Drier savannas store more soil carbon than previously realized 

Savannas and grasslands in arid climates globally are crucial in climate change mitigation by storing substantially more soil carbon than previously understood, according to a new study. These ecosystems help to slow climate warming by serving as valuable carbon sinks.

Focus of the study 

A team of 20 international researchers analyzed data from 53 fire-manipulation experiments worldwide.

“The majority of our sites were from fire-manipulation experiments where fire treatments were prescribed (43 out of 53). The fire frequencies ranged from one fire every 17 years to one fire every year. The low fire treatments were usually complete fire exclusion,” noted the study authors. 

The analysis was focused on how fire impacted carbon storage in topsoil across savanna and grassland ecosystems. 

What the researchers learned 

Drier environments were found to be notably sensitive to wildfire frequency variations compared to their humid counterparts.

Study lead author Adam Pellegrini is a professor at the University of Michigan’s Institute for Global Change Biology.

“The potential to lose soil carbon with very high fire frequencies was the greatest in dry areas, and the potential to store carbon when fires were less frequent was also the greatest in dry areas,” explained Pellegrini.

The study revealed that fire suppression stemming from human activities such as infrastructure development has led to smaller, less frequent wildfires in drier savannas and grasslands over the past twenty years. 

Unforseen carbon storage

The reduction in the size and frequency of wildfires promoted an estimated 23 percent increase in the amount of carbon stored across dryland savannas. 

This increase was not foreseen by most of the state-of-the-art ecosystem models used by climate researchers, noted study second author Peter Reich, director of the Institute for Global Change Biology.

As a result, the climate-buffering impacts of dryland savannas have likely been underestimated, said Reich. The experts calculated that over the past two decades, soils in savanna-grassland regions worldwide have gained 640 million metric tons of carbon.

Extensive carbon sink 

“Ongoing declines in fire frequencies have probably created an extensive carbon sink in the soils of global drylands that may have been underestimated by ecosystem models,” said Reich. 

“In other words, in the past couple of decades, global savannas and grasslands have slowed climate warming more than they have accelerated it – despite fires. But there is absolutely no guarantee that will continue in the future.”

During the same time frame, however, there has been a 25 percent decrease in soil carbon in humid savanna-grassland due to more frequent fires. Ultimately, the researchers determined that there was a net increase of 640 million metric tons of soil carbon globally.

Study implications 

Although the amount of carbon stored in these regions may seem insignificant, Pellegrini noted its importance in the broader context of global climate change mitigation efforts. 

“In the grand scheme of things, no, this is not really a massive amount of carbon that will put a dent in heat-trapping anthropogenic emissions,” said Pellegrini. 

“But no one region – neither the Amazon rainforest nor the U.S. Great Plains grasslands nor Canada’s boreal forest nor dozens of other biomes around the world – can alone store sufficient carbon to make a large contribution to slowing climate change. However, in aggregate, they can.”

“Plus, there are several savanna and grassland regions that have soil carbon-credit projects being developed, so understanding their capacity to sequester carbon is relevant to the region – even if it’s not a massive flux globally.”

The study is published in the journal Nature Climate Change.

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