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- A popular social media challenge in South Africa asks people to plant 10 spekbooms each. This is justified by the claim that the plant can capture and store “more than four tonnes of carbon dioxide per year per hectare planted, making it more effective than the Amazon rainforest”.
- While evidence suggests that spekboom can absorb carbon dioxide at these levels, it is only possible under very specific conditions. The plant must be in its natural environment.
- Available data shows that the Amazon rainforest captures and stores between 23.8 and 38.4 tonnes of carbon dioxide per hectare per year. This is substantially higher than the available estimates for spekboom.
Spekboom, also known as Portulacaria afra or porkbush, is a plant species common to southern Africa. The succulent, which has small juicy green leaves, is found across the South African provinces of the Eastern Cape, KwaZulu-Natal, Mpumalanga and Limpopo, as well as into Swaziland and Mozambique.
Spekboom has recently gained popularity because of claims that it can absorb relatively large amounts of carbon from the atmosphere.
In March 2019, Getaway magazine said that spekboom “can sequester more than four tonnes of carbon dioxide per year per hectare planted, making it more effective than the Amazon rainforest at sucking carbon dioxide out of the atmosphere”.
These claims have since been shared on social media and by a number of online publications. But are they correct?
| Summary The claim that spekboom “can sequester more than 4 tonnes of carbon dioxide per year per hectare planted” is misleading. According to research from 2014, spekboom can capture and store between 2.9 and 8.6 tonnes of carbon dioxide per hectare per year in its aboveground biomass. When the plant is considered in its entirety, this number can increase to 15.4 tonnes of carbon dioxide per hectare per year. But this is only under very specific circumstances. The available research shows that these rates were achieved where spekboom was being used in the restoration of a subtropical thicket in its natural habitat with other plants. |
Carbon sequestration is when carbon is captured out of the atmosphere and stored. One way to store carbon is in plants. Plants use solar energy to convert carbon dioxide and water into sugars and oxygen via a process called photosynthesis. A lot of this carbon remains trapped in the plant’s tissues and in the surrounding soil. This is aided by high rainfall, which promotes plant growth.
Jan Vlok, a botanist and consultant with Regalis Environmental Services based in Oudtshoorn in the Western Cape province, told Africa Check that spekboom's ability to capture and store carbon “varies greatly, depending on local environmental factors”.
Numerous botanists pointed us to a 2014 paper published in Restoration Ecology, that investigated using spekboom to restore degraded ecosystems.
The researchers found that in subtropical thickets in the Eastern Cape province, spekboom was able to absorb between 2.9 and 8.6 tonnes of carbon dioxide per hectare per year within an environment that was being restored. The rate of carbon dioxide absorbed into the plant varied, depending on its location and the area’s rainfall.
However, this is only for the plant’s trunk and leaves above ground. When the plant was considered in its entirety (which also includes the plant’s roots, debris and soil) the researchers found that plants in arid and semi-arid environments could absorb in the region of 15.4 tonnes of carbon dioxide per hectare per year.
But this carbon-absorption is only possible under certain conditions.
Spekboom must be in its natural habitat
Anthony Mills, one of the authors of the 2014 paper and CEO of Cape Town-based climate change consultancy C4 Solutions, suggested we speak to Kelly Vlieghe, an analyst at the company. Vlieghe said that that spekboom “is effective at sequestering carbon, but only within its natural habitat”. She said that spekboom planted outside of its natural environment is unlikely to be more effective at absorbing carbon than plants indigenous to an area.
The subtropical thicket, where spekboom is indigenous, ranges from the eastern parts of the Western Cape, near Still Bay, to beyond East London in the Eastern Cape.
To achieve high carbon sequestration rates, the plant must also be part of an ecosystem with other plants. “It is important to note that spekboom and carbon sequestration goes hand-in-hand with ecosystem restoration,” said Marius van der Vyver, a botanist at Nelson Mandela University, in Port Elizabeth, Eastern Cape.
“It is not only the spekboom itself that is sequestering the carbon within an ecosystem under restoration over time, but also the other, hard-wooded and thus much more carbon-heavy plants, such as the common canopy dominant trees and other large shrubs present within the ecosystem that spontaneously regenerate as spekboom is brought back in sufficient quantity into the system.”
Bob Scholes, a professor of systems ecology at the Global Change Institute, at Wits University in Johannesburg, told us that spekboom would only sequester more than four tonnes of carbon dioxide per hectare annually, as claimed, under special conditions. “Spekboom thicket, in a good rainfall year, on deep soils, without browsing [animals eating it], in the middle of their growth spurt, might be able to attain this number,” he said.
