On July 4, 2000, about 1145 hours Pacific daylight time, a Thorp T211, N91313, sustained substantial damage when it collided with trees while maneuvering near Grass Valley, California. The private pilot/owner was operating the airplane under the provisions of 14 CFR Part 91. The pilot and one passenger sustained serious injuries. The personal flight departed Auburn Municipal Airport, Auburn, California, about 1130, and was scheduled to terminate at the Alta Vista, California, airport. Visual meteorological conditions prevailed and no flight plan was filed. The pilot stated he was cruising the local area about 1,000 feet agl, at 2,500 rpm, when the airplane suddenly started to descend. Although he hadn't noticed any evidence of strong winds, he was on the lee side of a peak. He turned to avoid the perceived wind shear and applied full power, but his descent continued. The airplane collided with 100-feet tall trees in a private campground after he maneuvered to avoid hitting one straight on with the cabin area. One wing folded and the airplane came to rest on its side. The pilot suffered a broken leg, while the passenger suffered a broken arm. An examination of the airplane's engine by the manufacturer's representative, under the supervision of a Safety Board investigator, did not reveal any discrepancies that would preclude normal operation. The wreckage was released to the owner's agent.