The pilot and aircraft had been fighting forest fires earlier in the day. A late developing fire necessitated another dispatch. The aircraft arrived on scene and checked in with the lead aircraft circling overhead coordinating the fire fighting efforts. The tanker made 2 runs over the fire area following the lead aircraft. After the 2nd run, the aircraft banked left in a level turn to make another run without using the lead aircraft. During the 3rd run the tanker struck trees near the top of a 1800 ft ridgeline. Witnesses on the ground remarked how low the aircraft was. According to the pilot's peers, he took pride in doing his job well and always strove to put the chemical right on target. There had been occasions in the past where this zeal to succeed had resulted in his going lower than the 150 ft AGL minimum altitude mandated by forestry service regulations. He had, on occasions, returned to base with tree foliage stuck in his aircraft.