The pilot reported that he was conducting a practice flight for a short-takeoff-and-landings (STOL) competition. The intended flight profile was a timed course consisting of multiple takeoffs-and-landings on several turf runways to demonstrate the STOL capabilities of the airplane and the pilot flying. The pilot intentionally flew the airplane at a low altitude throughout the flight. The flight path was in a river valley with tall ridges on either side of the river and there was a known risk of encountering downdrafts while flying the course. While performing a turn toward one of the airstrips, and less than 100ft above the ground the pilot reported encountering a downdraft and descended rapidly. In response, the pilot increased the engine throttle from near idle to maximum power. The pilot reported that the engine “stumbled” briefly because of the rapidly increased throttle, so he leveled the airplane’s wings and lowered the airplane’s pitch to increase airspeed. As the airplane approached an open area for landing, the airplane’s left main landing gear struck a tree, and the airplane subsequently impacted the ground. The airplane sustained substantial damage to the fuselage and the right wing. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The weather conditions at the time of the accident included clear skies with light surface winds from the south.