The pilot was landing at his home airport and requested runway 32 since the winds were reported from 320 at 6 knots. Dusk lighting conditions prevailed. The final approach is initially over trees, then opens into a cleared area where approach lights are positioned. After crossing over the trees on short final, the airplane seemed to drop and the pilot added power. The airplane contacted the approach lights and the airport perimeter fence before coming to a stop in the grass adjacent to the runway. The FAA inspector examined the airplane after the accident and found no evidence of a mechanical malfunction or failure. The pilot concluded that he may have hit an air pocket.