You need to get a Brown cutter. Some models are designed to cut trees by backing into them and then mulching them up. Pretty heavy duty bush hogs. The state highway crews use them and along ROWs. Sometimes you can find them used.
http://www.brownmanufacturing.us/catalog/7
Go get a Brown tree cutter used for $7K and all you need is a 60 pto hp 2 wheel drive tractor with heavy ply tires. That will handle 5-7 trees at 3-6 acs per day. Put a heavy wire screen between the ROPS. You can back over them or use your front axel to bend them over. I have a 5 ft tree cutter and it is unbelievable what it can do.
Good luck!