The pilot reported that during a soaring competition, he found himself in an area of sink and proceeded to an open field that he had pre-selected, in the event that an off-field landing was warranted. He reported that he had a choice of two open fields, selected the field with the northeast to southwest orientation, overflew the field and noted that trees marked the northeast and southwest boundaries. He reported that he made his approach from the northeast, overshot the intended landing zone and impacted the trees that marked the southwest boundary. The glider sustained substantial damage to both wings and the empennage. The landing zone was about 800 feet long, from the northeast to the southwest boundary, and the trees were about 50 feet high. The pilot reported that there were no mechanical malfunctions or anomalies with the glider prior to impact that would have prevented normal flight operations.