Thank you for contacting AaE for help.

This is a common problem in winter.  The following is Master Gardener Barbara Harlan's answer to a similar question: There really isn't anything you can put on the trunk to fix the problem. Depending on how much of the bark was chewed off, you tree may or may not survive. If your tree has been complete girdled it probably will leaf out in the spring but will not be able to survive. If there is only partial damage, your tree may be able to survive but may be compromised. The following is a direct quote from a University of Ohio Fact Sheet on Rodent Damage: "The meadow and pine mouse (vole) feed on apples trees of all ages and may damage other fruit plants. Meadow mice eat the bark of the trunk and roots both above and below the ground level. Pine mice nest underground, eating bark from the roots near the surface. Mouse damage is easily noted by carefully removing soil from around the base of the tree and over the larger roots near the soil surface. Bark completely removed around the trunk or roots by gnawing of the rodents will girdle the tree and cause death or severe plant weakening. Keeping the soil bare and free of grass cover or else mulch around fruit trees or vines will help discourage mice from meeting nearby and feeding on fruit plants. Pea gravel (small stones) placed 1 inch below the soil surface and around the tree will discourage mice. When planting trees, allow the soil to settle one inch and apply the stone two inches deep."

Master Gardener Bob Bystrom adds: If your tree does not survive and you purchase a replacement, please following the steps given in Protecting Trees and Shrubs Against Winter Damage to prevent this happening again. Rabbits also feed on bark of trunks and exposed roots of young fruit trees. Rabbit guards, preferably wire screen, wrapped around newly-set fruit plants are suggested to prevent feeding damage. Where plants are extremely girdled by rodents, death of plants usually occurs shortly after growth begins in spring. As with other types of mechanical injury, weakened plants are more susceptible to drought, cold injury or insect and disease infestations. Rabbits will generally feed on apple trees and brambles when the soil is covered with snow for 10 to 14 days. With vinyl guards, put guards on newly planted trees in October and keep the guard on all year.

To which I will add:  It does looks like the tree trunk has been girdled.  If you purchase a new tree and use a vinyl guard, check that the tree trunk is not pushing against the guard each spring.  You want to allow the trunk to expand without hindrance as the tree grows.

I hope this helps.