The pilot reported that while on a pleasure flight the airplane's engine sputtered and lost power. He said that he checked the magnetos, applied carburetor heat and checked the fuel selector in an attempt to restore engine power. Engine power was not restored and the pilot executed a forced landing to a field. During the landing, the airplane struck trees, resulting in substantial damage. The pilot noted that after the accident there were no apparant preimpact visible defects that would account for the loss of engine power. A separate examination of the airplane subsequent to the accident revealed no anomalies that would account for the loss of engine power.