Green roots of the Incas

The Incas were a race of eco-warriors who built their empire on solid environmental foundations, according to new research.

Many of the great civilisations, such as the Roman and Mayan, ultimately fell victim to self-inflicted environmental destruction.

A seven-year study by Alex Chepstow-Lusty and Mark Winfield, plant scientists at Cambridge University, has revealed the extent to which the Incas of South America used the strategic management of a variety of native trees to rejuvenate, stabilise and enrich their land.

"Here we have a civilisation developing from a pretty-much ravaged landscape because they were strategically planting trees, as well as building terraces and fertilising soil," Chepstow-Lusty said.

The research, published in the latest issue of the journal Ambio , has brought together historical and archaeological evidence alongside detailed pollen analysis from two Peruvian lake beds.

This pollen record covers a 4,000-year period up to the present day, including the centuries in which the Incas ruled a realm of 30 million inhabitants that stretched from what is now southern Colombia to central Chile.

The principal site investigated was the sacred lake of Marcacocha, about 90km from the Inca capital Cuzco and at the heart of many Inca archaeological sites.

Here the scientists built up a detailed record of the area's flora over four millennia by identifying distinctive, wind-blown pollen from different plant species that had been trapped in 6m of layered mud.

This analysis showed that there were few trees in the vicinity of the lake before AD1100, while the presence of the shrub Ambrosia arborescens and an influx of inorganic sediment indicated quite severe soil erosion - evidence of the ill effects of a landscape cleared for agriculture and grazing.

After AD1100 came a period of global warming known as the Medieval Warm Epoch.

During this time, large amounts of pollen from a soil-improving native alder known as aliso were recovered from the Marcacocha mud. This was sustained throughout the following four centuries, unaffected by the rising Inca empire.

This adds weight to accounts in Spanish that record the importance the Incas gave to the planting of several species of tree, while excavations show that wood was in widespread use in heavily populated areas.

Chepstow-Lusty believes that this indicates an approach to planting trees that enabled the Incas to produce fuel and a spectrum of different food products.

This would not only preserve the land but also help defend the people against the vagaries of a climate that is regularly disrupted by extreme events such as El Nino.

Today, Peruvians are encouraged to grow eucalyptus trees rather than native species.

The native trees also protected wild relatives of many tuberous crops, creating one of the world's centres of crop diversity.