Is spekboom South Africa’s silver-bullet solution to becoming carbon neutral?

Over the last few weeks, a viral trend known as the #SpekboomChallenge has dominated South Africa’s social media timelines. The challenge? To grow 10 or more spekboom (Portulacaria afra) plants in 2020 to offset part of your personal carbon footprint. As the impacts of anthropogenic climate change become a lived reality, and with South Africa currently, the world’s 14^th highest emitter of greenhouse gasses, discussions around reducing our carbon footprints are certainly necessary. But is spekboom South Africa’s silver-bullet solution?

I have been interested in spekboom’s reputed carbon sequestration ability since my time as the prefect in charge of my high school’s environmental portfolio, where one of my projects was to plant a spekboom for every scholar and staff member. At the time I had been reading a number of articles in the popular press about carbon-trading with spekboom. Articles about spekboom’s ability to sequester carbon crop up in South Africa’s media on a regular basis, and over the years I’ve noticed the same claims being made, yet few articles providing references. The claims are namely that spekboom uses two types of photosynthesis, that a thicket can store more carbon than a tropical rainforest, and that it sequesters up to 10 tons of carbon per hectare per year. Working in the agricultural sciences has taught me to be inherently sceptical of anything that is presented as a silver-bullet solution for a complex problem like carbon footprints. The hype around the #SpekboomChallenge was the nudge I needed to assess my own prior beliefs in spekboom, and delve into the research to see if the popular press’ claims are supported by peer-reviewed studies.

What I found is that spekboom is somewhat unique in its ability to alternate between the C3 and CAM photosynthetic pathways, which allows it to sequester carbon as efficiently as possible under varying environmental conditions. However, the phrasing in many of the #SpekboomChallenge articles and social media posts has made it seem as though the plant employs both types of photosynthesis at the same time. This is not strictly true. Spekboom experiences seasonal shifts between the C3 and CAM pathways in response to temperatures, with the CAM pathway used during warmer periods to reduce photorespiration and unproductive water losses. This is an important adaptation for survival in semi-arid environments such as the spekboom-dominated Albany thicket, where we would expect the average rainfall of only 250-400 mm per year to keep biomass production quite low.

Biomass production is an important component of an ecosystem’s carbon storage capacity, although there a number of factors that play a role in carbon cycling. Spekboom thickets have an exceptionally high carbon storage capacity for a semi-arid environment, with intact thickets storing approximately 245 tons of carbon per hectare. In comparison to South African biomes with similar annual rainfall and temperatures, intact spekboom thickets store three times more carbon per hectare than the Succulent-Karoo and eight times more carbon per hectare than the Nama-Karoo. According to the same study even our grassland biome, which receives 900-1200mm of rainfall per year, only has a 70% the carbon storage capacity of an intact spekboom thicket. However, the media’s claim that spekboom thickets store more carbon than the tropical rainforests is not true. There are a number of ecosystems that store significantly more carbon per hectare than spekboom thickets, such as the rainforests of East Africa (330 tons of carbon per hectare), the peatbogs of Indonesia (2700 tons of carbon per hectare), or the Siberian permafrost (4500 tons of carbon per hectare). Carbon storage is also highly variable within different areas of thicket, due to a range of factors from local climatic conditions to differences in spekboom ecotypes. A study looking at spekboom thicket restoration reported restored thicket storing 161 tons of carbon per hectare after 27 years of growth at one site, but only 65 tons of carbon per hectare after 20 years of growth at a second site. However, while the carbon storage capacity of spekboom is highly variable and not as high as other biomes I once again want to emphasise that it is remarkably high for the environmental conditions under which it grows.

The third popular claim that we need to assess is if spekboom sequesters up to ten tons of carbon per hectare per year. The same study looking at carbon storage capacity of restored spekboom thickets also calculated the carbon sequestration rates at the two sites, and these were determined to be 4.2 and 1.2 tons of carbon per hectare per year respectively. Looking at the results of the most comprehensive review of published literature on tree growth and CO₂ removals the carbon sequestration rates of spekboom-dominated thickets at the higher end of the range are still comparable to forest landscapes. The study found that, in the first 20 years of growth, naturally regenerating humid forests sequestered carbon at a rate of between 3.0 and 5.1 tons per hectare per year (depending on the region), and the dry forests of South America and Australia at a rate of 2.8 and 3.8 tons per hectare per year respectively. So if spekboom-dominated thicket is able to sequester carbon at rates comparable to most forest landscapes and has a higher carbon storage capacity than other semi-arid and sub-humid South African biomes, should we go ahead and plant as much of it as possible? Not unless this forms part of restoring degraded thickets or tracts of land that historically supported spekboom.

