Stamping out savanna fires doesn’t bolster carbon sink by much, study finds

  • enerates smaller carbon sequestration gains than previously thought, an analysis published in the journal Nature found.
  • The data from a decades-long experiment in South Africa’s Kruger National Park raises questions about whether fire suppression in savannas can help in combating climate change, according to an accompanying commentary.
  • Shrubs and grasses that make up the savanna store more carbon below ground, on average, than forests, which is one reason why fires aren’t as damaging in these landscapes.
  • Even for plots in the national park subject to intense fire activity, the researchers found that root and soil carbon stores are largely preserved.

A nearly seven-decade-long experiment in South Africa’s largest national park is yielding surprising results about how fires mold savanna land.

The analysis from Kruger National Park, published in the journal Nature, found that stamping out fires generates much smaller carbon sequestration gains than previously thought — a mere 0.35 metric tons of carbon per hectare per year. This is a fraction of earlier estimates, some of them as high as 9.4 metric tons.

According to an opinion piece by Niall P. Hanan and Anthony M. Swemmer published along with the study, the results raise questions about whether fire suppression in such landscapes is efficient at combating climate change.

Scientists can miss the mark when calculating carbon storage potential because underground carbon storage is harder to get right. Plants stash carbon by turning it into biomass through photosynthesis. About a quarter of plant biomass is found underground, per a recent estimate. Shrublands and grasslands store more carbon below ground, on average, compared to forests — two and three times, respectively.

Savanna fire in West Africa. Shrublands and grasslands store more carbon below ground, on average, compared to forests — two and three times, respectively. Image by ETF89 via Wikimedia Commons (CC BY-SA 4.0).

Today, more than half of all greenhouse gas emissions linked to fires in the tropics come from the burning of natural grassland and savannas. The African savanna is the most expansive in the world. Fires are lit every year to prepare land for farming. In many cases, farmers circle back to the same plots after leaving them fallow for a few years.

Savanna lands are often thought of as a transition between thickly clad rainforests and desolate deserts. They receive less rainfall than tropical forests but enough to support some vegetation, sporting smaller, widely spaced trees. The ample sunlight reaching the soil allows grasses to flourish.

Kruger National Park is spread across 1.96 million hectares (4.85 million acres), four times the size of Grand Canyon National Park in the U.S. Most of Kruger is subtropical and tropical savanna. It hosts more than 200 grass species, from blue buffalo grass (Cenchrus ciliaris) to stinking grass (Bothriochloa radicans), and more than 400 tree and shrub species.

The “burning trials,” as scientists first called them, started at the park in 1954. The main aim was to understand how fires shaped the ecosystem, especially animal and plant diversity. When scientists first set up the experiment, fires were considered an obvious human threat to natural environments.

This understanding has become complicated over the years as more information about the role of fires in regenerating terrains has emerged. Controlled fires can also act as breaks and prevent the build-up of combustible material.

In recent decades, researchers have turned their attention to how fire activity modulates carbon storage. The new study by a team from South Africa and the U.S., led by Yong Zhou at Yale University, shows that fires may not significantly diminish the savanna’s ability to stash carbon.

Pretoriuskop landscape in Kruger National Park. The authors of the study selected plots in Pretoriuskop landscape, one of the rainiest parts of the park, getting about 700 millimeters (27.5 inches) of rain every year. Image by krugergirl2 via Flickr (CC BY 2.0).

The 7×7 hectare plots carved out as part of the trial are spread across four types of landscapes in Kruger with different soil compositions, vegetation and rainfall. The authors of the Nature study selected plots in one of the rainiest parts of the park called the Pretoriuskop landscape, which gets about 700 millimeters (27.5 inches) of rain every year.

These lots, separated by fire breaks, are subject to varying degrees of fire activity. The study authors collected data from four kinds of treatments: plots where fires were lit every year, every two years, every three years, and not at all. This last string of plots has not seen fire activity for almost 70 years, capturing what would happen if fires were banished altogether.

The three-year interval represents the normal fire activity for this region. Annual fires represent an extreme scenario. The researchers found that aboveground biomass decreases for plots burned every year, but the root and soil carbon stores are largely maintained.

Forest biomes are better studied, but estimates from these regions cannot be extrapolated wholesale to savanna land. As part of their work, the scientists excavated five resprouting silver cluster-leaf plants (Terminalia sericea) and found that belowground biomass was almost 20 times what was found aboveground.

A wildebeast in Kruger National Park. Many savannas are drier than the authors’ study site. While parts of South American and Australian savannas receive more rainfall than Kruger, they have different vegetation and soils. Image by H&T PhotoWalks via Flickr (CC BY-NC 2.0).

One drawback with the study is the authors only included plots that receive a lot of rain. They cited logistical reasons for doing so in the study. But they also noted that these regions could support denser vegetation.

“Many savannahs are drier than the authors’ study site, particularly those in Africa,” Hanan and Swemmer wrote in their accompanying commentary. While parts of South American and Australian savannas receive more rainfall than Kruger, they have different vegetation and soils. “Similar studies in other regions are needed to further explore the role of fire suppression and tree planting in fire-prone savannahs,” they wrote.

There’s another reason why fire suppression is unpopular among savanna ecologists. It’s often bundled with replantation efforts that crowd the land with trees to boost carbon storage. This can change the nature of the ecosystem and harm wildlife that is specially adapted to savanna conditions.

“It is important to balance the costs and feasibility of fire suppression or tree planting with the potentially small carbon-storage benefits estimated by Zhou and co-workers,” Hanan and Swemmer wrote.

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