I have noticed that when young trees are planted they have some type of plastic wrapped around the base of the trunk. It is usually about a foot long and starts from the ground up.

It's a spiral strip of heavier plastic which is both an insect guard and to keep lawnmowers and insects from damaging the bark. Eventually the trunk expands and the tree outgrows the strip.

Where I live, there is a huge problem with rabbits and other animals that eat the bark and trunk of young trees. In the winter when there is little grass available, some creatures will feast on your trees.