Of South Africa’s 1.69 million hectares of unbroken spekboom-dominated thicket canopy, 36% has been moderately degraded and 46% heavily degraded. This is largely due to the mismanagement of livestock, and has reduced the regions’ ecosystem carbon storage, rainfall infiltration, soil fertility, and carrying capacity. Several restoration programmes have already successfully used spekboom cuttings to restore degraded thicket to its original condition. In these instances, I believe the #SpekboomChallenge can be of immense value from a conservation perspective, by restoring habitat for a diverse range of fauna and flora in this species-rich region. However, South Africa currently emits over 456 million tons of CO₂ per year. Even if we hypothetically assume we could restore all 1.69 million hectares of spekboom thicket to a state that would sequester at the maximum recorded rate of 15.4 tons of CO₂ per hectare per year, these thickets would only be able to sequester 5.72% of South Africa’s current annual emissions.

There are also a number of reasons why planting spekboom outside of the thicket biome will be of little benefit. Some research suggests that one of the reasons spekboom thickets have such a high carbon storage capacity is that they have a low rain throughfall, which potentially creates a drier microclimate below the canopy. In theory this reduces carbon release due to lower rates of soil organic carbon mineralisation and microbial activity in the leaf-litter. Planting spekboom thickets in wetter environments (such as on our grasslands) would likely result in thickets with a lower carbon storage capacity, as higher annual rainfall would reduce the effect of the low rain throughfall. Converting other biomes into spekboom thickets also threatens the local biodiversity of these biomes, in the same way that any other land-use change such as forestry would. I specifically mention forestry because if the argument is that it would be beneficial for us to convert some of our natural spaces into carbon sinks then eucalyptus plantations, which sequester carbon at an average rate of 2.46 times that of the maximum-recorded rate of spekboom, are arguably a better option. This is, of course, a ridiculous argument to make, as focussing only on a single species’ carbon sequestration ability ignores the wider environmental impacts of planting it out of habitat.

So what does this mean for the #SpekboomChallenge?

I believe this is a valuable lesson in fact-checking. Article after article repeated claims with no research to support them, which presumably were copied from each other in a sensationalised written game of broken-telephone. These articles were so widely shared on social media that the claims were accepted as hard truths and this created false hope within South Africa’s climate movement. In the face of anthropogenic climate change, we need to be pragmatic and make evidence-based decisions and large-scale afforestation is not a viable climate solution for Africa.

To everyone who got their hands dirty planting spekboom, I wholeheartedly believe the intentions of the challenge were pure and that a number of benefits have come out of it. As a plantsman, there are few things I want to see more than everyone being excited about plants, as they are such an integral yet under-appreciated part of our lives. The #SpekboomChallenge was certainly successful in getting more people talking about the importance of plants, even though the facts were somewhat misconstrued. It brought us numerous examples of citizens, especially children, propagating spekboom for their local communities and getting involved in urban-greening projects. Spekboom is an excellent addition to any garden because it is water-wise, hardy, attractive, and incredibly easy to propagate. I would love to see the energy of the #SpekboomChallenge continued with all of us being more proactive in urban-greening projects, as they come with a number of benefits ranging from improving air quality and reducing energy costs to enhancing a community’s sense of social identity and improving mental health. South Africa has an incredibly diverse range of plant species, and we should draw on this diversity to meet the requirements of different projects in different settings.  

Lastly, for those looking for a solution to reducing our national carbon emissions: 82% of South Africa’s greenhouse gas emissions come from the energy sector. If we can take the same energy we saw with the #SpekboomChallenge, and combine it with our current load-shedding frustrations, I firmly believe we can place enough pressure on our government to divest in fossil fuels and move towards more sustainable power generation.