THE PLT REPORTED THAT HE MADE A LOW PASS OVER THE MEDICINE LAKE AREA, THEN AFTER CIRCLING FOR A FEW MINUTES, HE CLIMBED BACK TO 7500 FT, ESTABLISHED 2000 RPM & 70 KTS, & CONTINUED THE FLT. APPX 5 MIN LATER, HE NOTICED THAT THE TREES APPEARED TO BE GETTING CLOSER. HE CHECKED HIS VERTICAL VELOCITY INDICATOR & NOTED A 200 FT/MIN DESCENT. HE THEN APPLIED FULL THROTTLE, BUT DID NOT NOTICE THE RPM INCREASE & CONTINUED TO LOSE ALTITUDE. SUBSEQUENTLY, THE ACFT COLLIDED WITH A TREE & CRASHED AT AN ELEVATION OF ABOUT 6000 FT. DURING THE INVESTIGATION, THE ENG WAS EXAMINED & RUN ON A PORTABLE TEST STAND & IT OPERATED NORMALLY. THE DENSITY ALTITUDE OF THE CRASH SITE WAS ESTIMATED TO BE 8300 FT. REPORTEDLY, THE TEMP & DEW POINT IN THE AREA WERE 63 & 32 DEG, RESPECTIVELY. ACCORDING TO ICING PROBABILITY CHARTS, VISIBLE CARB ICING WAS POSSIBLE AT RATED POWER. ALSO, UPDRAFTS & DOWNDRAFTS WERE COMMON IN THE AREA & WERE REPORTED BY OTHER PLTS.