Good news! Tropical forests can regrow way faster than we thought

Humans have destroyed or degraded around two-thirds of the world’s tropical forests—mainly through cutting and burning trees to make way for agriculture. Roughly every six seconds, another football field-size area of forest disappears. In light of this crisis, many companies promised they would stop deforestation in their supply chains by 2020. They did not succeed.

That’s a major problem for the climate, both because deforestation releases huge amounts of greenhouse gas—the tropical forests lost in 2020 emitted an equivalent amount to 570 million cars—and because forests that are left standing play an important role in sucking up new emissions. But a new study in Science finds some cause for hope in reversing the problem: In some areas, though it takes decades for a forest to fully recover, a large portion can regrow naturally much more quickly if it’s left alone.

After 20 years, an average of almost 80% of the old-growth forests can regenerate. “That’s really surprisingly fast,” says Lourens Poorter, professor in functional ecology at Wageningen University in the Netherlands and lead author of the paper.

Because of the complexity of a rainforest, scientists expected that it would take much longer. “There’s tremendous species diversity,” Poorter says. “In one of the hot spots in the Amazon where I worked, for example, every second tree is another species. It’s mind-boggling. And that made us think that old-growth forests would take centuries to recover.”

[Photo: richcarey/iStock/Getty Images Plus]

An ideal study would track regrowth over at least a century, he says, but because there isn’t enough time to do that, the scientists studied various areas that had been cleared for farming, and then abandoned, at different points of time—10 years ago, 20 years ago, 40 years ago, 80 years ago. Then they compared the regrown “secondary” forests on the sites to old-growth forests nearby, looking at the mix of species, size of trees, composition of the soil, and other factors. Though the forests couldn’t completely recover, they made a huge amount of progress.

That’s partly because the warm, humid growing conditions in the tropics are ideal for plants; some trees can grow as much as 16 feet in a single year. If the forest can regrow naturally, it has advantages to an area that’s been replanted by humans. The trees tend to crowd together, naturally shading out weeds and grass, which makes it easier for other trees to grow. It also restores the area’s humidity. The regrown forest also has more biodiversity, with more species that can better survive drought or other threats, and better support wildlife.

Still, natural regrowth can’t happen on any piece of deforested land. If the remaining forest is too far away, seeds might not blow to the area, or not enough birds and other animals may be left to carry seeds. In some cases, tree planting may be necessary. In other cases, restoration efforts can use “assisted” natural regeneration. “It could be as simple as putting up a fence,” says Robin Chazdon, a coauthor on the study and emeritus professor at the University of Connecticut. “If you have cattle in the region, and they are walking in there and trampling seedlings and eating seedlings, that’s a problem.” Restoration projects might also be as simple as fighting fires or planting trees to attract seed-carrying birds, but otherwise leaving the land alone to regrow, requiring little effort or cost.

Making sure the new forest stays undisturbed is another challenge. In some cases, a farmer might burn a forest to make nutrient-rich soil, farm on the land, and then abandon it when the nutrients are depleted—but come back to it when the forest has regrown to a certain point. Others might cut down young trees because they’re worried that if the land looks untended, someone else will claim it. Even when countries enforce deforestation laws, Chazdon says, they often don’t apply to trees in newly regrowing forests. Nonprofits and governments working to stop deforestation will need to scale up programs that give low-income farmers the financial incentive to let young forests grow. Software that tracks forest regrowth from satellite images and other remote sensing can help ensure that projects are actually succeeding.

When young forests are given a chance to regrow, though, they’re a powerful tool: The young trees can suck up around 11 times more carbon than old-growth forests. Still, it’s equally important to find ways to keep original forests in place. “It’s always more effective to protect the existing forest,” Chazdon says. “You can’t restore anything in any reasonable time frame that resembles what was lost. So we don’t want to lose more forests. We  can’t easily recover [them]. Even if you can recover certain aspects, that’s not the same as bringing the original forest back.”