Pilot stated that while departing from a small 2000 foot long private strip the engine appeared to lose power. The aircraft would not climb and collided with a tree at the departure end of the runway. The aircraft then collided with several other trees prior to colliding with the ground. Post crash running of the engine revealed no evidence to indicate any precrash mechanical failure or malfunction. Witnesses stated that after lift-off the aircraft appeared to level off at 10 to 15 feet above ground level and flew straight ahead colliding with the trees. There was no attempt to avoid the trees which had clear areas on both sides. In a written statement, the pilot reported lifting off at 80 knots, but in a verbal statement, he said he had lifted off at 40 knots. Also, a witness said the gear started retracting just after lift-off. According to the pilot's handbook, the max performance climb speed was 72 knots and the gear should have been retracted after obstacles had been cleared.