After takeoff, the pilot noticed the tachometer reading 2,100 RPM and the airspeed was 45-50 miles per hour. He felt the climb performance was less than normal to reach 500 feet above the ground (AGL), where he began a climbing right turn to the crosswind. During the turn he noticed the altitude decreasing, so he tried to recover from the descent. At about 400 feet AGL he felt the airplane sink and start to stall in what he believed was a downdraft. He lowered the nose and started to recover from the stall when the airplane impacted trees. Density altitude was 2,867 feet. No anomalies were found with the airplane.