THE PILOT WAS ON A VFR CROSS COUNTRY FLIGHT WHEN THE AIRCRAFT DESCENDED RAPIDLY AND COLLIDED WITH TREES. SEVERAL WITNESSES REPORTED HEARING EXTENSIVE ENGINE SURGES. WITNESSES REPORTED SEEING THE ACFT MANEUVERING IN A HIGHLY ERRATIC MANNER, THEN BEGIN A NOSE DIVE FOLLOWED BY ACFT ROTATION. ALL THE WITNESSES STATED THAT THEY HEARD THE ACFT'S ENGINE SURGE TO A HIGH SETTING JUST PRIOR TO IMPACT. THE ACFT WAS HEADING WEST WHEN IT IMPACTED IN A 40 DEGREE NOSE DOWN ATTITUDE AND A 45 DEGREE RIGHT BANK. THE ACFT DEBRIS WERE SCATTERED THROUGHOUT A 100 BY 200 FOOT AREA OF THE DENSELY FORRESTED, HARD GROUND. THE AREA FORECAST INCLUDED MOUNTAIN OBSCURATIONS, MODERATE RIME OR MIXED ICING, AND TURBULENCE WITH VISIBILITIES BETWEEN 3 TO 5 MILES. THE LOCAL WEATHER WAS AS FORECAST ACCORDING TO LOCAL WITNESSES AND BY AIRLINE EMPLOYEES.