Growing plants are better able to capture and store large amounts of carbon than plants that have reached their maximum size. This is because carbon is a building block in their stems, trunks and roots, and because their respiration, during which plants release carbon dioxide, has not caught up with their growth, said Scholes.
Once plants are fully grown, they no longer require additional carbon for their internal scaffold. In a degraded ecosystem where spekboom is being reintroduced, it is able to grow quickly and capture more carbon than if the plant is at full size.
| Summary The claim that “spekboom is more effective than the Amazon rainforest at sucking carbon dioxide out of the atmosphere” is incorrect. It has been estimated that the Amazon basin rainforest has the ability to absorb and store about 15.4 tonnes of carbon dioxide per hectare per year in its aboveground biomass. When the forest’s trees, their roots, and the soil are taken into account the figure increases to between 23.8 and 38.4. In comparison, the available data suggests that spekboom can absorb between 2.9 and 8.6 tonnes of carbon dioxide per hectare per year in its above ground biomass. When the entire ecosystem is considered, this figure can increase to 15.4. These rates can only be achieved, however, in very specific circumstances. |
The Amazon basin forest is the largest tropical rainforest in the world. It covers between six and eight million square kilometres and straddles nine countries, although most of it is in Brazil.
The March 2019 Getaway article claimed that spekboom is “more effective than the Amazon rainforest at sucking carbon dioxide out of the atmosphere”. It quoted the Spekboom Foundation of South Africa as saying that that spekboom is “10 times more effective per hectare at carbon fixing than any tropical rainforest”.
Johan Swart from the Spekboom Foundation told Africa Check that he had received this information from Dr Alastair Potts, a botanist at Nelson Mandela University in Port Elizabeth. Potts pointed us to a 2006 study by Anthony Mills and Richard Cowling. (Note: Potts denies having made the statement or corresponding with Swart on the topic.)
Potts told Africa Check that the carbon absorbing ability of spekboom was not applicable outside of the subtropical thicket.
“The Mills and Cowling paper is measured from plants put into a barren landscape and protected from herbivory,” Potts told Africa Check in an email. “The growth rates are very, very high simply because the system is not saturated.”
Available research shows Amazon trumps spekboom
We could not find a single scientific study that directly compared the carbon absorption capabilities of the Amazon rainforest and spekboom.
The carbon sequestered per hectare per year by spekboom during thicket restoration is something closer to the Net Primary Productivity, or the amount of carbon that is absorbed by plants and enters the ecosystem. This mostly applies during the early growth phase of a recently established ecosystem, said Scholes.
One 2018 study, which limited itself to the Amazon’s aboveground woody biomass, put the figure at 4.2 tonnes of carbon per hectare per year. This measurement is in carbon per hectare, not carbon dioxide per hectare. It converts to 15.4 tonnes of carbon dioxide per hectare per year, calculated by multiplying by a factor of 3.66.
Another study from 2000 estimated that the Amazon rainforest absorbed and stored between 6.5 tonnes and 10.5 tonnes of carbon per hectare per year. Once converted, this translates to between 23.8 and 38.4 tonnes of carbon dioxide per hectare per year.
These are substantially higher than the available estimates for spekboom, which range from 2.9 to 15.4 tonnes of carbon dioxide per hectare per year.
However, this doesn't mean there's a carbon sink of this size, said Edward Mitchard, a senior lecturer at the University of Edinburgh, Scotland, who heads up a research group trying to quantify carbon stocks and change in tropical forests. A carbon sink refers to a natural system that captures and stores carbon from the atmosphere.
“Animals (especially insects) eat a lot of this Net Primary Productivity, and trees die and are consumed by fungi, animals, and bacteria,” Mitchard told us over email. “From a normal year-to-year assessment, the whole ecosystem in the Amazon is approximately in balance.”
Mitchard said that plants in the Amazon capture and store higher amounts of carbon when they are “young” and still growing. “This is probably also the case for your 'wonder plant' which is growing from nothing.”
Stat from 2006 may have been misinterpreted
We asked Mills how the link between spekboom and the Amazon rainforest arose. His colleague Vlieghe, to whom questions were directed, said that the 2006 paper written with Cowling “stated that at a particular site … the rate of carbon sequestration was ‘comparable to many temperate and tropical rainforest’”.
“The important point coming out of these comparisons is that, compared to many arid or desert plants, spekboom has a sequestration rate that is more akin to that of plants occurring in tropical or wet environments,” Vlieghe said.
She said she believes that the link between spekboom and the Amazon rainforest may have been caused by a misquote of the 2006 paper. “Where this error first arose in the media is unclear to us.”
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