IMPROPER PLANNING/DECISION BY THE PILOT, HIS PREMATURE ROTATION FOR TAKEOFF AND SUBSEQUENT FAILURE TO OBTAIN OR MAINTAIN SUFFICIENT SPEED TO CLIMB, AND HIS FAILURE TO ABORT THE TAKEOFF (WHILE THERE WAS SUFFICIENT RWY REMAINING). CONTRIBUTING FACTORS WERE: THE PILOT ALLOWED THE AIRCRAFT GROSS WEIGHT TO EXCEED THE MAXIMUM LIMIT, HIGH DENSITY ALTITUDE, UPHILL RUNWAY GRADIENT, DOWNDRAFT, TURBULENCE, AND TREES.