The floatplane sustained substantial damage on impact with trees and terrain following a loss of engine power during initial climb from a lake. The pilot and passenger reported no injuries. The pilot reported that he warmed the engine and taxied around the lake on step before that takeoff. He stated that the fuel valve was in the off position. He said that he put the floatplane between two small rows of pine trees about 20 feet apart. He reported no mechanical malfunctions with the floatplane in reference to that flight.