Johnny Appleseed’s heart was in the right place when he walked all over the early United States planting fruit trees. Ecologically, though, he had room for improvement: To create truly dynamic ecosystems that host a lot of biodiversity, benefit local people, and produce lots of different foods, a forest needs a wide variety of species. Left on their own, some deforested areas can rebound surprisingly fast with minimal help from humans, sequestering loads of atmospheric carbon as they grow.
New research from an international team of scientists, recently published in the journal Nature, finds that 830,000 square miles of deforested land in humid tropical regions — an area larger than Mexico — could regrow naturally if left on its own. Five countries — Brazil, Indonesia, China, Mexico, and Colombia — account for 52 percent of the estimated potential regrowth. According to the researchers, that would boost biodiversity, improve water quality and availability, and suck up 23.4 gigatons of carbon over the next three decades.
“A rainforest can spring up in one to three years — it can be brushy and hard to walk through,” said Matthew Fagan, a conservation scientist and geographer at the University of Maryland, Baltimore County and a coauthor of the paper. “In five years, you can have a completely closed canopy that’s 20 feet high.” Fagan said he’s walked in rainforests in Costa Rica whose canopies reach 60 feet up “that are 10 to 15 years old. It just blows your mind.”
That sort of regrowth isn’t a given, though. First of all, humans would have to stop using the land for intensive agriculture — think big yields thanks to fertilizers and other chemicals — or raising hoards of cattle, the sheer weight of which compacts the soil and makes it hard for new plants to take root. Cows, of course, also tend to nosh on young plants.
Secondly, it helps for tropical soil to have a high carbon content to nourish plants. “Organic carbon, as any person who loves composting knows, really helps the soil to be nutritious and bulk itself up in terms of its ability to hold water,” Fagan said. “We found that places with soils like that are much more likely to have forests pop up.”
And it’s also beneficial for a degraded area to be near a standing tropical forest. That way, birds can fly across the area, pooping out seeds they have eaten in the forest. And once those plants get established, other tree-dwelling animal species like monkeys can feast on their fruits and spread seeds, too. This initiates a self-reinforcing cycle of biodiversity, resulting in one of those 80-foot-tall forests that’s only a decade old.
The more biodiversity, the more a forest can withstand shocks. If one species disappears because of disease, for instance, another similar one might fill the void. That’s why planting a bunch of the same species of tree — à la Johnny Appleseed — pales in comparison to a diverse rainforest that comes back naturally.
“When you have that biodiversity in the system, it tends to be more functional in an ecological sense, and it tends to be more robust,” said Peter Roopnarine, a paleoecologist at the California Academy of Sciences, who studies the impact of the climate on ecosystems but wasn’t involved in the new paper. “Unless or until we can match that natural complexity, we’re always going to be a step behind what nature is doing.”
Governments and nonprofits can now use the data gathered from this research to identify places to prioritize for cost-effective restoration, according to Brooke Williams, a research fellow at the University of Queensland and the paper’s lead author. “Importantly, our dataset doesn’t inform on where should and should not be restored,” she said, because that’s a question best left to local governments. One community, for instance, might rely on a crop that requires open spaces to grow. But if the locals can thrive with a regrown tropical forest — by, say, earning money from tourism and growing crops like coffee and cocoa within the canopy, a practice known as agroforestry — their government might pay them to leave the area alone.
Susan Cook-Patton, senior forest restoration scientist at the Nature Conservancy, said that more than 1,500 species have been used in agroforestry worldwide. “There’s a lot of fruit trees, for example, that people use, and trees that provide medicinal services,” Cook-Patton said. “Are there ways that we can help shift the agricultural production towards more trees and boost the carbon value, the biodiversity value, and livelihoods of the people living there?”
The tricky bit here is that the world is warming and droughts are worsening, so a naturally regrowing forest may soon find itself in different circumstances. “We know the climate conditions are going to change, but there’s still uncertainty with some of that change, uncertainty in our climate projection models,” Roopnarine said.
So while a forest is very much stationary, reforestation is, in a sense, a moving target for environmental groups and governments. A global goal known as the Bonn Challenge aims to restore 1.3 million square miles of degraded and deforested land by 2030. So far, more than 70 governments and organizations from 60 countries, including the United States, have signed on to contribute 810,000 square miles toward that target.
Sequestering 23.4 gigatons of carbon over three decades may not sound like much in the context of humanity’s 37 gigatons of emissions every year. But these are just the forests in tropical regions. Protecting temperate forests and sea grasses would capture still more carbon, in addition to newfangled techniques like growing cyanobacteria. “This is one tool in a toolbox — it is not a silver bullet,” Fagan said. “It’s one of 40 bullets needed to fight climate change. But we need to use all available options.”
Correction: A quotation in an earlier version of this story mischaracterized the height of a specific tree canopy.
