DURING FLT, THE HELICOPTER PLT NOTED THAT THE MAIN ROTOR RPM BEGAN DECREASING & THE ENG RPM BEGAN INCREASING. HE INITIATED AN AUTOROTATION, BUT THE HELICOPTER LANDED IN TREES & WAS DAMAGED. AN EXAM REVEALED A BOLT LUG ON THE END OF THE LINEAR ACTUATOR (THAT ENGAGED THE BELT DRIVE) HAD FAILED FROM FATIGUE.