Whether your trees can be saved depends on the extent of damage. When bark is removed by animals, lawnmowers, or any other method, the photosynthates (sugars, etc.) produced in the leaves are partially or completely stopped from reaching the roots that rely upon them. The roots are still supplying water and nutrients to the twigs, buds, and leaves, but they are getting little to nothing in return. Gradually (or sometimes quickly) the root system declines to the point where it can no longer support the aboveground parts.
Complete girdling is almost always fatal, even though death may take months to two to three years (remember, the roots are still passing up water and nutrients through the undamaged wood). If 50 percent or more of the stem circumference is damaged, the plant will probably die or hang on in an unhealthy and ugly state for awhile. Less than 50 percent girdling, depending on the species and its relative health before the damage, usually does not spell disaster. You may find your trees much less vigorous than before the damage.
If you have less than 50 percent damage, your trees will need intensive care. Mulch the soil surface with 2 to 4 inches of coarse, organic mulch, to a distance of at least 3 to 6 feet from the plant stem. Keep the mulch several inches away from the trunk. Do not pile it up against the trunk. You will need to water regularly for the life of the tree if rain doesn't keep the soil uniformly moist. In some cases, bridge grafting may be done to save valuable trees and shrubs. Equally important, protect from girdling damage in the future by protecting them with hardware cloth in the fall.
Methods of Grafting
will provide step-by-step instructions on bridge grafting. Trees, Protecting from Winter Damage
has information on how to protect your trees from damage by rodents.