On February 9, 2012, at 1530 Pacific standard time, a Cessna 172M, N5100R, experienced a loss of engine power during cruise flight and the pilot made a forced landing in a forested area approximately 2 miles north of Cusick, Washington. The owner/pilot operated the airplane as a personal cross-country flight under the provisions of 14 Code of Federal Regulations Part 91. The pilot, the sole occupant, was not injured. The airplane sustained substantial damage to the wings and fuselage. The flight departed Coeur d'Alene Airport – Pappy Boyington Field (COE), Coeur d'Alene, Idaho, about 1500, and was destined for Sand Canyon Airport (1S9), Chewelah, Washington. Visual meteorological conditions prevailed for the local area flight, and no flight plan had been filed.According to the pilot, he performed a preflight of the airplane, to include sumping the fuel tanks and checking the oil before his departure from COE; there were no abnormalities noted. He also stated that the run-up was normal. On takeoff, he had 10 degrees of flap extended. The takeoff was normal, but the pilot stated that the airplane did not feel 'as spunky as normal'. All of the indications were good; it just did not seem to be producing power. The pilot stated that about 10 minutes from his destination airport, the engine began to run rough. He thought the engine may have carburetor ice, and he pulled the carburetor heat on; however, it did not clear up. He began looking for a spot to make a forced landing, while also attempting to troubleshoot the engine problem. The troubleshooting included switching from one fuel tank to the other, activation of the carburetor heat, use of the boost pump and primer, and all failed to restore power. The pilot stated that the engine kept missing like it wanted to quit. He saw a field that he thought he initially would make; however, it became apparent to him that he was not going to make the field. At that point, he saw an opening and flew toward it. The pilot recalls turning off the master, magneto, and electrical switches before the airplane struck trees. On scene investigation revealed that the left fuel tank was breached and empty from the impact. About 8 gallons of 100LL fuel were drained from the right fuel tank from wreckage recovery personnel. A post accident examination of the airplane and subsequent engine run revealed no mechanical malfunctions or failures that would have precluded normal operation.