The pilot elected to land a high skid, turbine-powered helicopter in a burned-out wooded area as part of a vegetation reclamation survey. The area he selected contained several rain-soaked, burned and fallen birch trees, which required that he land perpendicular to and on the fallen trees with the helicopter's skids. The operator reported that after landing atop the uneven, fallen trees, the pilot retarded the engine throttle to flight idle to begin the mandatory 2 minute engine cool down procedure. The helicopter subsequently tipped backwards and slid to the left, with the tail rotor striking a log. The pilot said there were no preimpact mechanical problems with the helicopter. The helicopter sustained substantial damage to the tail boom, tail rotor drive system, and tail rotor gearbox.