Science - October 15, 2011

Rainforest poised at tipping point

Have you ever seen an expanse of the tropics that consists of 60 percent forest? It is not likely that you have, because nature ‘forbids’ it, as Wageningen sciences have demonstrated in Science.

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Sixty percent forest cover in a tropical forest is what is known as a tipping point: a more or less forbidden situation in the tropical forest ecosystem. Once the forest has thinned to this critical point, it does not take much to push it over the edge and cause it to deteriorate into savannah. And savannahs have tipping points of their own, which form the transition point to an entirely treeless environment.

Bypassing
These tipping points become clear when the correlations between rainfall and forest cover are mapped out. Marina Hirota and her colleagues at Aquatic Ecology and Water Quality Management did this for the sub-tropical regions of Africa, Australia and South America. Rainfall levels are the key factor in the formation of forest and savannah. But it is not a straightforward linear correlation. Instead of a gradual increase in the number of trees as the rainfall rises, the researchers identified three fairly sharply delineated stable types of ecosystem: treeless, savannah and forest. These three are separated by 'forbidden' situations around the level of five percent and of sixty percent forest cover. The ecosystem can only change by bypassing those situations.

Resilience
'This is one of the most convincing pieces of evidence that alternative stable situations exist on a large scale in nature', says Professor Marten Scheffer, who led the research on tipping points. 'We are amazed ourselves at how strongly the data confirm this radical but increasingly influential theory.' Scheffer is one of the architects of the theory.
This discovery by the Wageningen team makes it possible to determine the resilience of tropical forest in the face of climate change. That resilience is negligible at the places where the ecosystem is approaching the tipping point. Which means things do not look good for the Brazilian rainforest. Hirota: 'Our results show that the area where the forest is most fragile coincides exactly with the zone where the most felling takes place.'

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