WITNESSES STATED THE PLT LANDED ON THE 3000 FT GRASS RWY, THINKING HE HAD LANDED ON A 4000 FT RWY ABOUT 5 MI TO THE WEST. THEY ADZD HIM NOT TO TAKEOFF IN THE 95 DEG TEMP. THE PLT DECIDED TO TAKEOFF AND USE ALL OF THE AVAILABLE RWY. HE SAID HE LEANED THE MIXTURE AND USED 1/4 FLAPS FOR TAKEOFF. WHILE CLIMBING OVER A ROW OF TREES AT THE END OF THE RWY, THE PLT BELIEVED THERE WAS A SLIGHT POWER LOSS. THE ACFT THEN SETTLED INTO A SECOND ROW OF TREES (60 TO 70 FT TALL) & CRASHED. THE WITNESSES RPRTD THE ENG WAS PROVIDING FULL POWER UNTIL IMPACT. THE ENG TACHOMETER WAS FOUND TO READ 150 RPM HI, BUT NO PREIMPACT PART FAILURE OF MALFUNCTION OF THE ENG WAS FOUND. THE FLT MANUAL INDCD THE ENG SHOULD NOT HAVE BEEN LEANED FOR TAKEOFF BELOW 5000 FT. PERFORMANCE CHARTS SHOWED THAT THE ACFT WOULD HAVE NEEDED 2025 FT TO CLR A 50 FT OBSTACLE.