Rabbits & hares love to chew on them to help keep their teeth down and to help bring certain bacteria into their gut.
If they haven't done too much damage , can you fit standard tree guards around the trunk or if the trunk is too big cut the bottom out of a plastic bucket and slit the side to make one add plenty of chicken wire to the bucket to Keep the away from further up the trunk
OR
Use 1/2 " chicken wire fastened on a triangle of four foot tall sticks hammered it to the ground .