Clearing Oasis Trees Felled Ancient Peru Civilisation

Reposted in full from New Scientist, 7 November 2009

'The ancient Peruvian Nazca people, famous for creating giant, elaborate lined images on a desert plateau that are visible from space, may have brought about their own destruction by cutting down trees that protected the land they lived on.

That's the verdict of new research into pollen remains in the Ica river valley in southern Peru, where the civilisation thrived for 500 years until the people started to disappear at the start of the 6th century AD.

The prevailing explanation for the Nazca people's demise is that a huge flood wiped out not only their settlements but also their delicate irrigation systems, leaving a desert where no one has lived since.

The new findings agree that the flood was what finished off the Nazca, but suggest the people would probably have survived it if they hadn't already cleared native huarango trees to make way for maize, cotton and beans.

With roots reaching as deep as 60 metres underground to seek out water, lifespans beyond 1000 years and leaves that trap airborne moisture, huarango trees (Prosopis pallida) were a "keystone" species that turned otherwise arid river banks in Peru into oases flanked by fertile flood plains. They also fertilised the otherwise poor soil by dropping leaves and fixing nitrogen.

Their extensive root systems physically anchored the oases in place, and protected them from periodic floods; their huge branches deflected the wind, which can be fiercer than 100 kilometres per hour. Once this protection was gone, the huge flood in around 500 AD destroyed the agricultural systems with which the Nazca people had replaced the huarango, turning the terrain into desert.

The civilisation is best known for the Nazca lines, a series of hundreds of enormous images including human figures, hummingbirds, fish, llamas, lizards, monkeys and spiders. They were created by scraping away red surface pebbles to reveal white rock beneath, and some are more than 200 metres across.

David Beresford-Jones of the University of Cambridge and Alex Chepstow-Lusty of the French Institute for Andean Studies in Lima, Peru, analysed 1.5-metre-deep profiles of pollen distribution in soil from Nazca oasis sites.

In the oldest, deepest layers, about 70 per cent of the pollen is from huarango trees. Around 1.2 metres down, pollen from crops such as maize and cotton joins that of the huarango, showing the beginnings of agricultural expansion.

And around a depth of 80 centimetres, corresponding to around 200 AD to 400 AD, the crop pollen starts to dominate, and huarango pollen rapidly diminishes, showing that most trees had been felled.

Suddenly, about 50 centimetres down – corresponding to about 500 AD – the only pollen is from plants of the Chenopodiaceae and Amaranthaceae families, which thrive in salty water, marking the flood that doomed the Nazca. Thereafter, the salty soil could no longer support crops.

The researchers also found hundreds of huarango stumps, confirming the trees had been chopped down.

They warn that protecting trees like the huarango that thrive in arid regions might be crucial as a fifth of the world's poorest people live on arid land, much of which has suffered degradation induced by humans. Planting trees, on the other hand, has helped reclaim desert in Niger.

What happened to the Nazca carries lessons for us, says William Laurance, an authority on deforestation at James Cook University in Cairns, Queensland, Australia. "As they say, those who don't learn the lessons of history are doomed to repeat them."'