While there are several approaches to repairing missing bark from a tree, New Mexico State University recommends using bark grafting. Since this process uses thumb-sized branches from the tree, it works even when the original bark is missing. It is especially useful in repairing damage from vehicles or rabbits.
With bark grafting, remove any loose bark and sharp edges around the injury, careful to avoid further damage to the tree. Cut the graft branches a few inches longer than the injury. Flatten one edge to lie against the tree trunk, and cut the other to form a wedge. Cut parallel flaps in the bark that are slightly longer than the bridge graft, and leave them attached to the tree. Lift the flap and insert the wedge-shaped edge under the attached bark. A successful graft restores the nutrient flow to the roots and increases the tree's chances of survival.
Removing a ring of bark from around the tree is a process known as girdling and normally kills the tree by cutting off the root's food supply. When forming a bridge graft, the bark should be oriented in the same direction as its natural growth as phloem tissue only transports nutrients in one direction.