When you wound a bonsai tree through the removal of a branch you create a wound. Depending on where you prune the branch back to will depend on what type of response the tree will have to the wound created. Some wounds will seal over completely, while others will remain with a hole or cavity or wound feature and this is down to where you prune.
If pruned too close to the main trunk, this will create a flush wound and this will create a hole behind the wound called a cavity. The reason for this is that the cells for creating wound wood / callous wood have been removed right down to the trunk and the wound wood forming will be only just around the edges of the wound and will be insufficient in most instances to fully seal over the wound as it grows leaving an open wound where decay can set in.
If pruned too far out from the main trunk, this will create a stub, which will slowly die back over time to the main trunk. Therefore the correct position to prune a branch is shown in the diagram below but this will vary with species as trees do not follow the books.
If pruned at the branch bark ridge in the correct position the wound will eventually seal over completely and during this process a ring of callous wood is formed around the edges of the wound. Here are a few examples of this.
When creating wounds on bonsai ensure that you apply cut paste to the wounded area afterwards to reduce the potential for moisture loss and to seal over the wound aiding with the tree response naturally. Cut paste contains beneficial hormones to aid with callous and wound wood formation and there are various products on the market for both coniferous and deciduous trees as well as the cut putty.
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