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Most dogs do this because they are bored and are looking for something to do. Be sure to provide him with plenty of activity toys so that he has enough to entertain him when he's outside.
The fear with the tree bark is that it will cause an intestinal obstruction (highly likely) so you want to nip this behavior in the bud as soon as you can.
If nothing else, you might have to remove the wood chips and put a liner around the tree to keep him from eating it.
Let me know if you still have questions.