Farming without fire in tropical regions, like indigenous populations did before 1492, may be the key to both feeding people and managing land more sustainably, according to new research.
Dr José Iriarte, an archaeobotanist at the University of Exeter in the United Kingdom, and colleagues, report their findings today in the journal Proceedings of the National Academy of Sciences.
For hundreds of years before Columbus arrived in Central America, indigenous people converted vast swaths of tropical savannas into agricultural fields with raised beds for growing crops - all without the use of slash-and-burn or other fire-intensive techniques, which are common today, and are a major contributor to greenhouse gas emissions.
But soon after 1492, there came a sharp surge in uncontrolled burns throughout the Amazon's coastal landscapes, finds the new study that has dug into more than 2,000 years worth of soil.
Those carbon-emitting burning practices, which continue today, contribute significantly to global warming.
The results - which were exactly opposite of what scientists expected to find - reveal an unexpected picture of pre-Columbian agricultural practices in an often overlooked and endangered landscape. In turn, the findings suggest that the secret to a sustainable future may lie in the past.
"In a time of climate change, we need an alternative way of managing these savannas that is fire-free, and this is a lesson we can learn from the past," says Iriarte. "They were managing the savannas in what we can say today was a sustainable way."
Savanna versus forest
In forested regions of the Amazon, which have received the bulk of scientists' attention, studies have shown that burning was common in pre-Columbian times. But the massive collapse of indigenous populations after European diseases arrived allowed the forest to take over, and burning quickly subsided.
Scientists have long assumed that the same pattern occurred in the coastal savannas of Central and South America, which make up 20 per cent of lowland areas in the region. To find out for sure, Iriarte and colleagues visited coastal wetlands in French Guiana. There, previous archaeological studies have revealed extensive pre-Columbian agricultural systems, including raised fields, canals and ponds that took advantage of seasonal flooding to irrigate crops.
The researchers began by digging up cores of sediment dating back 2,150 years. It was the perfect environment for sampling because wetland soil is oxygen-deprived, allowing pollen and other identifiable plant materials to survive for centuries without threat from bacteria.
Between about 1,000 and 1,200 years ago plant remains documented the arrival of raised-field farmers in the area, report the researchers. At the same time, to everyone's surprise, a near absence of charcoal in those early layers showed that there were very few fires during this time.
Then, just after the 1492 arrival of Columbus, a sharp rise in charcoal particles in the soil revealed a 100-fold increase in fire frequency compared to pre-Columbian times. It is the exact opposite of what happened in forested areas with the colonial transition.
The reason pre-Columbian farmers kept their savanna-dominated landscapes mostly fire-free, says Iriarte, was likely to prevent the loss of nitrogen and phosphorous from the soil and to keep the ground more fertile. Instead of burning, they opted for more labour-intensive practices to battle weeds and grow maize and other crops.
"They understood how to micromanage their environment for greater productivity," says Dr William Woods, an geographer at the University of Kansas in Lawrence, who adds that agricultural burning now accounts for 30 per cent of all the carbon released into the atmosphere around the world.
Restoring traditional methods could make tropical lowland farms more productive while also easing their environmental burden.
"If we look at old techniques and educate people to apply them," says Woods, "We've got the fix there."