The pilot and passenger departed early for a day of ballooning. As they approached the selected landing area, the pilot started a descent to slow the balloon in slower winds. As the balloon neared the treetops, the wind direction shifted slightly, which resulted in the balloon colliding with a tree. During the flight, and unbeknownst to the pilot, his lapbelt had become unsecured, and after the impact with the tree, he slid forward in the seat and held on to the uprights of the chair to prevent falling from the balloon. In this position the pilot could no longer control the balloon and remained holding onto the frame as the balloon went through the trees, which resulted in substantial damage to the envelope and frame. The pilot reported that the balloon had no preimpact mechanical malfunctions or failures that would have precluded normal operation.