CHILIBRE, Panama The land where Marta Ortega de Wing raised hundreds of pigs until 10 years ago is being overtaken by galloping jungle palms, lizards and ants.
Instead of farming, she now shops at the supermarket and her grown children and grandchildren live in places like Panama City and New York.
Here, and in other tropical countries around the world, small holdings like Ms. Ortega de Wing’s and much larger swaths of farmland are reverting to nature, as people abandon their land and move to the cities in search of better livings.
These new “secondary” forests are emerging in Latin America, Asia and other tropical regions at such a fast pace that the trend has set off a serious debate about whether saving primeval rain forest an iconic environmental cause may be less urgent than once thought. By one estimate, for every acre of rain forest cut down each year, more than 50 acres of new forest are growing in the tropics on land that was once farmed, logged or ravaged by natural disaster.
“There is far more forest here than there was 30 years ago,” said Ms. Ortega de Wing, 64, who remembers fields of mango trees and banana plants.
The new forests, the scientists argue, could blunt the effects of rain forest destruction by absorbing carbon dioxide, the leading heat-trapping gas linked to global warming, one crucial role that rain forests play. They could also, to a lesser extent, provide habitat for endangered species.
The idea has stirred outrage among environmentalists who believe that vigorous efforts to protect native rain forest should remain a top priority. But the notion has gained currency in mainstream organizations like the Smithsonian Institution and the United Nations, which in 2005 concluded that new forests were “increasing dramatically” and “undervalued” for their environmental benefits. The United Nations is undertaking the first global catalog of the new forests, which vary greatly in their stage of growth.
“Biologists were ignoring these huge population trends and acting as if only original forest has conservation value, and that’s just wrong,” said Joe Wright, a senior scientist at the Smithsonian Tropical Research Institute here, who set off a firestorm two years ago by suggesting that the new forests could substantially compensate for rain forest destruction.
“Is this a real rain forest?” Dr. Wright asked, walking the land of a former American cacao plantation that was abandoned about 50 years ago, and pointing to fig trees and vast webs of community spiders and howler monkeys.
“A botanist can look at the trees here and know this is regrowth,” he said. “But the temperature and humidity are right. Look at the number of birds! It works. This is a suitable habitat.”
Dr. Wright and others say the overzealous protection of rain forests not only prevents poor local people from profiting from the rain forests on their land but also robs financing and attention from other approaches to fighting global warming, like eliminating coal plants.
But other scientists, including some of Dr. Wright’s closest colleagues, disagree, saying that forceful protection of rain forests is especially important in the face of threats from industrialized farming and logging.
The issue has also set off a debate over the true definition of a rain forest. How do old forests compare with new ones in their environmental value? Is every rain forest sacred?
“Yes, there are forests growing back, but not all forests are equal,” said Bill Laurance, another senior scientist at the Smithsonian, who has worked extensively in the Amazon.
He scoffed as he viewed Ms. Ortega de Wing’s overgrown land: “This is a caricature of a rain forest!” he said. “There’s no canopy, there’s too much light, there are only a few species. There is a lot of change all around here whittling away at the forest, from highways to development.”
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.