In separate actions, Seattle Audubon Society and the Natural Resources Council of Maine are challenging green labels held by Weyerhaeuser and Plum Creek Timber Co., that certify their forests are managed in environmentally sustainable ways.

''Traditionally, environmental groups have used the courts to enforce the law,'' said David Ford, CEO of metaFore, a Portland-based nonprofit that helps companies assure the paper products they buy are produced sustainably. ''Now that certification has come along and really is a voluntary market-based mechanism, it appears that they are looking to these standards and using the existing process.''

Amid growing worries about global warming, major corporations - such as magazine publisher Time Inc., and lumber retailer Home Depot - are demanding wood and paper products be certified as sustainable.

Ford said suppliers without a green label are sure to be hurt in the marketplace.

''I know Weyerhaeuser and Plum Creek have put a lot of time and energy and money into these certifications and take them quite seriously,'' Ford added. ''I don't think it is something they would walk away from.''

The complaints were filed this month with the Sustainable Forestry Board in Arlington, Va., which oversees the Sustainable Forestry Initiative, an independent green label covering 90 percent of the private industrial forests in the country.

The Seattle Audubon complaint alleges Weyerhaeuser has been logging too heavily in spotted owl habitat in southwest Washington.

The Natural Resources Council of Maine alleges Plum Creek Timber Co. should lose its certification after repeated violations of state logging standards.