On August 24, 2002, about 1600 mountain daylight time, a Bell 206L-1, N832AH, registered to Hillsboro Aviation, operated by the U.S.D.A. Forest Service as a public use firefighting flight, collided with tree(s) located about 18 miles west of Wisdom, Montana. Visual meteorological conditions prevailed at the time and company visual flight following was in effect. The helicopter was substantially damaged and the commercial pilot, the sole occupant, was not injured. The flight had been operating in the area all day. During a post-flight walk around, the pilot noticed damage to both main rotor blades. One blade was punctured and both blades displayed tearing to the underside material of the blade. The pilot reported that several missions had been flown that day and he was unsure as to when the damage occurred. The pilot stated, "I believe that during one water drop the aircraft's altitude was insufficient to avoid contact with an unseen tree top which rose into the rotor near the opposite side of the aircraft from the pilot station. This tree top would have been taller than the surrounding timber." The pilot reported no mechanical failures or malfunctions with the aircraft at the time, and that he felt no vibration, heard no noise, or noticed any change in rpm. USFS personnel reported that it appeared that during the second to the last flight of the day, while the pilot was supporting firefighting personnel on the ground, the pilot descended into the trees with the 50-foot long line that held the water bucket. During this maneuver, the main rotor blades came in contact with tree(s).