The purpose of the flight was to inspect landing strips to be used for fire-fighting support. The pilot had inquired about the amount of rainfall in the past few days and determined that the surface conditions would be satisfactory. On arrival he judged the strip to be in firm condition after two low passes. After landing, the surface was found to be much softer than anticipated and the aircraft became mired while taxiing. Two attempts to take-off were made, the first being aborted when rotation speed could not be attained. On the second attempt, lift-off was too far down the runway and the aircraft collided with trees off the end after it stalled when the pilot attempted to pull it over the trees. The pilot used reasonable judgement when planning but the information about previous rainfall was inaccurate and affected his decision. The surface condition was too soft to provide safe obstacle clearance distance for take-off.