After a fifteen minute glider flight, the pilot entered the traffic pattern for a planned full-stop landing on a grass airstrip that ran adjacent to the active paved runway. He then extended his downwind leg in order to create a greater clearance between himself and a powered airplane that was landing on the paved runway. After rolling out on base leg, the pilot realized that he had extended his downwind leg too far to reach the grass strip under the current gusting wind conditions. He therefore decided to land in a field short of the grass runway. While maneuvering to land in that field, he failed to maintain clearance from a nearby tree. After the wing of the glider hit the tree, it cart wheeled into the field, and sustained wrinkling and crushing damage to both its wings and fuselage.