Wednesday, October 19, 2011

Tropical Forest and Savannah Tipping Points

October 19, 2011

Tipping points are always a hotbed issue in climate science, but in a new study it has been found that a region with tropical tree cover will jump quickly between a forested state to a savannah or treeless state.

The research was conducted by scientists from Wageningen University and published this week in the journal Science.

The study shows that the strength a forest or a savannah has of staving off becoming treeless – it’s resiliency – is related to rainfall in an almost universal way across all three of the continents that were studied; Africa, Australia and South America.

Based on a massive satellite dataset, the authors used the empirical model that they built from the data to predict where existing forest is most fragile. However, the model, in showing where forest is at it’s most fragile, also shows where a treeless or savannah situation may be tipped into a savannah or forest situation respectively.

“Tree cover is one of the most defining aspects of landscapes” says Milena Holmgren, one of the authors and a specialist on plant ecology. “It is therefore remarkable how poorly we understand what determines where we have forest, and where savanna or desert. Obviously rain is important. Deserts are found in the driest places and rainforest in the wettest. However, what happens in between has been very puzzling for scientists.”

The model showed that instead of a gradual increase in tree cover with increased rainfall, there appeared a ‘forbidden’ state around 5% and around 60% of treecover. Thus, the system jumps between three contrasting states” forest, savannah (which equates to roughly 20% tree cover), and a completely treeless state.

“This is one of the most convincing lines of evidence for the existence of large scale alternative stable states in nature” says Marten Scheffer, who leads the research program on tipping points. “We were surprised ourselves how well the data supported this influential, but radical theory.”

“As the system approaches a tipping point, it becomes increasingly fragile, in the sense that a small perturbation, such as a dry year or some small scale deforestation, may invoke a critical transition to the other state” says Scheffer, commenting on the studies ability to determine not only where a system is most fragile, but also where we can start regaining ground.

“Understanding potential impacts of climate change on the Amazon forest is one of the major challenges for scientists in the region today” explains author Marina Hirota. “Our study now shows that the forest is most fragile precisely in the areas where pressure from human activities is also the highest. This kind of information should allow governments to make better decisions, as it shows the risks and opportunities that are inherent to the stability properties of these ecosystems that still cover massive parts of the Earth.”

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