Is Missing Bark Dangerous?

Looking out your window, you observe the trees in your yard – flush with bright green leaves, delicate blossoms, and even the occasional bird’s nest.  Then you see a problem: some bark missing from the trunk.  Is this a problem?  Does it mean your tree is going to die and fall down?  Or can you just ignore it and assume everything will be fine?  The answer primarily depends on the cause of the missing bark.
The first reason for a tree to have lost its covering is damage.  Lightning strikes, automobile accidents, and even deer rubbing their antlers on the trunk can cause bark to fall off.  The general rule of thumb is if 25 percent or less of the circumference of the trunk is damaged, then the tree should heal, blocking off the wounded area and recovering fully.  More than 50 percent damaged will require specialized tree care to nurse the plant back to health.  For anything more than a minor wound, consult an arborist.
More than being the cause of danger, missing bark is usually a symptom of another danger.  Insect infestations and some plant diseases cause bark to disappear.  Certain fungal cankers result in the outer layer peeling away in strips.  Boring insects, such as the carpenter worm, eat their way through the wood, leaving open areas on the skin of the tree.
Any open wound on a tree may lead to disease and further injury.  Many insects and molds infiltrate plants through exposed pulp, and some damage can weaken nearby limbs.  Keep an eye on trees with even the smallest bit of missing bark.  With proper precautions and care, the injury should not be dangerous. 
Regardless of the cause, always call a tree care professional when you see missing bark.  If you do not, the insects or disease may kill not only that tree, but all the others in the area.