- A new study shows that 215 million hectares (531 million acres) of degraded and deforested land in the tropics could regenerate naturally.
- The researchers developed a model based on satellite images of where forest had regrown, using machine learning to filter out places where humans had planted trees.
- The analysis also incorporated a range of biophysical variables, such as rainfall, fire frequency, and the distance from the edge of standing forest.
- While natural regeneration could be cost-effective, scientists also say the permanence of regrown forests is critically important to the benefits it can provide to biodiversity and the climate.
Cleared or degraded tropical forests around the world covering a combined area larger than Saudi Arabia could regrow on its own, according to new research published Oct. 30 in the journal Nature.
Restoring 30% of the world’s degraded land by 2030 is a target of the 2022 Global Biodiversity Framework, which 196 countries have signed. Much of the current focus centers on planting trees in these areas. But restoration also includes allowing forests to recover on their own, a process scientists call natural regeneration, which can be more cost-effective than tree planting in some situations.
The Nature study suggests the ecological conditions are right for about 215 million hectares (531 million acres) of land in the tropics to regrow into forests. Tropical forests top the list for restoration because once they grow back, they provide habitat for a multitude of species and at the same time keep carbon out of the atmosphere where it would accelerate climate change. If all of the areas identified in the study grew back, it would pull 23.4 billion metric tons of carbon from the atmosphere in a 30-year period, the analysis found. That’s the equivalent of more than twice as much carbon dioxide as the world emitted for energy in 2023.
Study co-author Matt Fagan, associate professor in geography and environmental systems at the University of Maryland, said the paper’s aim isn’t to present tree planting and natural regeneration as an “either/or” choice. Both approaches have their place, he said. But tree planting can be expensive, fire risk can be a problem, particularly when using nonnative species, and it often doesn’t support as much biodiversity as naturally regrown forest, Fagan said.
“The nice thing about what we’re proposing is that it’s incredibly cheap relative to planting trees,” he added.
Fagan and his colleagues had previously used satellite data to pinpoint areas where forests had returned. For this study, they used machine learning to weed out spots with planted trees, and then used these natural regrowth areas as input data for their model. They also integrated a suite of “biophysical” variables that make land conducive to regrowth, including measures of soil chemistry, their proximity to other forests (which serve as a critical source of seeds and rainfall), and how often the area had burned between 2001 and 2017, to identify places where regrowth seems most likely.
“Here’s areas that, if you just let them go, will pop back into rainforest quickly,” he said.
“It’s incredibly impressive,” Fagan added. “[At] 3 years old, it’s over your head; 5 years, you’re under shade.”
Overall, the results are a “first step,” Fagan said, to help identify areas that could regenerate at minimal cost.
“If you’re looking for ways to invest your money, where to target your effort, and the potential of a region, I feel like this is a really good study,” he added. It’s the kind of data that could be used to boost reforestation efforts in countries like Brazil, where 43.7 million hectares (108 million acres) of forest could regenerate naturally, according to the analysis. The team has shared its data set online.
Brazil, which holds the most tropical forest on Earth, recently committed to restoring some 12 million hectares (30 million acres) at the U.N. biodiversity conference (COP16) in Cali, Colombia, prior to the release of this study.
The study’s results showed that Brazil, China, Colombia, Indonesia and Mexico accounted for 52% of the land that could regenerate forests naturally. Fagan said that finding was “really exciting.”
“They represent an opportunity to work with a very small number of governments to get half the job done,” he added.
“There’s no map like that until now,” Jonah Busch, an environmental economist and independent researcher who was not involved in the study, told Mongabay. “It’s helpful to have a baseline.”
Busch and his colleagues recently published a paper looking at the cost effectiveness of tree planting compared to natural regeneration across 138 countries. Had this new study been available, he said, his team would have used it in their research.
Still, Busch said these numbers are “more of a floor than a ceiling.”
“These are the places where natural forests are likely to come back on their own if you do nothing,” he added. “There’s way more that could happen if effort was put into it.”
Fagan and his colleagues’ study focused primarily on biophysical factors influencing natural regeneration, though they did examine other socioeconomic variables like the distance from water or roads. These factors can make it more likely, for example, that fledgling forests would be cut down for farming before they can fully regrow. Fagan said he’s excited to see subsequent analyses that will focus on the human side of the equation and how that boosts or stymies reforestation.
Karen Holl, professor of environmental studies at the University of California, Santa Cruz, who was also not involved in the research, noted that it’s critical to include local communities in regeneration efforts. Holl’s own research in Costa Rica has shown that people often see natural forest regeneration as “messy,” and they tend to place more value on planted landscapes.
What’s also important is how long these forests last once they’ve regrown, Holl said.
“We sort of assume that the land is growing back, and the large-scale data suggests that it’s not as permanent as it seems,” she added. “The problem is that you can’t map people’s decision-making at large scales.”
Fagan’s own research in Costa Rica revealed that people had cleared 50% of secondary forests within 20 years in the team’s study area.
“The only way a young forest sticks around is if local people are on board with letting that young forest stick around,” he said.
“This [study] just shows possibility. Persistence is the other half of it, and persistence is through policy,” Fagan added. “It’s through working with people. It’s through building partnerships and finding ways for people to extract money out of young forest and make them a good financial investment. They’re cheap to establish. They’re not necessarily cheap to keep around.”
Banner image: Capuchin monkeys in a lowland tropical rainforest in Costa Rica. Image by Bernd Dittrich via Unsplash (Public domain).
John Cannon is a staff features writer with Mongabay. Find him on Bluesky and LinkedIn.
Citations:
Williams, B. A., Beyer, H. L., Fagan, M. E., Chazdon, R. L., Schmoeller, M., Sprenkle-Hyppolite, S., … Crouzeilles, R. (2024). Global potential for natural regeneration in deforested tropical regions. Nature. doi:10.1038/s41586-024-08106-4
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Busch, J., Bukoski, J. J., Cook-Patton, S. C., Griscom, B., Kaczan, D., Potts, M. D., … Vincent, J. R. (2024). Cost-effectiveness of natural forest regeneration and plantations for climate mitigation. Nature Climate Change, 14(9), 996-1002. doi:10.1038/s41558-024-02068-1
Reid, J. L., Fagan, M. E., Lucas, J., Slaughter, J., & Zahawi, R. A. (2018). The ephemerality of secondary forests in southern Costa Rica. Conservation Letters, 12(2). doi:10.1111/conl.12607
Crawford, C. L., Yin, H., Radeloff, V. C., & Wilcove, D. S. (2022). Rural land abandonment is too ephemeral to provide major benefits for biodiversity and climate. Science Advances, 8(21). doi:10.1126/sciadv.abm8999
Holl, K. D., Ashton, M. S., Bukoski, J. J., Culbertson, K. A., Curran, S. R., Harris, T. B., … Vincent, J. R. (2022). Redefining “abandoned” agricultural land in the context of reforestation. Frontiers in Forests and Global Change, 5. doi:10.3389/ffgc.2022.933887
Schwartz, N. B., Mitchell Aide, T., Graesser, J., Grau, H. R., & Uriarte, M. (2020). Reversals of reforestation across Latin America limit climate mitigation potential of tropical forests. Frontiers in Forests and Global Change, 3. doi:10.3389/ffgc.2020.00085
Zahawi, R. A., Reid, J. L., & Holl, K. D. (2014). Hidden costs of passive restoration. Restoration Ecology, 22(3), 284-287. doi:10.1111/rec.12098
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