On April 12, 2004, about 1430 Pacific daylight time, an Aeronca 65-TAC, N39515, experienced a loss of directional control, veered off the runway, and collided with trees while on the landing roll at the Alta Sierra Airport (09CL), Grass Valley, California. The passenger/owner was operating the airplane under the provisions of 14 CFR Part 91. The commercial pilot and pilot rated passenger were not injured; the airplane sustained substantial damage. The personal local flight departed Watts-Woodland Airport, Woodland, California, about 1330, with the planned destination of 09CL. Day visual meteorological conditions prevailed, and a flight plan had not been filed. In a written statement, the pilot reported that he was attempting to land on runway 01. He stated that runway 01 is solely used for landing at 09CL, due to runway 19 having an excessive upsloping gradient. During the approach, the pilot noted tailwind conditions and opted to perform a wheel landing. While on the landing roll, the airplane encountered a gust of wind from the right, resulting in the tail being pushed to the left. The airplane veered to the right and began a 45-degree arc off the right side of the runway. The pilot applied full left rudder pressure and left brake to counteract the veer. The airplane departed the right side of the runway and collided with trees. The pilot stated that on the right side of the runway are two groups of dense trees. In between the tree groups, about halfway down the runway, air funnels through creating wind gusts. He thought this was why he encountered sudden crosswind gusts. The pilot reported no preimpact mechanical malfunctions or failures with the airplane. The airport does not have a weather reporting station. The pilot reported that the winds were from 190 degrees at 5 knots.