A branch collar is the attachment structure in woody plants that connects a branch to its parent branch or to the trunk. The branch collar consists of overlapping wood fibers according to Alex Shigo's model of branch attachment. Research now disputes this model (Slater and Harbinson, 2010).
During plant growth cycles, wood on smaller branches forms first, according to Shigo's model. Wood at the base of the branch extends slightly over the face of the trunk, forming the branch collar. Then, the wood of the parent branch forms over the top of the basal branch wood, usually forming a circular structure called the trunk collar. Together, the branch collar and trunk collar in Shigo's model are simply referred to as the branch collar. Branch collars can also be flat or somewhat recessed into the trunk or parent branch, as in some conifers.
This model is not considered satisfactory by some current researchers, as it does not provide an adequate explanation as to how the anatomy of a fork-like junction is formed.
The accretion of layers of wood behind the branch collar is a conical decay-resistant structure called the branch core. The knot found in lumber is this branch core.
When woody plants naturally shed branches because they are nonproductive, usually from lack of light, these branches die back to the branch collar. Insects and fungi decompose the dead branch, and it eventually falls off, leaving the exposed branch core. The branch core resists the spread of decay organisms into the parent branch or trunk during the time it takes for the woundwood, or callus, to seal over the wound.
Events such as storms or pruning may damage the branch collar, thus defeating the naturally-occurring defense of the branch core and exposing the trunk to decay. Understanding the branch collar anatomy is important in tree pruning. Pruning practices that mimic natural branch shedding avoid unnecessary damage to the plant's defensive anatomy.
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