With 3.4 billion years of experience under their belts, plants are the most tried-and-tested technology to mop up industrial-era carbon pollution. But with only a few decades to clean up the atmospheric ‘landfill’, they aren’t the silver bullet to reverse climate collapse. New studies nevertheless show that the carbon-sink potential of South Africa’s natural plant communities may have been underestimated. Protecting the carbon-capturing services offered by the country’s semi-deserts, savannahs, and grasslands has never been more urgent, ecologists say.

Back in the 1990s, something peculiar was happening in places like Hluhluwe-iMfolozi and other parks in KwaZulu-Natal. The savannah landscapes known for their Big Five and other charismatic wildlife were changing. The veld – mostly roomy, grassy areas with the occasional tree – was beginning to get crowded in by something that could only be described as woody thickets. Photographs taken near Hluhluwe in the mid-50s, compared with others taken in the same place four decades later, confirmed it.

These old savannahs were turning into something else entirely – an impenetrable thorny bush that was good for neither grazing, nor game viewing. Farmers started to fret about the threat to their livestock herds. Park officials worried about the possible loss of tourism. After all, who would travel all that way if they couldn’t see the wildlife which these savannahs were so famed for?

What was going on? Could it be that land owners were simply mismanaging their veld, causing this crowding-in of trees? For the past 15 million years, the main gardeners in southern Africa’s savannahs have been plant-eating animals, and fire. Moving through the veld, they cut down many of the small saplings that sprout in the veld. Only the odd tree would escape these pruning processes and make it into adulthood, leaving wide open grassy spaces in between, with the picture-book animals we’ve come to associate with them.

Maybe land managers needed to allow more regular fires to burn through the veld to mow down the saplings, or change their livestock or game densities?

But botanists at the South African National Biodiversity Institute (SANBI) had another idea. Could rising carbon dioxide (CO2) in the atmosphere from burning fossil fuels have something to do with it? After all, trees grow by drawing in carbon from the air around them and converting it to sugars to build their woody parts. If there’s more of this gaseous food available to them, surely they’ll grow faster? Could rising CO2 levels be fertilising the trees of the African savannah?

To test the idea, then SANBI scientists Barney Kgope, now director for carbon sinks and mitigation at the Department of Environment, Forestry and Fisheries (DEFF) and Guy Midgley, now a professor with Stellenbosch University, popped a few young indigenous acacia trees (Acacia karoo) into growing chambers, and started tweaking the CO2 concentration levels. They grew some of the trees in an atmosphere that simulated pre-industrial and glacial conditions, where CO2 was at 180 to 280 parts per million (ppm). Others were grown at 370 ppm, which was what global CO2 levels were at, at the time of the experiments, in the early 2000s. Others were grown at 550 ppm, which is what they estimated we’d see by mid-century. Some were grown at 770 ppm and 1,000 ppm.

The results were clear as day: the higher the CO2 in the chambers, the faster the trees grew, both above ground and in their root mass. Saplings grown at 180 and 280 ppm grew extremely slowly. When the scientists clipped the trees back to simulate fire or grazing damage, those with more CO2 in the chambers bounced back faster, because their carbon-storing root systems had developed more quickly.

Their measurements confirmed the hypothesis: higher atmospheric carbon was super-fuelling the trees, allowing youngsters to grow faster and escape the fire and grazing “traps” that had been keeping them in check out in the savannahs for millions of years. Now, growing stronger and faster, they could muscle in on the grasslands to create these dense woody thickets.

A new study published last week confirms that plants around the world are responding in a similar way, which is also allowing them to ratchet up their carbon mop-up operations in response to higher levels of atmospheric CO2. The study also suggests that this might boost plants’ role in putting on the handbrake to slow the climate emergency.

The findings, published in the journal Global Change Biology, calculate that since 1900, land-based plant growth has sped up by 31%, meaning the equivalent amount of increased banking away of carbon through photosynthesis. More recently, as the CO2 concentration has doubled, plant growth has increased by 47%.

This 30% increase in plant growth during the past century is double what previous estimates had calculated.

According to the authors, these findings suggest that plant growth will play a bigger role, in locking away carbon than previously thought. If economies cut their carbon emissions drastically in the immediate future, it will allow plants to have a greater effect in stabilising global heating and avoiding pushing beyond the 2°C upper-limited warming target set out in the Paris Agreement.

This highlights the urgent need to protect and restore land-based carbon sinks, they say.

This is good news, says an up-beat Midgley, who is usually far more grim on matters relating to climate. It doesn’t solve the carbon problem, but it does show that carbon-capturing ecosystems can buy us urgently needed time as global economies try to make radical society-wide changes needed to level off and then reduce emissions to avoid dangerous levels of warming.

