The pilot flew a visual traffic pattern to a grass runway utilized by a combination of light sport aircraft, powered parachutes, and radio controlled aircraft. While on downwind at 400 feet above ground level, the pilot noticed three ground vehicles parked in a lot often used by pilots when they flew radio controlled aircraft. As he searched for possible radio controlled aircraft, the pilot became distracted and allowed his airspeed to decrease, which resulted in an aerodynamic stall. The pilot was unable to regain control and the airplane subsequently impacted a line of trees.