Since even a cub has sharp teeth, anything that will hold a bears weight that he can climb. :roll:
From beech tree scars in the Adirondak forest most that I have seen were 10-18". But then piss one off and I would not hold on to that figure. One climbed the ladder stand of the junior high principal last year and swatted his foot. I have also seen many on the Outdoor Channel climb ladders and such.
Climbing a tree to get away from a bear is not he best plan if that is the plan.