In an ancient grove at the bottom of the world, an American lumber company and a group of Chilean scientists are attempting a risky experiment: to see whether ''sustainable development'' will work in one of the few relatively unaltered ecosytems left in the world. The Trillium Corporation, a forestry and real estate firm based in Washington State, wants to harvest a pristine 10,000-year-old temperate rain forest in remote Tierra del Fuego, a wind-swept island only 800 miles from Antarctica. Chileans, meanwhile, are hotly debating the future of their country's fast-disappearing forests. In the past decade, scores of paper-pulp and lumber companies from the United States, Japan, and Europe have descended on southern Chile, clear-cutting thousands of acres of native old-growth trees they can no longer cut down at home and replacing them with imported species such as pine and eucalyptus. Amid this debate, Trillium has hired a commission of 100 of Chile's leading scientists to produce a $9-million environmental-impact plan that would permit the harvest of 350,000 acres of lenga forests with limited damage to the ecosystem. Commission members hope this rare project - Latin America's first major collaboration between scientists and business executives - will become a model for responsible corporate behavior on how the region manages its last native forests. Commission member Claudio Donoso, Chile's best-known forest ecologist, explains why some of the planet's most unique trees have to be cut. ''The best solution to avoid their destruction is proper management,'' Mr. Donoso says. ''And if we don't work with these people, there won't be any [trees] left.'' Experts say that southern Chile and the US Pacific Northwest are the only two temperate rain forests with large tracts of relatively unaltered ecosystems remaining in the world. With its snow-capped mountains, glaciers, and deep bays, Tierra del Fuego is an ecological trove of 140 species of lichen, 65 species of fish, 30 types of insects, and 171 species of birds, as well as such exotic wildlife as the endangered red fox, flightless rhea, condor, and albatross. Trillium's interest, however, is in the island's potential cash cow: a reddish-brown hardwood from the lenga tree, (Nothofagus Pumilio) a type of beech. The company plans to spend $200 million to export finished wood, furniture, doorjambs, and window sills to the US, Europe, and Asia. To be sure, Trillium expects to profit by an international trend to buy ''politically correct'' wood. Company executives say that each product will carry a green seal of approval to show they were harvested in an ecological manner. Chilean government officials are well aware of the extent of forest destruction in recent years, as well as the fact that that lumber and paper industries have played a major role in Chile's economic miracle. These firms now represent the nation's second-largest export, after copper, earning more than $1.6 billion in 1994. In just the first four months of 1995, Chilean forestry exports rose about 60 percent, to $521 million, compared with the same period last year. Trillium executives, on the other hand, have pledged to refrain from clear-cutting or replacing the lenga with imported trees. They pledge to implement a permanent monitoring system and halt the venture if the lenga stops regenerating. Instead of clear-cutting, Trillium workers will shelter cut, whereby 30 percent of the forest is maintained in age, type, and distribution to allow the lenga to naturally reseed the ground. Commission research shows that the tree grows faster when selective cutting allows more sun into the groves. ''I know it can work,'' says Mary Kalin Arroyo, a botany professor at the University of Chile in Santiago and coordinator of the Trillium science commission. ''After all, we don't want to go down in history as the destroyers of these forests. There are a lot of careers on the line.'' Yet Mrs. Kalin's commission is under attack by conservationists who say Tierra del Fuego's ecosystem is too fragile to withstand any logging operation, even one that is based on sustainable-development principles. Specifically, they argue that because the Fuegian soil is so thin, the lenga roots are never deeper than 18 inches, and that the cold climate, high winds, and lack of water cause the tree to grow slowly, taking between 80 to 120 years to reach maturity. Heavy machinery, roads, or the slightest disruption, they say, could cause the trees to die quickly. ''You don't have to be a rocket scientist to know that it will put that forest in decline and that lenga trees don't come back,'' says US environmentalist Doug Tompkins, a Chilean resident who heads the San Francisco-based Deep Ecology Foundation. ''The lenga forests should not be touched,'' adds botanist Adriana Hoffman, coordinator of the Defenders of the Native Forest, a vocal Chilean environmentalist group. ''We will do our best to stop this enormous project.'' For Donoso, however, a researcher at Chile's Austral University in Valdivia, Trillium's approach represents a ''common sense'' option between those who would ban all logging and those who would place few controls on private corporations. Under ideal circumstances, Donoso says, the government would preserve Chile's remaining 24 million acres of forest lands, but ''that's not our reality,'' he laments. About 19 million acres (79 percent) of Chile's woodlands are in private hands, while the state controls only 5 million acres (21 percent), according to the National Forest Corporation (Conaf), the state forest agency. And even with that small percentage of land under its control, Conaf officials concede they don't have sufficient manpower to enforce the nation's environmental protection laws. Of the agency's 140 inspectors, only 50 currently patrol the forests part time, according to Fernando Olave, director of forest control. Some observers say they foresee no improvement until the Chilean public, which is slowly awakening to the problems of the environment, lobbies Congress to fatten Conaf's budget or pass legislation making sustainable development a requirement of doing business in Chile. ''Chileans don't see themselves as a forest country,'' says Alexander Wilde, the Ford Foundation's Chile representative. ''Mining in the north and agriculture in the center dominate its historical image of itself.'' In a clearing near a forest with 600-year old lenga trees, scientists are studying leaf and bark specimens under microscopes and recording data on computers in shipping containers converted into labs. In her log-cabin office, Professor Kalin says the major question for her team of scientists are: ''How much disturbance can the ecosystem manage? How far can we push it?'' Those questions are expected to be answered this month when commission recommendations are sent to a regional environmental agency. If they give the US corporation the green light, Trillium will begin early next year on its ''Project Condor,'' which includes the construction of a sawmill, port, power-generation plant, processing plant, residential village for 850 workers, and hundreds of miles of roads. Across the Strait of Magellan in the port city of Punta Arenas, Trillium Vice President Jean Gorton proudly states her firm's commitment to the environment. ''There are not many companies doing what we are doing,'' she says. ''We are on the cutting edge.'' US conservationists, however, say it wasn't always that way. In the past seven years, Trillium - which also owns timber lands and real estate in 18 US states, two Canadian provinces, and Argentina - has been attacked publicly by US-based environmental groups such as Habitat Watch and Earth First! for clear-cutting, building improper roads, employing dangerous herbicides, and killing beavers. According to the Bellingham (Wash.) Herald, Trillium has been the target of 12 stop-work orders between 1990 and '94 by the Washington State Department of Natural Resources. ''We made some mistakes,'' Trillium Vice Chairman Ken Hertz told the Herald last year. And just last December, Argentine authorities put a stop to Trillium's plan to export wood chips from its 75,000-acre property on the Argentina side of Tierra del Fuego, after 6,000 protesters signed a petition describing the lenga-forest harvesting plan as destructive to the environment. ''Trillium has spent a lot of money marketing itself as 'green,' '' says Manuel Baquedano, the president of Chile's Institute for Environmental Policy, a nongovernmental environmentalist organization, ''but nobody really knows what this company is about.'' In northern California, Kiko Anderson of Ancient Forest International questions what would happen if Trillium's Tierra del Fuego venture were to go awry. ''What would prevent them from abrogating the agreement down the road if it cuts into profit potential?'' he asks. ''Or from turning around and selling it to someone who won't follow the same principles?'' Donoso, however, says Trillium executives have operated in good faith, and he has no doubt that they will adhere to his commission's strict standards. ''If they don't,'' he says, ''I will be the first to denounce them.''