A patrol car's flashing lights cut through the fog-shrouded forest, and an 18-wheeler loaded down with ancient tree trunks ground to a stop.

Gerardo Antonio Alvarez hoisted his ample frame out of the cab and into the cool air. He knew the routine and already had the certificates - stamped with all seals in order - that showed his wood was cut legally.Alvarez fired up his truck again, rumbling toward the capital and away from the front line of a battle Costa Rican officials claim to be winning. They say that loggers cleared 110,000 acres a year a decade ago, while today the country plants 30,000 acres more than are cut down in a year.

But many environmentalists play down the value of replacing forests grown over thousands of years with young trees of limited ecological value. And even Environment Minister Rene Castro Salazar - who ecstatically declared this month that Costa Rica has reversed deforestation - acknowledges the numbers don't tell the whole story.

The man who conducted the study for the government, Arturo Sanchez of the University of Costa Rica, said Castro inflated the results.

"The minister systematically confuses the ecological value of natural forests," he said. "We're not necessarily reforesting."

The study found that from 1986 to 1997, Costa Rica had cut 405,000 acres of forest, slightly more than 10 percent of its forest land. During the same time, 314,000 acres had regrown or been replanted. The latter figure, however, included everything from trees to overgrown coffee fields because of the low resolution of the satellite images.

Castro added in 395,000 acres of land he said had been reforested but didn't show up on satellite images. His conclusion: Costa Rica has 300,000 acres more forest than it did in 1986.

"We find the statement pretty irresponsible," said Luis Diego Marin Schumacher, director of the Preservationist Association of Forest Flora and Fauna. "Mr. Minister says deforestation is zero, which we believe to be ridiculous."

He and other environmentalists pointed out that the study didn't differentiate between primary forests and tree plantations. And selective cutting thins remaining forests and lessens their biological diversity.

"If you look at the satellite image, there's still a patch of forest, but the quality of that patch is deteriorating day by day," said Javier Baltodano, a biologist with the Association of Costa Rican Environmentalists. "A forest that once had 120, 130 species of trees could now be dominated by two or three."

Officials also say illegal logging has dropped to almost zero in most of the country. They cite enforcement by police officers such as Luis Chacon Mendez, who checked the paperwork on Alvarez's load.

But Chacon Mendez pointed out that his shift ends at 9 p.m. "After that, none of us are left," he said. "Is there illegal cutting? Too much."

Still, Costa Rica is doing better than most countries. Even if the pace of deforestation has only slowed - not stopped - that is no small feat. Worldwide, 42 million acres of forest are being destroyed each year, according to the World Wide Fund for Nature.

Reforestation is economically important to Costa Rica, where tourism is the nation's top industry. An estimated 60 percent of tourists come to visit the country's spectacular rainforests and waterfalls.

Costa Ricans also hope to cash in on the forests in another way.

In a time of concern about global warming, Costa Rica is pushing for a sort of commodities market for tree-cover in which rich countries could buy the right to exceed pollution quotas from countries covered in trees.

Trees absorb carbon dioxide, which accumulates in the atmosphere and contributes to global warming, so Costa Rica could sell pollution credits based on its ample tree cover.

Costa Rica plays host this week to an international conference on climate change in which 135 governments will send representatives to discuss how to combat global warming.

Marin Schumacher, of the Preservationist Association, says environmentalists want to make sure Costa Rican official's upbeat claims about reforestation aren't unquestioningly accepted at the conference.

"After we give our point of view," he said. "It will be tough for them to persuade the public not to have its doubts."