Often times during winter the bark on young trees can be damaged by deer rubbing their antlers on them, or by rabbits feeding on the bark tissue.  The injury to your trees could take years to heal or in cases where the trunk has been completely girdled, cause the tree to die within a few years. The best defense is to put a corrugated plastic tree guard on the tree.  Ideally keep it on from November through early spring.  They are commonly found at hardware stores and garden centers.  I recommend using them for the first several winters until the trees develop thicker bark.  Deer and rabbits cannot resist thin bark, especially those found on young maple, linden and crabapple trees. Other times of year deer and rabbits might also graze on your landscape plants and shrubs.  It’s recommended to choose plants they dislike, although they will eat anything if they are hungry enough.  Spray on products that smell or taste bad can deter them to a degree, but if you live in an area with large deer or rabbit populations, some foraging is common.
Tree Damage girdle

Girdle #1

Deer Girdle

Deer Girdle

Deer Girdle Tree Damage

Deer Girdle #2

Tree Damage

Trunk Guard

David Anderson Shadywood Tree Experts ISA Certified Arborist   Additional reading: http://www.extension.umn.edu/garden/yard-garden/wildlife/deer-in-the-home-landscape/ http://www.extension.umn.edu/garden/yard-garden/wildlife/rabbits-and-trees-and-shrubs/