According to the pilot, while enroute, the engine rpm began to decrease. The pilot began to troubleshoot the partial loss of engine power by verifying the cockpit switches were in the correct position, verifying the fuel selector position, switching between the magnetos, and applying carburetor heat, but the engine power did not return. The airplane was unable to maintain altitude, so the pilot made a forced landing into a field, during which the airplane collided with a tree. The airplane sustained substantial damage to the fuselage and right wing. Following the accident, the pilot reported that there were no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation and stated that the partial loss of engine power the airplane experienced on the accident flight was likely the result of carburetor icing. The weather conditions at the time of the accident were conducive to the formation of serious icing at glide engine power settings.