For the heavy stuff, I've seen a railroads that have used a rapidly rotating steel beam about 4-5 feet long, like a big lawnmower blade but really a club that wacks limbs off trees using blunt force. These may be called flail mowers, I'm not sure. Leaves a mess, though, and the remaining trees are really ugly and rough looking, like a tornado hit the side of them.
I think these forestry mulchers are in wider use now. Pretty neat to watch.