“What’s also important to note, is that this study doesn’t include the work that we have been doing in this area in South Africa for the past two decades. The models they use don’t reflect the impact of savannahs turning to thickets because of super-fuelled seedlings escaping the fire and grazing trap, and the higher levels of carbon draw-down because of that. If the models in this study had included this mechanism, they would likely have shown even more positive results for carbon sequestration.”

Another local study, published in the journal Nature Communications in 2018, shows a greening of 8% over sub-Saharan Africa over the past three decades. This increase in woody vegetation, seen in high resolution satellite images, is partly explained by rising CO2, along with a warmer, wetter climate, and a decrease in fires and browsing animals in parts of the subcontinent.

This is a double-edged sword, though. Faster woody growth can clean carbon from the atmosphere and benefit the global community, but at the cost of the richness of wildlife and the subcontinent’s vast grazing potential. Midgley and colleagues agree – this service to the world comes at a cost to southern Africa, and is that fair?

The sorcery of plants won’t solve the crisis alone

Coal is a wonder of the natural world. The colour of molasses, the consistency of crystal, it is, in essence, fossilised sugar. It started out as living, breathing plants which, and over the course of 400 million years when the dinosaurs reigned, slowly sucked gaseous carbon from the air around them. Under the glow of the sun, they turned this carbon dioxide into carbohydrates to build their woody branches, stems, and roots through the strange alchemy of photosynthesis. Dying plants, trapped in the Earth’s ageing skin and pressured by heat and time, slowly transformed into rock.

A potent, energy-dense, burnable rock. Oil is that, in liquid form, conjured by the same strange magic in ocean-living algae.

The fossilised sugars that have been dug up and burned over the past 200 years are the leftovers of a global carbon mop-up operation by plants that took tens of millions of years to do, local ecologists point out. Hoping that today’s plants will single-handedly clean up the same amount of carbon in just a few decades, to stop global heating from creeping above the 2°C temperature threshold, is an exercise in number-crunching that even a struggling maths matriculant can see doesn’t add up.

But while industrial humans have been conducting their grand geo-engineering project on the climate, plants have been toiling away in the background, slowing its progress, these studies show. Without plants’ free carbon-banking services, atmospheric CO2 would be way higher, and the global average temperature would be well above the 1°C that’s already making the global climate so unstable, according to Midgley.

“Globally, we emit about 10 gigatons of carbon from fossil fuels every year,” explains Midgley. “But only about half of that stays in the atmosphere permanently.”

About 2.5 gigatons get drawn down into the ocean. The other 2.5 gigatons are taken up by land-based plant life.

“Without that atmospheric regulation ecosystem service chugging away in the background, we would be much closer to 500 ppm in atmospheric carbon,” says Midgley. This is well above the current 415 ppm which has pushed global temperatures up by 1°C since the start of the industrial age.

Trees aren’t the only plants doing the clean-up

Of all the carbon-hungry plants out there, trees are the most charismatic, and have become the darlings of nature-based carbon draw-down initiatives. The Bonn Challenge is a United Nations-led global initiative to restore 150 million hectares of deforested and degraded land around the world by 2020, with plans to take that to 350 million hectares by 2030.

Bringing the Bonn Challenge to Africa is the African Forest Landscape Restoration Initiative (AFR100), which aims to restore 100 million hectares across the continent by 2030. AFR100 has already locked down US$ 1 billion for the next decade of work, and so far 28 African countries have signed up, including South Africa.

The backbone of the Bonn Challenge is the Forest Landscape Restoration (FLR) initiative, which is “the ongoing process of regaining ecological functionality and enhancing human well-being across deforested or degraded forest landscapes”.

The FLR is about more than “just planting trees”, the initiative says, it’s about “restoring a whole landscape to meet present and future needs and to offer multiple benefits and land uses over time”.

At first reading, the FLR looks relatively benign: even if Africa is the continent with the vast grasslands and savannahs, the initiative says it will only target naturally forested areas that have been denuded of trees.

But local ecologists are nevertheless nervous, with some levelling a broad-side critique in science journals and the “grey literature” press late last year.

The problem, they argue, is with the global definitions of what constitutes a forest, a savannah, or a grassland, and how one measures their levels of health or degradation.

Dr Caroline Lehmann, a specialist in savannah biogeography at the University of Edinburgh and associated with the Wits University Centre for African Ecology, explains why the local interpretation of the global Bonn Challenge goals could be so damaging to African ecosystems.

“Grassland ecosystems are fundamentally misunderstood,” she says, writing in The Conversation last year. “The (UN) Food and Agricultural Organisation defines any area that’s half a hectare in size with more than 10% tree cover as forest. This assumes that landscapes like an African savanna are degraded because they have fewer trees and so need to be reforested.”

The result is that savannahs and grasslands around the world are often viewed as being “degraded forests” and therefore need to be restored with tree planting.

