It may be a long shot, but do you know WHY he is digging? For instance, do you possibly have moles or other subterranean critters? If that is the problem, you need to use a pesticide that will kill the grubs and other food sources for the moles, and then they go away and the dog doesn't have a "target" anymore. If he's digging just for fun, you may have a problem on your hands, with no easy solution. Sorry, or Good luck, whichever the case may be.
I have been catching him in the act now and send him a page on his e-collar and has been leavin it alone more and more now that i have looked at the tree more i think it is because it has a nest at the top?