THE ACFT CRASHED ON THE ARPT AFTER A TURN OF APRX 120 DEGREES TO AVOID 60 FT TREES AT THE DEPARTURE EDGE OF THE ARPT. THE PLT HAD USED THE GRASS BESIDE THE AIRSTRIP BECAUSE OF STANDING WATER AT PLACES ON THE RWY. THE TAKEOFF RUN WAS STARTED ABOUT 500 FT FROM THE BEGINNING OF THE RWY ON THE HAY STUBBLE ADJACENT TO THE HANGAR. AUTO FUEL WAS BEING USED WITHOUT STC AUTHORIZATION. THE PLT STATED THAT THE ACFT SEEMED TO STOP CLIMBING AT ABOUT 50 FT AGL AND HE THOUGHT HE WOULD NOT BE ABLE TO CLEAR THE TREES AHEAD. DURING THE TURN THE ACFT STALLED AND WAS LANDED WITH THROTTLE STILL FULL OPEN ACCORDING TO THE PLT. THE THROTTLE WAS CLOSED AS THE PLANE APPROACHED SMALL TREES AND A HEDGE ROW. THESE TREES WERE HIT BY THE RT WING AND THE ACFT SWUNG RT TO A STOP. THE PLT SAID LATER THAT HE FOUND THE ELEVATOR TRIM ALMOST ALL THE WAY NOSE DOWN WHICH MADE HIM THINK THAT THE ACFT WOULD NOT CLIMB. HE SAID THAT THE 200 LB ACFT OWNER HAD FLOWN THE ACFT ON ITS LAST FLT AND SINCE HE SAT IN THE REAR SEAT HAD TRIMMED THE ACFT NOSE HEAVY.