The forest products giant Weyerhaeuser said yesterday that third-quarter earnings fell 86 percent because of low wood prices, duties on imports of Canadian softwood lumber and merger-related charges. The company, based in Federal Way, Wash., reported a profit including one-time items of $13 million, or 6 cents a share, compared with a profit of $91 million, or 41 cents a share, a year ago. Weyerhaeuser, which announced a major timberland sale and two plant closings on Monday, is suffering in a downturn in the wood and paper industry that has lasted more than two years.