The pilot of the glider reported that the takeoff and aerotow were normal, and he released from tow at 3,000 ft above ground level (agl). He stated that he kept the airport in sight for most of the flight; however, at an altitude of about 1,500 ft agl, he lost sight of the airport and was unable to locate it. At an altitude of 1,100 ft agl, he decided to look for a suitable field in which to land off-airport. As he approached the field, he recalled flying over a tree, but did not recall the accident sequence. A witness stated that he watched as the glider descended toward the field, “clipped” a tree, and collided with the ground nose-first. The glider’s vertical stabilizer and both wings were substantially damaged during the accident and the pilot was seriously injured. The pilot reported that there were no preaccident mechanical malfunctions or anomalies with the glider.