Until around 2,500 years ago, river valleys on Peru’s south coast were filled with dry forests. But as they cleared trees for agriculture the ancient Nasca inadvertently exposed this landscape to a sequence of powerful El Niño events, thereby sowing the seeds of their own collapse. Now, long-term studies by Cambridge archaeologists show that lessons from the past can help parts of the desert bloom again.

We have been studying a whole watershed. We wanted to understand the relationship between the people and the landscape, and how by harnessing the water for their agricultural systems they either maintained that landscape or helped turn it into desert.

Professor Charles French

The impact of our research has been demonstrable. From local and regional government policy and education to impact on the ground, it has been used by our Kew collaborators to change practice among local farmers and landowners in Peru, which is quite something.

The Ica Valley in southern Peru stretches from the sand dunes of the Pacific to the snow-capped high Andes. Less famous, perhaps, than its southerly neighbours known for the huge landscape-scale figures known as the Nasca Lines, the Ica Valley has fascinated Cambridge archaeologists for more than 20 years. Here on the coast, the river meanders along a long course cut into a desert tableland through gorges, opening here and there into wide, cultivated basins.

After crossing the Pan American Highway, the valley rises into the foothills of the Andes. “At first it is rubbly and desert like, full of outwash from ancient river channels and deposits blown by the wind,” says Professor Charles French of the McDonald Institute for Archaeological Research at Cambridge.

“Reaching the top, at 4,600 metres, in the background are the snow-capped Andes and you’re looking far into the Amazon watershed. Up there, amid huge bog systems, you find remnants of former pastoral systems, settlements and irrigation features like dams and corals.”

For French and his colleague Dr David Beresford-Jones, it is the story of how humans shaped this landscape and its river – and how their lives were in turn shaped by the changes they set in train – that is the object of their research.

To read this human impact story, French and Beresford-Jones used a variety of approaches, from archaeology and palaeo-environmental studies, to soil sampling, topographical surveys and GIS modelling.

“Archaeological sites are useful because they give us a time control, revealing when certain events took place,” says French. “And by sampling sections of the eroded valleysystems – the layer cake that is the Holocene history of the infill of the valley – one can read the profile a bit like a book and discover how it relates to land use inland and upstream.”

Their research revealed that desertification began in earnest from about 1,200 years ago, partly driven by changes in the intensity of El Niño causing less frequent but more intense rainfall in the Andean hinterlands, but also due to changes set in train by humans.

“It’s the sheer density of human exploitation that made agriculture unsustainable,” says French. “They gradually removed trees in the valley basins, which exposed the landscape to El Niño climate perturbations and triggered wind and river erosion, while the development of massive field systems in the highlands removed so much water from the system that it caused stress in the lower parts of the valley.”

By around 1,000 AD, previously extensive irrigation systems in the lower Valley had been abandoned, as was the farmland they had once serviced in the lowland basins such as Samaca.

Uncovered by the Cambridge research, these lessons of the past are now being used to repair some of the damage through habitat restoration projects implemented by collaborators at the Royal Botanic Gardens, Kew. The local government of the Ica Valley region has introduced regulations prohibiting the felling of native Prosopis trees and their burning for charcoal. Lessons about the importance of these trees to sustainable agriculture are now taught in schools in the Ica Valley, where replanting programmes are well underway.

“Prosopis are like large mesquite trees,” says French. “With their massive gnarled trunks and huge, scraggly boughs they look like something from a woodland you might find in a Harry Potter novel. They have a huge life span, living for up to 1,000 years, and they are incredibly tough – they can be much abused, blown down, half felled and yet still they regenerate.”

French explains: “The impact of our research has been demonstrable. From local and regional government policy and education to impact on the ground, it has been used  by our Kew collaborators to change practice among local farmers and landowners in Peru, which is quite something.”

Central to this success has been Alberto Benavides, a local landowner and co-sponsor of Cambridge’s project in the Samaca Basin. Over the past 20 years he has planted some 4,000 of these extraordinary trees, helping retain water and rebuild soil along the edge of the river.

Inspired in part by Cambridge’s archaeological work, this sympathetic landowner is helping restabilise the landscape and, two decades  on, a change in the system is becoming visible. French concludes:

“Benavides has turned the Samaca Basin into a fantastic organic farm. The land is lusher  and better managed than it has been for millennia. It’s like a beautiful oasis in the lower part of the valley system.”