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.

... 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 ...

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.

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Globally, one-fifth of the world’s carbon emissions come from the destruction of rain forests, scientists say. It is unknown how much of that is being canceled out by forest that is in the process of regrowth.

In Latin America and Asia, birthrates have dropped drastically; most people have two or three children. New jobs tied to global industry, as well as improved transportation, are luring a rural population to fast-growing cities. Better farming techniques and access to seed and fertilizer mean that marginal lands are no longer farmed because it takes fewer farmers to feed a growing population.