The pilot reported that he initiated a soft field takeoff on a wet and soft grass strip. The airplane lifted off and the pilot leveled the airplane at 10 feet above ground level to increase speed. The pilot stated that the wind shifted to a tailwind and the airspeed dropped 10 knots. The airplane would not clear small trees at the end of the runway, and the pilot aborted the takeoff after colliding with the first trees. The airplane then landed approximately 300 feet from the end of the runway.