The pilot reported that he was preparing to land and the wind favored an approach to the north. While on approach to land, he observed another glider being pulled out onto the field. Concerned about obstacle clearance during the landing, he continued on a downwind leg to the north to set up for a landing to the south. The last thing he recalled prior to the accident was a “very strong sink.” The glider collided with a cedar tree, then the ground, about 1,016 feet from the north end of the runway. A witness reported that the glider appeared to climb and then banked hard to the right and rolled upside down before the collision. An inspector with the Federal Aviation Administration inspected the wreckage and confirmed substantial damage to the empennage. His inspection of the wreckage revealed that the glider was airworthy at the time of the accident. The pilot reported no mechanical malfunctions or failure that would have precluded normal operation of the glider.