While new forests may absorb carbon emissions, he says, they are unlikely to save most endangered rain-forest species, which have no way to reach them.

Everyone, including Dr. Wright, agrees that large-scale rain-forest destruction in the Amazon or Indonesia should be limited or managed. Rain forests are the world’s great carbon sinks, absorbing the emissions that humans send into the atmosphere, and providing havens for biodiversity.

At issue is how to tally the costs and benefits of forests, at a time when increasing attention is being paid to global climate management and carbon accounting.

Just last month, at climate talks held by the United Nations in Poznan, Poland, the world’s environment ministers agreed to a new program through which developing countries will be rewarded for preventing deforestation. But little is known about the new forests — some of them have never even been mapped — and they were not factored into the equation at the meetings.

Dr. Wright and other scientists say they should be. About 38 million acres of original rain forest are being cut down every year, but in 2005, according to the most recent “State of the World’s Forests Report” by the United Nations Food and Agriculture Organization, there were an estimated 2.1 billion acres of potential replacement forest growing in the tropics — an area almost as large as the United States. The new forest included secondary forest on former farmland and so-called degraded forest, land that has been partly logged or destroyed by natural disasters like fires and then left to nature. In Panama by the 1990s, the last decade for which data is available, the rain forest is being destroyed at a rate of 1.3 percent each year. The area of secondary forest is increasing by more than 4 percent yearly, Dr. Wright estimates.

With the heat and rainfall in tropical Panama, new growth is remarkably fast. Within 15 years, abandoned land can contain trees more than 100 feet high. Within 20, a thick rain-forest canopy forms again. Here in the lush, misty hills, it is easy to see rain-forest destruction as part of a centuries-old cycle of human civilization and wilderness, in which each in turn is cleared and replaced by the other. The Mayans first cleared lands here that are now dense forest. The area around Gamboa, cleared when the Panama Canal was built, now looks to the untrained eye like the wildest of jungles.

But Dr. Laurance says that is a dangerous lens through which to view the modern world, where the forces that are destroying rain forest operate on a scale previously unknown.