The commercial pilot departed a 3,200 foot-long runway on a hot day in his 65 horsepower rated single-engine airplane. When he reached an altitude of 150 feet and an airspeed of 55 knots, the pilot made a left hand turn so he could clear the area for the other aircraft waiting to back-taxi/depart. While in the turn, the airspeed decreased and the pilot was unable to maintain altitude. The airplane collided with trees south of the runway. The pilot reported that there were no mechanical discrepancies with the airplane or the engine, and he should have continued straight out on departure versus turning.