Defined as a branch collar continuing to develop around the remains of a dead branch.

In the image shown, the branch failed, possibly as a result of storm damage, and the remaining section of branch died. You can see at the base of the branch the live section of the branch that continues to grow around the dead branch. In this instance we would recommend selective pruning of the branch to remove the dead stub and give the tree the opportunity to compartmentalise the wound. If the branch is left in its present state, the limb will deteriorate and there is the risk of this limb failing or the tree will be exposed to pests/diseases and placing the health of the tree at risk. In instances where there is evidence of a good habitat hollow in the stub, we will prune it back to minimise the risk of limb failure but also enable the limb to be used as a habitat hollow.

Trees develop collars as they grow as a mechanism of separating the trunk from the stem if the branch died. Every year the new growth envelopes the existing branch collar, within this collar a tree develops a protection zone that contain antimicrobial substances, when a branch dies the branch inhibiting organisms (decay/dieback) grow downwards towards the collar, the protection zone restricts the access of these organisms into the trunk effectively stopping the decay at the collar. (Alex Shigo - A New Tree Biology, pg 13).