The pilot reported that he was performing a normal takeoff in the short take-off and landing (STOL) airplane on a short narrow grass runway. After the airplane became airborne about midfield it encountered unexpected crosswind turbulence near the edge of a tree line. The airplane drifted to the side, contacted the top of a tree, entered an aerodynamic stall, impacted terrain and came to rest on its nose resulting in substantial damage to the engine mount, forward fuselage, both wings, and both flaperons. The pilot credited the shoulder harness installation with saving him from more serious injury. The pilot also reported that there was no preimpact malfunction or failure.