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Carbon sequestration role of savanna soils key to climate goals

  • Savannas and grasslands cover a vast area, some 20 percent of the earth’s land surface — from sub-Saharan Africa, to the Cerrado in Brazil, to North America’s heartland. They also offer an enormous and underappreciated capacity for carbon sequestration.
  • However, the role of forests in storing carbon has long been emphasized over the role of savannas (and savanna soils) by international climate negotiators, resulting in policies such as REDD+ for preserving and restoring forests, with no such incentives for protecting grasslands.
  • Scientists warn that the planting of trees, such as nonnative eucalyptus in Africa and Brazil, could be counterproductive in the long term, potentially contributing to climate change emissions while harming grassland biodiversity and altering ecosystems.
  • As participants prepare to meet for the COP23 climate summit in Bonn, Germany next week, grassland scientists are urging that policymakers turn an eye toward savannas, and begin to develop incentives for preserving them and their carbon storing soils. More research is also needed to fully understand the role savannas can play in carbon sequestration.
Elephant on the savanna, Kruger National Park, South Africa. Photo by Bobby Amoroso

JOHANNESBURG, South Africa – When biologist Sally Archibald scans the vast savannas and grasslands of Kruger National Park some five hours northeast of her university office, she sees something many environmentalists and climate activists do not.

“I live in a country with 3 percent forests,” says Archibald, who studies the dynamics of savanna ecosystems at Witwatersrand University. “It’s always seemed to me that these ecosystems are important and fascinating. But most people see savannas and grasslands as empty, useless things.”

She sighs and looks away for a moment. She is a scientist, not an activist. Carefully, she adds, “I suppose I need to get better at explaining this. These impressions [people have] are wrong. We need these ecosystems as they are,” to curb climate change and protect biodiversity.

That’s a message, she agrees, that needs to be understood by political leaders, negotiators and climate activists from 196 countries as they prepare to meet in Bonn, Germany for the 23rd annual United Nations Climate Summit (COP23) from Nov. 6-18.

The most urgent goal of COP summit participants will be to ramp up the voluntary national carbon-reduction pledges made at the 2015 Paris summit, and to raise billions to assist vulnerable nations with climate adaptation strategies, such as renewable-energy installations and water-retention initiatives, objectives confirmed in teleconferences last week.

COP23 participants will also likely again focus on forests in the race to store carbon and slow global warming. Yet they should also look more closely at the unheralded, but important, role of savannas and grasslands in climate change mitigation, according to experts.

Sally Archibald points out details on a South African grasslands map. Scientists are only now beginning to understand the key role savanna soils can play in carbon sequestration. Photo by Bobby Amoroso

Savannas and grasslands cover a vast area, some 20 percent of the earth’s land surface — from sub-Saharan Africa, to the Cerrado in Brazil, to North America’s heartland. They can also be relied on generally as stable, enormous and underappreciated areas for carbon sequestration. In grasslands, carbon is mostly stored below ground in soil, which plays the same pollution-absorbing role, though often with less intensity, as tropical rainforests.

Grasslands are also rich in biodversity. In Africa, they’re home to some of the world’s largest, most iconic animals. Lions, cheetahs, elephants, rhinos, hippos, wildebeest, giraffe, zebra and antelope are among the charismatic megafauna that do not thrive in forests. And the Brazilian Cerrado, south and east of the Amazon, is considered the world’s most biodiverse savanna.

“If you want to keep global temperature rise under 2-degrees Celsius [1.8 degrees Fahrenheit; the Paris Agreement goal], then you need forests growing,” agrees biologist Bob Scholes, a systems ecologist at Witwatersrand.

“I love trees,” he adds. “But the fact is, for a lot of reasons, they are not always the answer.”

Savannas as targets for tree planting

The lively discussion that is emerging over the comparative role of forests and grasslands in curbing climate change is largely being driven by decisions made at past COP summits and by the global policy response to climate change.

The Paris Agreement of December 2015 emphasizes two significant goals: vastly reduce the world’s burning of fossil fuels, and provide economic incentives through programs like REDD+ to protect forests, reduce deforestation, and restore forests on degraded land. Grasslands, however, received no such attention.

As a result “empty” savannas might seem at first glance to offer a rich target for planting huge numbers of trees to sequester carbon, while also allowing for nations and corporations to claim carbon offsets.

