The commercial pilot stated that he was flying his Schleicher ASW-27B glider in a soaring competition, which consisted of flying to a given point and returning, while being timed. The pilot turned around at the specified location, and flew along a ridge on the return to the airport of origin. An on-board computer indicated that the glider would arrive at the airport at an altitude of 100 feet above ground level (AGL). The pilot continued to "work the ridge" in order to gain altitude, since his standard personal minimum was to arrive at an airport no lower than 600 feet AGL, and 1,000 feet AGL was his "ideal" arrival altitude. When still short of the airport, the pilot realized that the ridge was no longer producing lift, and that he would not make either the last suitable landing site behind him, or the destination airport. The pilot slowed the glider down so that he could "fly into the trees." The left wing impacted trees first. The glider fell through the trees, impacted the ground right wing first, and came to rest inverted. The glider was substantially damaged. The pilot noted that there were no mechanical anomalies with the glider.