I was mad at the time when a rodent did quite a bit of chewing on the trunk of one of my Japanese maples (naturally, they picked the one that is furthest along in its development of the ones that were originally landscape nursery stock, not pre-bonsai when I got them). Now, after another growing season, I'm actually pretty ok with what happened as the tree survived just fine and it gave the bark on the trunk some character that looks very natural. Still, I'm not tempting fate again. I wrap the trunks of my deciduous trees with copper mesh before I take them down off the benches and set them on the ground for the winter. Lesson learned.Some of the best bonsai are the result of browsing or weather damaged trees. This may slow the thickening a little but may just add loads of potential in the long run.