A branch collar is the often visible swelling in a woody plant that forms at the base of a branch where it is attached to its parent branch or to the tree's trunk. The top of the branch collar consists of dense interlocking wood grain, which provides mechanical support to the branch attachment. Branch collars can also be flat or somewhat recessed into the trunk or parent branch, as in some conifers.
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 often resists the spread of decay organisms into the parent branch or trunk during the time it takes for the increment growth of the trunk, 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 external appearance and anatomical importance of a branch collar is key to correct tree pruning. Pruning practices that mimic natural branch shedding avoid unnecessary damage to the plant's defensive anatomy.
- Slater, D. Bradley, R, Withers, P. and Ennos, A. R. (2014) The anatomy and grain pattern in forks of hazel (Corylus avellana L.) and other tree species, Trees 28 (5) 1437-1448
- Tree pruning guide prepared by the US Forest Service for the US Department of Agriculture features a diagram of the branch collar.