On July 24, 1995, at 1330, central daylight time, a Wag-Aero, Sport Trainer, N642KW, sustained substantial damage when it impacted with trees while maneuvering after takeoff from LaCrosse, Wisconsin. The commercial pilot reported minor injuries to himself and to the passenger. The 14 CFR Part 91 departed LaCrosse, Wisconsin, en route to Caledonia, Minnesota. Visual meteorological conditions prevailed and no flight plan was filed. The pilot stated that he was flying low in a valley. He was unable to achieve sufficient altitude to clear the trees at the top edge of the valley which was about a 150 foot elevation change. He stated the air temperature and light breeze created a downdraft at the top of the valley which contributed to the airplane's insufficient climb and airspeed. The pilot also stated that he demonstrated poor judgment to be flying low in a valley.