THE FLT TURNED BASE WITH PARTIAL SPOILERS EXTENDED THEN TURNED FINAL, & ENCOUNTERED A 'SEVERE DOWNDRAFT.' THE PLT RETRACTED THE SPOILERS BUT THE DESCENT CONTINUED UNTIL IMPACT WITH TREES APPROXIMATELY 300 FT SHORT OF THE RWY. HE HAD ACCUMULATED 3 HRS TOTAL TIME MAKE & MODEL DURING 8 DUAL & 12 SOLO FLIGHTS IN THE PRECEDING 30 DAYS.