During the takeoff run the engine developed a maximum of about 5800 rpm, almost 1000 rpm lower than expected. The aircraft became airborne, but did not achieve the climb performance necessary to clear trees along the takeoff path. The observer called to the pilot that he was assuming control, closed the throttle and turned off the fuel and ignition systems. The aircraft collided with the trees and fell to the ground. No mechanical defect was subsequently found with the engine or systems of the aircraft. A number of reports have been received concerning induction icing in this type of engine, in which there is no provision for directing warm air to the carburettor. It was considered likely that induction icing occurred on this occasion. The pilot had not abandoned the takeoff attempt when the loss of power became apparent.