Ancient farming method may help conserve savannahs

A fire-free farming method practiced by early inhabitants of the Amazonian savannahs could help inform efforts to conserve and rehabilitate these important ecosystems around the world, a study has found.

The research provides greater historic context for findings presented at a conference earlier this year (26 January), which suggested that slash-and-burn — in which trees are felled, left to dry and then burned to prepare land for farming — provides better growing conditions for valuable trees such as mahogany.

This latest study, published in Proceedings of the National Academy of Sciences (9 April), found that 800 years ago, prior to European settlement of Latin America, indigenous farmers had developed a technique known as 'raised-field' farming to manage land sustainably without using fire.

The method involved constructing small agricultural 'mounds' which promoted drainage, soil aeration and moisture retention. It conserved soil nutrients and organic matter, and preserved soil structure.

The ancient technique was studied by researchers, who have created the first detailed picture of land use by documenting the very low frequencies of charcoal particles — an indicator of fire — in the savannahs of French Guyana.

"Modern indigenous groups living in savannah environments regularly burn the savannahs. From there paleoecologists extrapolated that all savannahs have been burned regularly in the past. But this was not the case," José Iriarte, researcher at the University of Exeter, United Kingdom, and leader of the study, told SciDev.Net.

"Building small mounds was the only way that they could cultivate these seasonally flooded savannahs. They were creating an elevated, dry, fertile platform," Iriarte said.

The researchers say that the study gives a unique perspective on land-use before as well as after the first Europeans arrived, and reveals that the high fire incidence in Amazonian savannas was a post-settlement phenomenon.

Iriarte says raised-field and slash-and-burn are techniques developed to suit different environments, but commented that burning resulted in the long-term depletion of soil nutrients. "In the long run, if [land] is not left fallow for a long period [of up to 20 years, it] is impoverished," he said.

The authors say their findings suggest that raised-field agriculture could also provide better water infiltration and storage in soil and crop roots, and promote crop growth. They also say their findings may apply in flooded savannah settings elsewhere in the world.

"We think that raised-field agriculture, which would require careful management of organic matter, could be applicable in all seasonally flooded savannas," said Doyle McKey, professor at the Université de Montpellier II, in France, who also participated in the study.

Link to full article in PNAS