On March 11, 2000, at 1210 central standard time, a Piper PA-18- 150, N150AM, experienced a loss of control while landing on runway 36 (2,450' x 20') at Brennand Field, Neenah, Wisconsin. The pilot intentionally ground looped the airplane and it collided with trees. The commercial pilot and passenger were not injured. The airplane was substantially damaged. The 14 CFR Part 91 flight was operating in visual meteorological conditions without a flight plan. The flight originated from Oshkosh, Wisconsin, at 1135 cst. The pilot reported that he checked the flight controls before the flight and they operated normally. He reported that just prior to touchdown he tried to apply "full up elevator, but the control stick hit a resistance." He reported the airplane bounced on landing and yawed to the right. The pilot reported he applied full left rudder and brake, but the airplane continued toward the trees. He then applied right rudder and brake to intentionally ground loop the airplane in an attempt to miss the trees. However, the left wing contacted the trees and the airplane spun around to the left prior to stopping. Post accident inspection of the airplane revealed the rear seat cushion was mounted on plywood. The plywood seat bottom was not fastened to the seat structure/airframe. There were snaps mounted on the seat pan for securing the cushion. There was only one snap on the plywood which did not match the position of the snaps on the seat pan. This allowed the seat cushion to move forward, restricting the movement of the control yoke.