Eating the bark of trees is not ideal for horses. Some horses eat other things (i.e., soil, fences, etc.) because they are bored; others may have a nutrient deficiency causing them to crave odd things in their diet. Make sure he has plenty of hay or other palatable horse-appropriate feeds that would help keep him from eating the bark on trees. If this continues, try covering the lower 6 feet or so of the trees with chicken wire to prevent the stripping of the trees.
