The pilot reported that he was departing from a wet and soft off-airport site, in a tailwheel-equipped airplane, during gusty wind conditions. He stated that shortly after takeoff, as the airplane neared the departure end of the site, the headwind abruptly decreased, and the airplane subsequently descended into an area of brush-covered terrain at the end of the site. The airplane sustained substantial damage to the wings and fuselage. The pilot reported there were no preaccident mechanical malfunctions or anomalies that would have precluded normal operation.