The pilot described the takeoff from the 2,000 ft-long turf runway surrounded by tall trees. He stated the airplane was making “takeoff power” and that all instruments were “in the green.” At the departure end of the runway the airplane encountered “windshear or a downdraft” and that the leaves in the treetops were being “pushed down.” The 6,000-hour pilot said he had never encountered windshear or a downdraft like it before. He said he adjusted the pitch attitude to best angle of climb, but that the airplane settled into the trees and descended to the ground while substantially damaging the wings, fuselage, and the empennage. The Federal Aviation Administration inspector who responded to the accident site discussed the forecast and PIREPS for low-level windshear with the pilot. The pilot said he had reviewed the weather, but as it was “nothing he hadn’t flown in before,” he elected to depart. The pilot reported that there were no pre-accident mechanical malfunctions or failures of the airplane that would have precluded normal operation.