damaging the collar means that the branch protection zone is inhibited.

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