Lehmann and Midgley recently published their critic of the AFR100 plans, along with two other local natural scientists – Prof William Bond from UCT’s Department of Biological Sciences and research associate of the South African Environmental Observation Network (SAEON), and Dr Nicola Stevens from Stellenbosch University’s Department of Botany and Zoology – pointing out that the 1 million hectares being targeted for restoration across the continent are largely aimed at grassy biomes, a strategy which is based on this “erroneous assumption that these (grasslands) are deforested and degraded”.

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What does the AFR100 commitment mean for South Africa, where 3.6 million hectares are earmarked for Bonn funding? According to the initiative’s website, the focus here will be to prioritise “water retention and landscape stability (erosion, combating desertification), clearing sparse and dense stands of invasive plants and bush encroachment, re-vegetation, soil and donga rehabilitation and restoration, and additional interventions to be determined through restoration opportunity assessments”.

But ecologists and ecosystem scientists who have asked the Department of Environment, Forestry and Fisheries (DEFF) to expressly state which ecosystems or specific sites will be targeted, or whether any of these interventions will also involve “reforesting” in grasslands or savannahs which may have been erroneously judged as “degraded”, they have been met with silence. Several of the ecologists approached by Daily Maverick said they were still in the dark as to exactly how the government intended rolling out the AFR100 ecosystem restoration work here.

Daily Maverick requested comment from the DEFF by email and telephone several times between mid-November 2019 and end-January 2020, and as yet has received no response.

How will the DEFF interpret “forests” or measure the health of grasslands and savannahs? What does “re-vegetation” mean, in this context? Will the government guarantee ecosystem-appropriate restoration, rather than changing the tree density in savannahs and grasslands?

Lehmann argues that the definition of “forest” needs to be reworked, to ensure savannahs and grasslands are recognised as the important systems that they are, and which give “their own irreplaceable benefits to people and other species”.

“It’s essential people know what degradation looks like in open, sunlit ecosystems with fewer trees” and that restoration work to degraded ecosystems is done with the necessary sensitivity to the nuances of these different systems, she says.

Banking on grasslands

Only about 5% of South Africa has naturally-occurring forest ecosystems. The bulk of the country is covered by plant communities that have relatively little above-ground carbon storage, by comparison to the forests of the world which get most of the air time: the Nama-Karoo biome, grasslands, and savannahs.

At first glance, grasslands and savannahs may not look like an impressive carbon sink, but because they cover such a vast area, if they’re managed right, they can lock carbon away beneath the ground in a relatively stable state.

Some local ecologists and conservation bodies are investigating whether good grasslands management might be a way to draw carbon-mitigation funding into the country. If private landowners or communities in charge of communal lands can show the carbon-storage potential of their grasslands, and manage them to store that carbon in a measurable fashion, it may be a way to draw funding in from the global North to support ecosystem restoration and conservation.

As one ecologist explains, grasslands and savannahs help lock carbon away beneath the ground. While leaves and roots also store carbon, it is relatively less ‘stable’ because plants die during the course of their life cycles. It takes centuries for stable soil carbon to form, but as plants suck CO2 from the atmosphere and take it underground through their roots, it eventually becomes locked into the soil. Time, organic matter, grazing, fires, microbes, and mineral properties of soils all help build stable soil carbon. As long as fires are not too frequent or hot, and areas are not over-grazed, grasslands sequester meaningful amounts of carbon globally.

There are some pilot soil carbon-mapping projects happening in parts of the country, which may draw together rangeland restoration initiatives in communal grazing areas with carbon credit opportunities. If researchers can show the carbon banking potential of grasslands, communal farmers may be able to apply for carbon funding which can help pay for better livestock management and the cost of restoration work.

Given that the plant communities dominating the African continent, and South Africa – where forests aren’t the main carbon sinks across much of the region – aggressive reforestation driven by the Bonn Challenge will come at a cost to biodiversity and ecosystem services on the content. The carbon mop-up services offered by Africa to the world in return will also be relatively small, on the scale of the global efforts needed to avoid dangerous levels of warming, and therefore don’t justify that cost, argue Bond and his colleagues in the paper The Trouble with Trees. The attention should rather be on no longer burning the fossilised carbon banked away by plants 400 million years ago.

The UK, in one year alone, was able to cut emissions by 12 million tons of CO2 equivalent, they say, simply by throttling back on coal-fired electricity generation between 2016 and 2017.

“That equates to 3.3 million hectares of open (African) ecosystems turned into plantations,” they write.

“Given the land use change envisaged for tree planting, over enormous areas, sustained for decades, with such poor gains in carbon reduction, we find it difficult to understand why afforestation is so widely supported.”

Rather conserve and restore the ecosystems that have evolved here on the continent, and let them keep ticking over in the background as they work to scrub the world’s polluted atmosphere clean. DM