Swidden agriculture - often called "slash-and-burn" - has been practiced on every inhabited continent throughout human history, and for decades it has been blamed for the destruction of the world's rainforests. However, a new study of more than 18,000 forest patches around the world found that the environmental impact of swidden is more nuanced, and less destructive, than the term "slash-and-burn" suggests - and that unwritten social rules in small societies, not government oversight, often regulate land use closely enough to support both harvests and forest regeneration sustainably over time.
The findings have practical implications for conservation and climate policy.
"The takeaway for policymakers is that small-scale rural communities have the ability to manage community forests in sustainable ways, and they need to be supported to solve problems using their own customary practices," said lead author Sean Downey, associate professor of anthropology at The Ohio State University. "It's going to look different in Belize than it does in Africa or Indonesia."
Swidden is one of humanity's oldest farming systems, practiced in tropical and subtropical forests for around 10,000 years. In Belize, where much of Downey's fieldwork has taken place, it involves several steps: Farmers clear a patch of forest and leave the fallen trees to dry out. Then the dried trees are burned, which releases the carbon stored in the wood back into the soil where it serves as fertilizer. When the rainy season begins, crops (often corn) are planted and harvested once or twice before the soil's nutrients start to run low. Then the field is fallowed, essentially given time to rest while the forest regrows.
Downey has spent much of his career studying how small communities balance the demands of farming against the long-term health of the forests they depend on for their livelihood: "It's a cyclical pattern of reciprocal exchanges of agricultural labor, social norms that help regulate land use, and fire and ecosystem services that regenerate the fertility of cleared forest patches."He first became interested in Belize's swidden agriculture in the early 2000s, and later discovered that satellite imagery revealed a stark contrast at the Guatemalan border: The forest had been cleared into pasture on the Guatemalan side, and there were intricate, patchwork mosaics of tropical forest on the Belizean side where Mayan communities were clustered.
A new study, published recently in the Proceedings of the National Academy of Sciences, found that the mosaics could originate from something as commonplace as neighbors deciding whether to help each other plant corn.
When a farmer asks neighbors to help clear and plant a new field, the scale of the request makes a difference. If it's a reasonably sized plot, most will show up to help. But "if a farmer asks for too much - say, a 20-acre field instead of a two-acre field - fewer people show up to help," Downey said. And if fewer people show up, the farmer can't clear the larger plot of land. "Over time, this guides people to ask for the right amount of land, reducing overall land use," he added.
This process, where shared expectations of right and wrong guide people's behavior, is known as normative reasoning. It is connected to the work of Nobel Prize-winning economist Elinor Ostrom, who showed that small communities can protect shared local resources through social sanctions - in this case withholding cooperation - rather than formal rules or government enforcement.
Downey's interdisciplinary research team - which included experts in anthropology, physics, applied mathematics and remote sensing - built a computer model and analyzed remote sensing images from real-world farming communities in Central and South America, sub-Saharan Africa, and South and Southeast Asia to study how people share labor, use land and select sites for swidden agriculture. They found that when farmers naturally coordinate their land clearing efforts, the result is a mosaic of swidden fields with fractal, self-repeating patterns. At the same time, cultural and social norms help reduce the total amount of land cleared.
Together, these things help communities balance their farming needs with long-term environmental sustainability. "Until now, there haven't been many formal models that can show how small-scale swidden communities regulate land without top-down rules," Downey said.
Importantly, the researchers also found that while clearing too much forest will turn it into grassland, the biodiversity of a forest increases when it experiences intermediate levels of disturbance.
"When you create swidden patches, you create new ecosystem niches during the fallow stage where different species thrive," Downey explained, and "you end up with higher levels of biodiversity across the landscape than you do in a mature forest alone."
In other words, the study found that swidden doesn't automatically lead to degradation. On the contrary, when used in moderation, it can enhance the ecosystem - a benefit, Downey said, that hasn't yet been incorporated into policy.
Co-authors include Shane A. Scaggs, Jensan Lebowitz, Rongjun Qin, Denis Tverskoi, Xinyi Wu and Zaarah Syed from the Ohio State University; and Stefan Thurner from the Complexity Science Hub, the Medical University of Vienna and the Santa Fe Institute. The work was supported by the National Science Foundation, including an NSF CAREER Award Downey received at Ohio State in 2017.
