On May 28, 2000, approximately 1730 mountain daylight time, a Cessna 172, N7902B, collided with trees while climbing out from Johnson Creek Airstrip, Yellowpine, Idaho. The commercial pilot and his passenger received minor injuries, but the aircraft, which was owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which was departing Johnson Creek for Caldwell, Idaho, was being operated in visual meteorological conditions. No flight plan had been filed. The ELT was activated upon impact. Because of rising terrain to the south of this airstrip, the Idaho Airport Facility Directory recommends that pilots "...takeoff runway 35 when wind conditions permit." On previous trips to this facility, the pilot had always followed the recommendation and taken off to the north. But on this trip, because the winds were blowing from the south at about 10 to 15 knots, he felt he should take off to the south. After takeoff, the pilot turned left and positioned the aircraft over the river to establish his initial climb. But as soon as he was positioned over the river, he sensed he was in an area of "sinking air," so he turned to the right over slightly rising forested terrain. As he continued to the south, he encountered more downdrafts, and it soon became obvious to the pilot that the aircraft was not going to be able to climb fast enough to stay above the trees on the rising terrain. Eventually the wings of the aircraft began clipping the treetops, and as it slowed, it fell down through the trees and impacted the ground. According to the pilot, the aircraft was heavily loaded, and he did not remember to lean the fuel mixture for best power prior to takeoff. The temperature at the time was approximately 65 degrees Fahrenheit, and the field elevation is 4,933 feet above sea level (MSL). Calculations using Macswain Enterprises Winvest determined the density altitude to be about 6,500 feet. In his written report, the pilot stated that, "The sinking air over the river and the trees was too much for the 145 hp engine to overcome in its nearly full capacity."