I wonder if in the winter, food is scarce for them or moisture if everything is frozen and that is why they are eating the bark. I can't imagine that bark would be the food of choice for a rabbit. If I realized when it started, I would try putting out food for them and see if they ate that instead of eating the trees? I think you might also try a live trap, I don't know if it would work to relocate a rabbit, but it might scare it enough that it wouldn't visit again. Or you could wrap some kind of wire around the first foot or so of the trunk around the ground in the fall.
