The pilot reported that he visually checked both fuel tanks prior to takeoff and noted he had 40 gallons of usable fuel for the flight. About an hour into the flight, the engine lost all power and he conducted an off-field emergency landing. During the landing, the airplane collided with a tree and damaged the left wing. After the emergency landing, while securing the airplane, the pilot discovered fuel streaks on the left horizontal stabilizer. He also discovered that the left tank fuel cap was missing. Although he said he thought he replaced the fuel cap after checking the fuel, he contacted the departure airport, and the fuel cap was located on the ramp. The pilot did not report any other mechanical anomalies. An examination of the wreckage by a Federal Aviation Administration inspector revealed structural damage to the left, outboard wing. The fuel tank was not breached, and the left fuel tank fuel cap was missing. The left and right fuel tanks were checked for fuel and both tanks were found empty. The fuel selector was discovered in the both position. The forward sump was drained, and a slight amount of fuel drained from the sump.