Remains of a Nazca canal cross a desertified landscape in the lower Ica Valley, Peru.

A paper by David Beresford-Jones and others (Latin American Antiquity 20, 303–33) that argues that deforestation, particularly of the deep rooted huarango, caused the Nazca civilization of Peru to loose their resilience to El Niño floods.  On the BBC David Beresford-Jones says:

The huarango tree (Prosopis pallida) is a unique tree with many qualities and played a vital role in the habitat, protecting the fragile desert ecosystem, the scientists say.

“It is the ecological keystone species in the desert zone enhancing soil fertility and moisture and underpinning the floodplain with one of the deepest root systems of any tree known,” Dr Beresford-Jones says.”

Nature News reports Native American culture sowed seeds of its own collapse:

Ice-core records suggested that severe storms — a mega El Niño — hit the Peruvian Andes around the time the Nazca’s fall began, but this had not been corroborated in the coastal valleys where the Nazca once lived. Beresford-Jones and his colleagues, focusing on the lower Ica valley, solidified this evidence when they discovered a flood layer that sat directly on top of a Nazca rubbish dump. The authors then recreated the flood using a computer simulation, demonstrating that a flood that left such a layer could have caused the damages to the Nazca canal system known to have occurred around 500 AD.

“But that’s not the end of the story,” says Beresford-Jones. “The landscape was only exposed to the effect of the El Niño because of what the Nazca were doing to their river valleys.”

Preserved tree trunks are scattered across the now-deserted lower Ica valley, about 200 km south of Lima, indicating a significant landscape change. To investigate this further, team member Alex Chepstow-Lusty of the French Institute of Andean Studies in Lima analyzed the pollen that had been blown to the edges of the basin by strong winds. For much of the older portion of the record, the pollen came from riparian trees, like huarango, which once created woodland oases that lined the rivers in the otherwise desert landscape.

But as Chepstow-Lusty moved forward in time through the pollen record, he found a gradual decrease in huarango pollen and a concomitant increase in pollen from agricultural sources, like cotton and maize, indicating that the Nazca were cutting down woodland to make room for farms. The records show that agricultural plants dramatically disappeared and were replaced by weeds; eventually the weeds died and the land became the lifeless desert it is today.

Beresford-Jones says that when the Nazca cut down the trees they destroyed the root system that had been anchoring the landscape.

“When the El Niño came it cut into the floodplain because it was no longer supported by woodland. That caused erosion and made the irrigation system useless,” he explains. “Storms like this should have just replenished the water table and wouldn’t have hurt them, but [the Nazca] exposed their own land.”