The commercial certificated pilot reported that during the initial climb after takeoff, as he was making a crosswind to downwind turn, the engine began to run rough and lose power. The airplane was unable to maintain altitude and collided with trees along a ridgeline, about 1.5 miles from the departure airport. Rising terrain is located on either side of the airport's location in a river valley. Due to the remoteness of the area, a postaccident examination of the engine was not conducted.