On July 7, 2003, at 1834 mountain standard time, a Sikorsky S-64E, N6979R, impacted trees with the tail rotor while hovering near Tucson, Arizona. The U.S. Forest Service was operating the helicopter as a public-use flight under the provisions of 14 CFR Part 133 for firefighting operations. The airline transport pilot and copilot were not injured; the helicopter sustained substantial damage. The flight departed Tucson International Airport (TUS), Tucson, about 1715. Visual meteorological conditions existed during the flight and a company visual flight rules (VFR) flight plan was in effect. In a written statement, the operator reported that the helicopter was equipped with a 2,000 gallon fixed tank with snorkel hover fill capability. After a series of water drops, the crew refilled the tank from a dip tank. The dip tank was located in mountainous terrain and there were two mesquite trees near it. During the refilling operation, the tail rotor made contact with the mesquite trees, but the pilots were unaware of the strike. When the helicopter returned to Tucson, it was refueled. During the refueling operations, damage was discovered to the number one exhaust stack. The helicopter was shutdown and examined. The tail rotor blade damage was discovered and initially thought to be a result of debris from the exhaust stack coming into contact with the tail rotor blades. In a telephone interview with a representative of the operator, he stated that closer inspection of the helicopter revealed that a portion of the exhaust stack was collapsed, but no portions were missing. The pilot reported to him that, when dipping the tank, he flew perpendicular to the road, into the wind with the tail of the helicopter in the trees. A tail rotor blade tip cap and small section of a mesquite tree were found at the dip tank site. A Federal Aviation Admininstration designated repair station inspected three of the tail rotor blades and found spar damage on the leading edge of one blade approximately 14.5 inches from the tip end. The two other blades examined were noted to have minor damage.