this is all i could find right now..
Chewing wood: For pure chewing enjoyment and immense pleasure, there's nothing better than a nice piece of soft wood to gnaw on...for some rabbits. Many of my rabbits aren't the least bit interested in a piece of plain old wood and many never gnaw. But most do love a good piece of apple tree twig and will gnaw the bark from the twig as though it were a delicious treat. (Caution: Not to be fed are branches from apricot, cherry, peach, plum and redwood trees, listed as toxic by poison centers.)