Bark is an integral part of any tree. It is part of the nutrient network which takes water and nutrients from roots to leaves. It also provides a protective covering for the softer wood inside. The following guide can help you understand the causes and concerns of different types of bark damage as well as what you should do about it.
The amount of concern you should have over bark loss depends greatly on the cause. Some types of trees, like paper birch for example, naturally shed bark. Most, though, do not. Strips of missing bark are often the result of animals like mice or deer feeding on the bark or animals scratching at the tree. Bark loss can also be caused by mechanical damage, such as hitting a tree with a lawnmower or tying a rope around a tree and leaving it to rub. The concerns with localized bark loss are primarily in that it gives an opening for insect or fungal infections to occur.
Holes are either caused by human intervention, like putting a nail in the tree, or by insects burrowing into the bark. Less commonly, holes can be caused by woodpeckers and other birds, but they generally only peck at trees with severe insect infestations. If you put a hole in a tree with a nail, it's usually a better option to leave the nail in then to pull it out and leave the tree exposed to insect infestation. If you notice holes from insects or birds, then you need a tree service out to treat for pests.
Girdling refers specifically to the removal of a strip of bark around the entire circumference of a tree. A common cause of this is placing mulch right against a tree trunk. In winter, the mulch may retain moisture, which causes bark to rot and fall off. Or, more likely, mice or other rodents bed in the protection of the mulch and chew the bark off in a ring around the trunk near ground level. Girdling can also occur from other animals like deer or beavers stripping the bark or from the rubbing of a line around the trunk, such as a clothes line.
What to do:
If bark damage hasn't resulted in a complete girdle, a tree service like S.A. Total Tree Service can treat the underlying cause of the damage by doing things such as treating for insects or caging the trunk against animals. Then they will prune back the canopy so the tree can continue to support a reduced amount of leaves while the wounds heal. If girdling has occurred, the tree likely can't be saved and will need to be removed.