A new analysis shows that the heavily-deforested Amazonian region of Mato Grosso is particularly susceptible to ‘savannization’ due to repeated burning that has likely depleted the region’s soils of precious nutrients. According to the study, published in the Journal of Geophyscial Research, savannization, or the process of tropical ecosystems shifting to savannah, is likely in northern Mato Grosso even if no further deforestation occurs.
Dr. Marcos Costa, one of the study’s authors, describes the savannization tipping point in an ecosystem as such: “the coupled atmosphere-biosphere system would shift from the present rainy climate-rainforest situation to an alternate drier-savanna situation…It involves the coupled vegetation-atmosphere-ocean system, but the evolution of this system apparently depends on other feedbacks that are usually neglected. These feedbacks are associated with the deforestation and agricultural practices, like soil nutrient limitation and fires.”
Aerial view of slash-and-burn agriculture in the Amazon. Photo by: Rhett A. Butler.
These feedback systems, soil nutrients and fires, are particularly important to an area like Mato Grosso, where large-scale burning has been used for decades by settlers seeking more land for ranching and agriculture. The repeated burning causes a direct loss of soil nutrients in the ecosystem.
“Frequent fires volatilizes significant stocks of [nitrogen], provoking a co-limitation of this nutrient in forests recovering from repeated fire,” the authors write. “Nutrients in the remaining ash might be lost by ash transport, and leaching to surface and groundwater.”
The authors point to previous study which showed that tropical forests suffering from repeated burning—five or more times—“accumulate biomass at an average rate lower than 50% of forests that burned only once, and are more susceptible to further burning.”
Why is the loss of soil nutrients through burning important? “Any plant must capture nutrients from the soil to grow,” explains Costa. “If soil nutrients are depleted, trees grow at a slower rate, because they can’t use efficiently the light and water available.”
Highlighted in pink the tropical forests of Mato Grosso have long suffered largescale deforestation and degradation.
This loss of nutrients can gradually change ecosystems. If plants are unable to grow quickly and abundantly, rainfall will decrease—due to the fact that precipitation is proportional to deforestation and plant biomass—changing an area that was once rainforest into savannah.
Another factor that contributes directly to Mato Grosso’s future is the length of its dry season.
“In present days, northern Mato Grosso climate barely supports the existence of a rainforest,” Costa remarks. “This is because the dry season is very long, almost as long as in the nearby cerrado. Local deforestation may increase the dry season by a few weeks or a month, which may make it more difficult for forests to regrow there.”
The study found that in the future Mato Grosso’s dry season could extend from May to September, whereas currently it usually ends in August, concluding that future savannization would be “mainly
caused by a longer dry season, although this process only starts in the presence of soil nutrient limitations,” the authors write.
Due to combination of this long dry season with low soil nutrients “in a marginal region [such as Mato Grosso] regrowing a rainforest similar to what it used to be is very unlikely,” says Costa. “If the current high deforestation rates and agricultural practices continue in northern Mato Grosso, we may expect an irreversible vegetation change in this region during the next 50 years. The rest of the Amazon is more resilient, but other regions in Amazonia may be at risk as well.”
Costa believes that the great vulnerability of Mato Grosso’s existing tropical forests make them an important target for conservationists, although he notes “this forest does not store as much carbon and does not have as much biodiversity as the more equatorial Amazon forests.”
With continuing pressure for soybean production, ranch land, and biofuels the outlook for Mato Grosso’s forest is increasingly bleak. According to the study: “over northern Mato Grosso, the rainforest does not recover on the timescale of 50 years, no matter how much is deforested.”
Citation: Monica Carneiro Alves Senna, Marcos Heil Costa, and Gabrielle Ferreira Pires. 2009. Vegetation-atmosphere-soil nutrient feedbacks in the Amazon for different deforestation scenarios. Journal of Geophysical Research, Volume 114.
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