On June 15, 2003, about 1500 Pacific daylight time, an unpowered Schleicher ASW-20 glider, N34LB, impacted trees after a loss of thermal lift near Idyllwild, California. The pilot/owner was operating the glider under the provisions of 14 CFR Part 91. The private pilot, the sole occupant, sustained minor injuries; the glider sustained substantial damage. The personal local flight departed Hemet-Ryan Field (HMT), Hemet, California, about 1230. Day visual meteorological conditions prevailed, and no flight plan had been filed. The primary wreckage was at 33 degrees 44 minutes north latitude and 116 degrees 44 minutes west longitude. The pilot reported he was released from the tow airplane near mountains, in gusty wind conditions, about 3,000 feet above ground level (agl), 4,500 feet mean sea level (msl). He climbed, using thermals, to 6,000 feet msl. The first goal was to ascend Mount San Jacinto (10,804 feet msl). The usual procedure to complete the ascension over the mountain range consisted of a "stepping stone" approach along or between the mountains. The pilot had done this maneuver on many past flights. After a third attempt to ascend Mount San Jacinto, the pilot did not find lift, and cruised in a southeast direction at 6,400 feet msl and 55 knots. Suddenly, he felt the glider slow, and noticed his airspeed had dropped from 55 to 40 knots and the glider had lost 100 to 200 feet. The pilot lowered the nose to prevent a stall. There was a dead tree to the left of the glider so the pilot executed a right, nose down turn downslope in order to maintain speed and regain terrain clearance. He heard a "bang," then a series of "booms and shocks" as the glider hit trees. Thereafter, he lost control of the glider. The glider went between two trees, spaced about 6 feet apart. The wings were sheared off and remained in the trees' branches, about 20 feet agl. The fuselage went between the two trees, and came to rest nose down, inverted, on the ground. The pilot worked his way out of the glider, and spent the night in the mountains. After an extensive hike, he followed a creek bed to a highway where he was later rescued. The pilot did not report any control malfunctions prior to the mishap.