A callus on a bonsai tree is a sign of healing after a wound has been made by pruning or removing a branch. A callus is a layer of new tissue that forms around the edge of the wound and gradually rolls inwards to close the gap. A callus can help prevent infection and improve the appearance of the bonsai tree. To promote callus formation, it is important to make clean cuts with sharp tools and avoid tearing the bark. Some bonsai enthusiasts also use concave cutters to create a slightly hollow wound that can heal more evenly. Sometimes, a wound may need to be reopened or scraped to stimulate more callus growth if it is not healing well. A callus can take months or years to fully heal, depending on the size of the wound and the species of the bonsai tree.