Zebra on an African savanna. In the past, the role of trees in storing carbon was overemphasized as compared to the role soils play in sequestration. One reason for this early bias was that it is easier to use satellites to monitor carbon storage in the aboveground tree canopy than belowground in soils. Photo by Bobby Amoroso

The Bonn Challenge offers a compelling example. The initiative aims to reforest 150 million hectares of deforested land worldwide by 2020 and a total of 350 million hectares by 2030. Website maps for the initiative, launched in 2011 and overseen by the Global Partnership on Forest Landscape Restoration, identify degraded or deforested rain forests in Brazil, Peru, Indonesia and the Democratic Republic of the Congo, among other nations. Many areas on the maps would clearly benefit from tree planting.

But a 2016 study that defends the value of grasslands notes that the Bonn Challenge also targets, among other savannas globally, the sparsely forested Brazilian Cerrado: “The initiative to identify and map opportunities for landscape restoration is undoubtedly well intended. Nevertheless, the implementation of a reforestation program in savannic formations — that is, afforestation — of this large region would be an act of extraordinary folly,” say study authors.

Here’s why: scientists who study savannas worry that grasslands have been targeted for tree plantings by policymakers without a solid understanding of how those initiatives may harm biodiversity by further shrinking habitats for many threatened species, and could also negatively impact carbon sequestration in soils.

“Most people don’t know anything about soil or soil carbon,” notes biologist Mark Ritchie, a soils expert at Syracuse University. “It took almost 15 years for people to realize that the carbon pool in the soil under tropical rainforests is [equivalent to] 60 percent of the pool above ground in trees. Where there is less rainfall, you get quickly to 50-50 above-and-below-ground carbon. And in drier areas, like savannas and grasslands, the vast majority of carbon is absorbed below ground.”

Grasslands are rich in biodiversity, and home to some of the world’s largest, most iconic animals, including rhinos like those seen here, lions, cheetahs, elephants, hippos, wildebeest, giraffe, zebra and antelope. Photo by Bobby Amoroso

Lack of knowledge

In looking at the carbon sequestration question, it’s important to realize that researchers do not view the debate as a competition between the carbon storage value of forests versus grasslands. Scientists now largely agree that negotiators at COP23 will need to be sensitive to both.

Instead, researchers warn that any decision that policymakers make without a thorough understanding of the role savannas, grasslands and soils play in carbon storage could prove counterproductive to curbing global warming, and to conserving biodiversity.

Scholes notes, for example, that the widespread planting of nonnative eucalyptus plantations in sub-Saharan Africa for timber does work as a temporary carbon sink. However, those thirsty, fast-growing trees, originally from Australia, also dry up nearby rivers where water is already scarce. Also, chemicals in eucalyptus leaves poison ground vegetation, reducing the carbon held in the grassland pool over time, Ritchie adds.

Archibald stresses that the soil-disturbing act of replacing grasslands with trees, such as eucalyptus, often causes the release of soil carbon back into the atmosphere, escalating emissions and worsening climate change.

“In Tanzania, they are losing more carbon plowing the ground for planting trees than you can ever accumulate in the tree being planted,” Archibald explains. “That fact isn’t taken into account when some group is requesting carbon credits” for planting trees.

Archibald explains that “Increasing the area covered by woody vegetation would increase the total amount of carbon stored. But it would have many ecological, social and economic consequences. It’s also a risky way to store carbon in areas with fire, tree-eating animals and a growing population,” like much of the wild African savanna. Similar carbon storage questions have been raised in Brazil’s Cerrado, where nonnative eucalyptus trees have also been planted.

Grassland scientist Bob Scholes (left) with the author. Scholes expresses concern over the introduction of Australian eucalyptus, and other nonnative tree plantations, to African and Brazilian grasslands. These introduced trees alter ecosystems and may not efficiently sequester carbon over the long run. Photo by Bobby Amoroso

Increasing recognition

Chris Meyer, an Amazon forest policy expert with the Environmental Defense Fund who will be in Bonn, says the concern of soil scientists is justified. He confirms that soil experts opted out of early COP discussions of forest polices such as REDD+, putting them and grassland ecosystems on the sidelines as forest preservation grew steadily in stature as an international climate change-mitigation priority.

