On the slopes of northern Ecuador's Quijo Valley, perpetual clouds shroud the canopy of a seemingly pristine tropical forest. But the beauty of the cloud forest hides a violent, tragic history. A new study of sediments from the valley's Lake Huila reveals centuries of indigenous agriculture that came to an abrupt end in warfare and fire around 1588.

Population collapse

From about 1400 to 1532, the Quijos Valley marked the eastern frontier of the Incan Empire. Although they were subjects of the empire, the people of the Quijos Valley maintained a distinct cultural identity from the Incas, and historical and archaeological records show that the valley was a conduit for trade between Incan territory and the peoples of the Amazon Basin.

The first Europeans to set foot in the Quijos Valley were Spanish expeditions in 1538 and 1541, who arrived in search of gold and cinnamon. They estimated that about 35,000 indigenous people lived in the region. By 1577, about 11,400 people had clustered around the Spanish town of Baeza, which the colonizers built in 1559 alongside the indigenous community of Hatunquijos. But by 1600, three out of four of these people were dead.

Many died of European diseases, which ravaged unexposed New World populations. Others perished under the Spanish encomienda system of forced labor, and many more died fighting in a series of uprisings against the Europeans between 1560 and 1578. Just three small huts remained in Baeza by the mid-1800s.

Hidden history

So it's little wonder that European travelers who passed through the Quijos Valley around that time marveled at its cloud forests and assumed the landscape was untrodden by human feet and unworked by human tools. But the mist-shrouded trees and flowers of Quijos' cloud forests conceal a long history of human settlement in the valley, only ended by conquest, disease, and violence. And every chapter of that history is preserved in the sediment at the bottom of Lake Huila, a 98-foot-wide depression on a lava terrace 8,500 feet above sea level.

Ecologist Nicholas Loughlin of the Open University and his colleagues took two core samples from Lake Huila. The top 20 inches of each core holds about 700 years of sediment that washed into the lake, carrying with it pollen from local plants, charcoal from fires, and bits of broken pottery.

In the layers deposited before European arrival, the lake sediments held pollen reminiscent of an open landscape, dotted with amaranth, meadow rue, grasses, and cultivated maize fields. When people started growing maize in the Quijos Valley is not clear, but there's archaeological evidence of maize agriculture in the neighboring Amazon lowlands dating back 6,000 years.

The pre-Columbian lake sediments also contained high concentrations of charcoal, along with spores from a charcoal-loving fungus genus called Neurospora. That's evidence that "local burning occurred in an environment where intact montane cloud forest rarely burns naturally," wrote Loughlin and his colleagues. That implies that people were setting fires in the valley, either to clear fields, cook food, or make pottery. The presence of pottery sherds in the ancient lake sediments offers more proof that people were settled in the Quijos Valley centuries before Europeans showed up.

End of an era

But around 1588, the bottom of Lake Huila holds evidence of a catastrophe. The amount of charcoal in the sediment cores suddenly increases by two orders of magnitude in those layers, which happens to coincide with historical records of open warfare in the Quijos just after the largest of the uprisings. Documents from the period describe attacks on Baeza and the total destruction of two nearby settlements, Archidona and Avila. The charcoal in the lake sediments is most likely the residue of those fires.

After that, maize pollen abruptly vanishes from the sediment cores, giving way to grasses, aquatic plants, and the types of pioneer trees and shrubs that begin to move in when a cleared landscape suddenly stops being cleared.

Those microscopic bits of pollen paint a picture of abandoned farmland being reclaimed by nature with surprising speed. By 1718, less than a century and a half after the devastation of 1588, the pollen record shows that the cloud forest, with its trademark trees, shrubs, and flowering plants, had returned to the slopes of the Quijos Valley. And by the mid-1800s, where there had once been maize fields and open grassland, visitors saw only "a dense forest, impenetrable save by trials" in a valley "unpeopled by the human race."

From an ecological perspective, that's an oddly reassuring story. It means that, given a little less than 150 years, tropical cloud forests can recover from centuries of clearing and cultivation.

"Transition from an intensely managed cultivated landscape to that of a montane cloud forest therefore alludes to the ability of an Andean montane cloud forest to recover to a structurally intact state over a period of around 130 years," wrote Loughlin and his colleagues.

Surprising ecological consequences

But the resilience of the cloud forest hasn't gone unchallenged. By the time those visitors to the Quijos Valley were writing home about its pristine forests, Ecuador had won its independence from Spain, and people were already moving back into the valley and clearing its land for cattle grazing and settlements. The lake cores contain less forest pollen and more from grasses and flowering meadow plants, mixed with the spores of fungi that thrive on cattle dung.

"The apparent wilderness described during the 19th century represented a shifted ecological baseline," wrote the authors. The cloud forests they saw on the slopes of the Quijos Valley looked pristine, but they had already begun to be disrupted by human activity.

Even so, people in the 1800s made less of an environmental impact on the valley than the people who lived there before colonization. Loughlin and his colleagues compared the pollen species in sediment layers from the last 700 years to the pollen species in layers of lake sediment deposited long before the first people moved into the region thousands of years ago. They found that the period before 1588 was most strikingly different from the original pre-human cloud forest. Even the settlement and cattle grazing since 1819 disturbed the cloud forest less than pre-Columbian agriculture.

It has been difficult to understand exactly how and to what extent pre-Columbian people in Central and South America modified their environment with agriculture, irrigation, and construction. Different cultures left different marks on the landscape, and archaeologists are sometimes still surprised by what they find beneath the dense vegetation. But Loughlin and his colleagues say this study suggests that overall, we may have been underestimating how drastically pre-Columbian people in South America impacted their environment.

Today, the floor of the Quijos Valley is mostly cattle pasture, interspersed with islands of forest, while the cloud forest still clings to the steeper slopes of the valley, high above the banks of the Rio Quijos. It's a strikingly beautiful landscape—but there's more here than meets the eye.

Nature Ecology and Evolution, 2018. DOI: 10.1038/s41559-018-0602-7 (About DOIs).