After the helicopter had been transitioned to forward flight, the pilot felt a vibration through both the collective and cyclic controls. During his attempts to stop the vibration, the helicopter was allowed to descend. As he then selected a climb attitude the helicopter yawed to the right. The pilot was unable to correct the yaw and the tail struck a tree which slowed the yawing and allowed the pilot to land the helicopter. Initial inspection revealed that the intermediate flexplate in the tail rotor drive system had disintegrated. A metallurgical examination of the flexplate indicated that it probably failed due to overload. The examination also revealed that prior to the application of the overload that resulted in the ultimate failure, the flexplate had been cracked and weakened by another previous overload. The cause of the overload that resulted in the pre-existing crack could not be determined. However, examination of the tail rotor indicated that the ultimate failure most likely occurred as a result of a minor tail rotor strike whilst the helicopter was transitioning into forward flight.