Raised-Fields Back from the Dead?

With Shrinking Forests and Expanding Savannas, Amazonia’s Future May Benefit from Past Lessons

Prehistoric Amazonian farmers suppressed rather than encouraged grassland fires, according to a report in Monday’s PNAS (Proceedings of the National Academy of Sciences). Researchers claim that applying the indigenous farming techniques offers a route to sustainably develop the region while simultaneously reducing carbon emissions.

From 800 to 500 years ago, farmers on the French Guiana coast of South America 

Map from Iriarte et al. 2012

mounded muck from the seasonally flooded savannas to form raised-fields. This farming strategy “could have supported large and concentrated populations,” according to the study. Suppressing fires from breaking out prevented wasting valuable soil chemicals. While the fields are still visible today, they were abandoned roughly at 1500 AD with the arrival of Europeans.

Europeans’ arrival in 1500 meant big changes for lowland South America, which includes both expansive rainforests and grasslands. The “Columbian Exchange” of crops, animals, and people between New World and Old also included infectious diseases which tragically lowered indigenous populations by as much as 95%. When the farmers died, the raised-fields they tended – which require enormous maintenance and care – fell out of use with the new, European-style farmers.

The scientists, from institutions like the University of Exeter and the Natural History Museum of Utah, used a core drilled from a coastal swamp next raised-fields and archaeological sites to reconstruct the environment from the last 2,150 years. By analyzing pollen, charcoal, and phytoliths (silica from plant structures that can identify specific plants) in the core, they noticed three distinct periods in land use.

The first phase, from approximately 2,150 years ago to 1200 AD, features pollen and phytoliths from a classic savanna environment. There is little charcoal from burning. In the next phase, from ~1200 AD to 1500 AD, new savanna plants arrive along with evidence of maize cultivation. This is when raised-field building was occurring, as also observed in the archaeological record. At 1500 AD, however, raised-field construction stops and the swamp core shows steadily increasing amounts of charcoal.

This research is particularly relevant to debates about deforestation, which, as it progresses, leaves behind new (but steadily degraded) savanna environments in its path. The charcoal produced from slash and burn agriculture – the cause of much of Amazonian deforestation because it is practiced on a massive scale – and the charcoal from burning savannas leads to increased carbon emissions. Couple this feedback loop of more carbon emitted (by burning) and less taken in (by trees) with grassland being used for methane-producing cattle, and you have a pretty short shrift for future inhabitants of the planet and lowland South America.

Adapted from Iriarte et al. (2012), the top half of the image shows the study area after it was burned. Below, different grass species occupy the original raised-fields (in light green).


That serves to explain why media coverage of the article and its authors themselves emphasize the potential these farming techniques offer to the savannas’ sustainable development. The University of Exeter’s press release quotes lead author Dr. José Iriarte (a respectable Amazonian archaeologist, by the way) on the possibilities:

             “This ancient, time-tested, fire-free land use could pave the way for the modern     implementation of raised-field agriculture in rural areas of Amazonia. Intensive raised-field agriculture can become an alternative to burning down tropical forests for slash and burn agriculture by reclaiming otherwise abandoned and new savannah ecosystems created by deforestation. It has the capability of helping curb carbon emissions and at the same time provide food security for the more vulnerable and poorest rural populations.”

As Dr. Iriarte explains, there’s a lot to conditionally hope for from this research. However, the economic and political realities of deforestation and land-use in Brazil and Amazonia should temper that optimism.

Bolivians have tried raised-field before. Clark Erickson (another respectable South American archaeologist) and numerous NGOs encouraged indigenous farming communities to reconstruct the fields in the late 1980s, but despite initial success, most of those farmers went back to the plow by the mid-90s. Why? Raised-fields, as the article and news release frequently mention, are honestly and truly labor-intensive. What’s more, raised-fields are not productive enough to justify that intensity. The article cites experiments that claim raised-fields can yield between 2 and 5.8 tons/hectare of maize (that’s corn, folks, and the data was collected archaeologically). Compare that with the average US yield of 8.93 tons/hectare and Brazil’s of 3.38, according to 2004/5 estimates from the USDA. From an economic perspective, subsistence farmers will find the initial construction and continuous maintenance of their raised-fields an unwieldy substitute for the slash and burn they practice right now.

Most agriculture in Brazilian savannas (natural or recently deforested) is practice by large agribusinesses that produce soybeans or farm cattle. Fortunately for sustainable development, these companies have the necessary equipment to efficiently create raised-fields on a large scale. But, their ease of harvest (imagine a combine harvesting crops in a field of speed bumps) and usefulness in cattle ranching (in a semi-submerged field) make them a prospect that Brazilian businesses will likely pass by.

Still, for all the hype, the prospect of a productive, sustainable, and low-carbon producing Amazon is a worthy goal worth pursuing for any sustainability entrepreneurs in Brazil and researchers there and elsewhere. In addition, it’s a great example of how archaeology can be made relevant in issues of great relevance to people other than archaeologists – and 800 year old farmers.