One simple reason for this forest favoritsm is technological: it is easier to measure carbon storage above ground in the tree canopy than it is to accurately estimate below-ground carbon storage over large areas. As a result, satellite monitoring is today the primary means used for qualifying to receive monetary incentives from programs like REDD+.

Meyer notes some good news: there is growing recognition in the COP community for a need to develop REDD-like climate adaptation and mitigation strategies to protect savannas and grasslands, and to understand fully their importance for carbon sequestration as well as biodiversity.

Those discussions are ongoing, he says, and will likely get attention at COP23.

“If we are going to meet the [Paris] net zero goal [for carbon emissions] by 2050, it can’t all be emission reductions. So where are you going to get it? Forest or soil?” Meyer asks. “We need to maximize the potential of both. But we still need a lot of science to figure this out.”

Meanwhile, two recent studies underscore the urgency of Meyer’s sentiment when it comes to the complexity of understanding the role of savannas, grasslands and soil, and how they are being impacted.

Mixed savanna, like this one in Kruger National Park, are valuable for their biodiversity and are increasingly being seen as important to carbon storage. People often tend to dismiss the value of savannas and grasslands, maybe partly because these ecosystems appear uninteresting to the eye. Photo by Bobby Amoroso

Ongoing research

New research continues to deepen understanding of savannas. For example In September, Bill Hoffmann, a plant and microbial biologist at North Carolina State University, published a study in Science Advances that found that active fire-suppression in the tropical savannas of southern Brazil is allowing for greater tree growth and cover. This imperils a range of plants and animals, like the maned wolf (Chrysocyon brachyurus) and greater rhea (Rhea americana), that live in those grasslands.

“We may be getting more carbon pulled from the atmosphere,” Hoffmann says, “but there is a trade off: these are still important areas for preserving biodiversity.”

Also in September, soil scientist Jon Sanderman at Woods Hole Research Center in Massachusetts, co-authored a report in PNAS that used computer models to estimate that agriculture has led to the loss of 133 billion tons of soil carbon from the top two meters of soil over the past 200 years, nearly equal to the total carbon loss from global deforestation, a stunning estimate.

“Trees gain carbon seven to 10 times faster than soils,” Sanderman says. “But soils can get on an equal footing because the cost is cheaper (by simply leaving savannas and grasslands alone). In agriculture, if you improve carbon [storage] levels, you can improve crop yields, use less fertilizer and retain more water. But it’s a real challenge to get farmers to alter the way they farm their land.”

Ritchie, the soils expert at Syracuse, returns the discussion to Africa where he does the majority of his research. There, in places like Kenya, Zambia and Tanzania, he is working to conserve savannas and grasslands by encouraging small-scale farming strategies that enable poor Africans to live off the land while leaving soil carbon undisturbed. This approach also allows birds and animals a greater range of movement on open lands, which they need to thrive.

There are many reasons to pay much closer attention to soils, how we treat them, what we plant on them, and what we don’t, Ritchie says: Carbon capture. Biodiversity protection. Poverty reduction in our poorest places. Each is a goal of every UN climate summit, and an objective in which savannas can play a key role.

“Those are three wins you get from preserving savannas and grasslands,” Ritchie says. “We just need more people to understand this.”

Justin Catanoso is a regular contributor to Mongabay and a professor of journalism at Wake Forest University in North Carolina, USA. He will provide coverage of COP23 from Bonn, Germany. Follow him on Twitter @jcatanoso

The Brazilian Cerrado, south and east of the Amazon, is considered the world’s most biodiverse savanna and is now known to be important to carbon sequestration. This vast biome is one of the world’s most threatened, as native grasslands are converted to soy plantations. Photo credit: Paulo Ellery via Visualhunt / CC BY-NC

Citations:

Abreu R., Hoffmann W., et al. (2017) The biodiversity cost of carbon sequestration in tropical savanna. Science Advances. DOI: 10.1126/sciadv.1701284

Fernandes G.W, et al. (2016) Afforestation of savannas: an impending ecological disaster, Natureza & Conservação

Sanderman J., et al. (2017) Soil carbon debt of 12,000 years of human use, PNAS. DOI: 10.1073/pnas.1706103114

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