The flight was planned as a fire spotting flight. All systems were normal during the pre-flight, start and after start checks. The aircraft took off towards the east.The wind was reported as blowing from a southerly direction at approximately 10 knots. Witnesses reported that, after lift off, the aircraft climbed at a very steep angle and very slow airspeed to approximately 300 feet above ground level. At that point the aircraft entered a flat spin to the left and completed four turns prior to impact 110 metres to the left of the runway centreline and 300 metres from the start of the takeoff role. The engine ran at a high power setting throughout the flight. A post accident inspection of the wreckage did not disclose any aircraft failure that might have contributed to the accident. The inspection did determine that the aircraft was not fitted with an aural stall warning device, similar to those fitted to all the other PA18 aircraft used by the Operator. A row of trees, designed to act as a wind break, is located along the southern side of the flight strip. These trees may cause some turbulence when a southerly wind is blowing. The pilot advised that he was concerned about possible turbulence from the trees and the crosswind effect on the aircraft. As a result he attempted to climb the aircraft above the trees as soon as possible after lift off. During the climb the pilot was distracted by the aircraft's proximity to the trees and he did not monitor the airspeed closely. He allowed the airspeed to reduce and at approximately 300 feet above ground level the aircraft stalled and entered a flat spin to the left. The pilot had no warning of the stall. There was no pre-stall buffet and the aircraft was not fitted with an aural stall warning device. The pilot attempted to recover from the stall but the aircraft impacted the ground before his attempt had any effect. It is possible that had an aural stall warning been fitted to the aircraft it would have alerted the pilot in sufficient time to prevent the loss of control.