The student pilot reported his glider was about one mile from his destination airport and thought he had enough altitude to make the field, but then he encountered a sink rate. He realized that he would not be able to clear power lines at the edge of the runway so the student pilot decided to make an off field landing in a large open field bordered by several obstructions. During the landing the right wing struck a small tree when the glider was still several feet above ground. The glider landed abruptly and the impact resulted in substantial damage to the right wing. The student pilot added that the accident could have been prevented if he had earlier recognized the need to land out and if he had prepared for the event at a higher altitude.