I hunt public land as well and had a deer shot 15 yards away from me as I had the crosshairs on his shoulders! W.T.F.
Turns out some kid was hiding behind a tree, he knew where I was (I had no idea he was there) and he shot straight in my direction. I gave him a pass but man...sucks to have a deer shot from under you.
As far as your case goes, I would introduce yourself and say "well... if we're going to be hunting this close to each other we might as well know who we're hunting by." I don't think 70 yards is too bad with a bow. But if it's a decent sized woods there should be no reason he can't find someplace else. On the other hand it's public land, he has just as much right to it was you do. That's public land for ya.