After practicing maneuvers at 5,000 feet, the student pilot returned to the airport. He entered the downwind leg at an altitude about 850 feet above the ground. He continued to descend, using airbrakes, until he realized he was too low, about 250 feet above the ground. He then tried to "cut the corners" of the traffic pattern in an attempt to make it to the runway; however, the glider's right wing impacted a tree.