How much bark does a tree need to survive?

Dam !

Any of the trees that the horses ate the bark down through the cambium layer-the greenish white inner bark-all around the circumference of the trunk will die. The cambium is the vascular system that provides nutrients and water to the new growth of the tree.

You're in luck, they need none. The bark is a protective layer with no other use. As long as bugs and birds dont cause havoc than you are fine. The roots and the leaves (for photosynthesis) will do its magic and develop bark over time. Actually, the science of genetics was found by a monk who simply cut stems from trees and plants to close them, he just cut stems off and planted them, they grew back healthy and similar to their parent plant.

well... alll of it u see bark keeps out disease that will kill them quite quickly once it gets in so u need to fence them off from ur horses and then make sure that it stays disease free