The pilot was engaged in spreading fertilizer on a forest. The airstrip being used was in a valley and to fly to the area of operation the aircraft had to cross a ridge line. As the aircraft approached the ridge the pilot stated that he noticed that the aircraft was descending, he applied full power and jettisoned the load. However, the stub wing of the aircraft struck some tall trees below the top of the ridge and a short distance later the forward flight of the aircraft was arrested when it collided with a large tree. The aircraft slid down the tree and became wedged between it and two other trees. The pilot was able to evacuate himself from the wreckage and walk to the airstrip. The pilot had taken 19 loads across the ridge line en-route to the treatment area prior to the accident flight. He had been clearing the tree- tops on the ridge by 30-60 feet on each run and planned to do the same this time. However, as the aircraft approached the ridge line the pilot reported that it encountered a downdraught. The presence of downdraughts in the area could not be substantiated by either other pilots or meteorology reports.