area where a subdominate branch joins another branch or trunk that is created by the overlapping vascular tissues from both the branch and the trunk. Typically enlarged at the base of the branch.

branch collar (Wikipedia)
A branch collar on a common oak (Quercus robur L.)

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. The collar consists primarily of tissues connected directly to the tree's main trunk or stem, not to the branch.

As a lateral branch typically grows more slowly in diameter than the upright stem to which it is attached, the base of the smaller branch becomes occluded, eventually forming a knot. It is this process of occlusion that gives rise to the swollen branch collar visible on many branch attachments.

When woody plants naturally shed branches because those branches are nonproductive, usually from lack of light reaching them, these lower branches typically die back to the branch collar. Insects and fungi decompose the dead branch, and it eventually falls off, leaving the exposed end of the branch at the point of its attachment at the branch collar. This arrangement helps to resist the spread of decay organisms into the parent stem or trunk during the time it takes for the increment growth of the trunk to seal over the dead branch stub.

Events such as storms or pruning may damage the branch collar, thus reducing the naturally occurring defenses of a branch attachment and exposing the trunk tissues adjacent to the wound to disease or 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.

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