Brown makes a heavy duty brush cutter, a "tree cutter " I think it is called. Seriously heavy duty and probably has a price tag to prove it. I have seen them used, never in use, or sitting ready to be used. One version had a spring loaded back part of the mower deck that would retract as you backed the blade into a tree, basically keeping the blade covered at all times.(Sort of a 6'version of PoppaMike's mower, with a spring loaded plate covering the blade.) Something to keep in mind if you do build one, is the "kick" when the blade enters or catches while cutting. Remember how much a Skillsaw kicks when it sticks? Multiple that many times over for a blade like this! If you use a lawnmower, consideration for constantly breaking the key on the crankshaft might be needed!
David from Jax