THE PILOT INITIATED A TAKEOFF FROM A ROUGH TURF AIRSTRIP THAT WAS 2400' IN LENGTH. THE RUNWAY HAD AREAS OF SOFT DIRT INTERSPERSED WITH AREAS OF GRASS THAT RANGED FROM 1 TO 8 INCHES IN HEIGHT. DURING TAKEOFF, THE AIRCRAFT COLLIDED WITH TREES NEAR THE DEPARTURE END OF THE RUNWAY. THE TREES WERE ABOUT 75' TALL; IMPACT OCCURRED ABOUT 50' ABOVE THE GROUND. CALCULATIONS SHOWED THAT THE DENSITY ALTITUDE WAS ABOUT 4700', AND THAT WITH THE PREVAILING CONDITIONS, ABOUT 2213' TO 2616' OF RUNWAY WOULD HAVE BEEN NEEDED FOR THE TAKEOFF GROUND RUN.