The glider pilot reported that after an uneventful tow to 4,000 ft msl (about 2,300 ft agl), he proceeded to fly north with the intention of reaching a mountain peak about 40 miles away. Soaring conditions were good, and the glider had climbed to about 10,500 ft msl by the time he reached a peak at the halfway point. On the second leg, the glider started to encounter sink, and the pilot continued to fly along mountain ridges to regain altitude. The pilot managed to climb the glider back to 8,000 ft, but again began to encounter sink and decided to work along mountain ridges back to the south with the intention of landing at an intermediary airport. He was not able to maintain altitude, and ultimately decided to perform an off-airport landing into a valley clearing. During the landing roll the right wing was substantially damaged when it struck a tree. The pilot reported that there were no mechanical failures or malfunctions with the glider that would have precluded normal operation.