Fire and Drought Turns Amazon Into Savannah Says Study

04/16/2014 - 08h55

Transitional forest on the edge of the Amazon is relatively resistant to fire, but when affected by extreme drought, the biomass dominated by trees starts being taken over by savannah.

Such is the conclusion of an experimental burn that was started ten years ago and suggests that the Amazonian rainforest is more vulnerable to climate change that was originally thought. The study was published by "PNAS", a scientific magazine.

The test was conducted in Querência (MT), in areas within the property belonging to the Grupo Maggi, linked to former governor Blairo Maggi. Scientists from the Amazon Environmental Research Institute (IPAM) and the United States´ Woods Hole Research Center, had access to three areas of 50 hectares each where they conducted controlled fires.

The scientists treated each plot differently. One plot was burned several times throughout the year. Another was burned every three years. The last one was not burned, in order to allow for comparison.

"First we burned the rainforest for a year and nothing happened," says the Ipam´s Paul Brando, who led the final phase of the experiment. "But when we burned it again during a year of drought [2007], the system was transformed and dry grass started appearing, modifying all the functional aspects of the rainforest and the processes of its ecosystems."

Boosted by a drought- that struck again in 2010- the fire swept through the jungle, making way for grass and what was once the Amazon started looking more like savannah.

Satellite images show that the area burned every three years ended up suffering more degradation than the area burned annually, since it had more "fuel", that is to say, more wood to burn during the driest period.

MODELS

Other experiments had already shown that drought increases tree mortality by 30% but the rate still seemed too low to transform a landscape. By blending droughts with burns the system reached a turning point.

This is important to understand how jungle will be affected by global warming since droughts are thought to become increasingly frequent in the Amazon.

A study led by Peter Cox from Exeter University brought together a compilation of 22 models to simulate how the rainforest would respond to drought. Only one suggested that the forest could turn in to savannah. That was one of the reasons the studies that projected that global warming was a great threat to the rainforest were considered "moderately reliable" in the last IPCC report (the UN Intergovernmental Panel on Climate Change.)

According to Brando, the problem with the simulations is that none of the models took in to account the harmful effect of the interaction between drought and burns.

According to Cox, who has visited the experiment in Querência "It looks like it´s the combined effected of the alternation between fire and regional drought that can make the forest reach its limit."

Translated by MILLI LEGRAIN

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