On May 12, 2001, at 1600 eastern daylight time, a Cessna 172N, N1674E, was substantially damaged during a forced landing to wooded terrain near Ashland, Maine. The certificated commercial pilot sustained minor injuries. The flight originated from Caribou Regional Airport, Caribou, Maine. Visual meteorological conditions prevailed and no flight plan was filed for the fire patrol flight conducted under 14 CFR Part 91. In a written statement, the pilot reported that the airplane was refueled by the operator at 1430, and at 1500 he completed his pre-flight inspection. After completing all of the appropriate checklists, he departed at 1513. The pilot climbed to 600 feet agl and began his normal fire patrol route. After departing one of the checkpoints, he observed light rain showers ahead of him with visibility greater than 20 miles and cloud cover near 4,000 feet msl. He decided to return to Caribou due to the weather. After initiating a turn back toward Caribou, the engine began to lose rpm and run rough. The pilot checked the fuel selector valve, mixture, primer, and pulled the carburetor heat on, which had a "limited effect." However, he "suspected carburetor icing", and left the carburetor heat on. While looking for a suitable place to land, the pilot reported that the engine rpm dropped from 2,000 to 400-800. He was unable to maintain altitude and attempted to land on a logging road. The airplane came to rest in wooded terrain about 45 nautical miles west of Presque Isle, Maine. The operator responded to the scene and recovered the airplane. According to the operator, the fuselage was compromised, the firewall was wrinkled, and the engine mounts were broken. One of the propeller blades was bent aft. Both wings, the empennage and right horizontal stabilizer exhibited impact damage. Engine continuity was established by manual rotation of the propeller. The right wing fuel tank was breached, but the operator was able to drain 20-25 gallons of fuel from both wing tanks. He also reported that the fuel strainer bowl was filled with fuel. Fuel extracted from the wing tanks and fuel strainer bowl was absent of debris and water. The pilot reported a total of 518 flight hours, of which, 222 hours were in make and model. He also reported that there were no mechanical deficiencies. Weather at Presque Isle/Northern Maine Regional Airport, Presque Isle, Maine, at 1555, was reported as winds from 140 degrees at 11 knots gusting to 16 knots, visibility 10 statute miles, sky clear, temperature 73 degrees F, dewpoint 48 degrees C, and altimeter 29.72 inches HG. Interpolation of a carburetor icing probability chart revealed that conditions for moderate icing at cruise power and serious icing at glide power existed.