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Hungry Maoris burned forests to grow food

Man and the land Polynesian people began burning the forests of the South Island of New Zealand almost as soon as they arrived about 800 years ago, researchers have found.

Within as little as 200 years, they had removed 40% of the moist closed canopy forests of the South Island, according to research published today in the journal Proceedings of the National Academy of Sciences.

The study of lake sediments reveals that dense beech and podocarp forests were rapidly replaced with tussock grassland and fern shrubland.

"It was quite a remarkable transformation - quite spectacular," says co-author Dr Janet Wilmshurst from Landcare Researchin Canterbury, New Zealand.

The researchers analysed cores taken from 16 lakes across the South Island, to a sediment depth representing a 1000 year time span.

They traced the amount of charcoal (indicating fire), pollen (indicating what kind of plants were growing at the time) and planktonic life in the lakes (indicating how much nutrient was in the lakes).

They found that the amount of charcoal increased 20 fold after the arrival of people. This would have meant that fire frequency went from one or two fires per 1000 years to several fires each century.

More frequent fires led to a cascade of effects: as the trees burned, the ground was left exposed, which meant more erosion, more instability and more nutrients washing into rivers.

To see if the climate at the time had played a role in the mass burning, the researchers examined tree rings, but found no evidence of a particularly warm or dry period.

This led them to conclude that it was human-set fires which were responsible for the loss of New Zealand's forests in the South Island.

Increasing the land's productivity

Wilmshurst says one of the main reasons people burnt the forests so quickly was that they were under pressure to make food more available.

She says the hungry new arrivals couldn't reach the inaccessible fruit of the native trees and also needed to find a source of starch, driving them to burn the forests to create an entirely new landscape.

"They were basically making the land more productive," she says.

"They were trying to 'make a living' in a forest that was designed for birds with small fruit high up - not like in Europe where you've got nuts and fruits growing at hand height that you could pick and store."

She says by burning the forests, the Maori encouraged the regeneration of bracken fern, the rhizome (root) of which was a very important source of food starch.

Restoration ecologist Professor Richard Hobbs from theUniversity of Western Australia says ecologists have been re-thinking the definition of natural landscapes, as more evidence emerges of how humans have shaped the landscape for hundreds or even thousands of years.

"Americans for example have this view of how pristine the country was before Columbus arrived and it all got messed up. But the North American native peoples had already changed it substantially: in the abundance of game species, modification through cultivation and so on.

"Humans are often the prevailing force and there's a move towards thinking that what humans did in these systems was valuable as well.

"Trying to put ecosystems back to something pre-human is sometimes counter-productive and you can end up with some nasty surprises you don't want."