Treebeard's voice is done by the actor playing Gimli. So I've heard, and I think he'll do really well. I like him a lot as an actor. Any other "Sliders" fans here? I used to watch that show religiously, until he left. After that...well, he made the show for me, so I didn't watch it much anymore.
Love John Rhys-Davies; loved his wonderful, fulsome voice since Raiders on the Lost Ark. He'd be a great Treebeard. He hated the makeup for Gimli, though, and it hated him. If ever an actor was "unrecognizable under his makeup," that'd be JRD.
Dan