On February 4, 2002, about 1215 hours Pacific standard time, a Schweizer SGS 2-33A, N34292, collided with trees on final landing approach and landed hard on runway 26 at the Warner Springs Gliderport (CL35), Warner Springs, California. The glider, owned by Associated Gliders of Southern California and rented by the pilot, was operated under the provisions of 14 CFR Part 91. The glider sustained substantial damage. The student pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed for the local area instructional flight and no flight plan had been filed. The flight originated from CL35. The Safety Board investigator interviewed the student pilot. The pilot stated that he was doing pattern work. The takeoffs were to be circuit tows, and he was to practice frontal slips and normal landings. He stated that this was the first tow of the morning. He was about 800 feet above ground level (agl) and the approach seemed normal; however, there was a little turbulence. He turned on final and attempted a frontal slip to a landing. The glider started to drift to the left. The student pilot stated that he must have cross-controlled the glider and could not return the glider to runway centerline. He maneuvered to avoid a tree and stalled the glider as the left wing contacted the tree. The student pilot stated that the glider "pancaked into the runway." He further stated that there were no mechanical discrepancies with the glider.