Spekboom (Portulacaria afra) is a soft-wooded, semi-evergreen shrub that is usually between 2.5 to 4.5 or more meters tall. The plant is the dominant species within the subtropical thicket biome, as well in biomass as in cover and relative abundance. As such, the plant is a keystone species and even has been called a miracle plant (www.c4es.co.za).
Portulacaria afrabelongs to the family Portulacaceae (www.plantzafrica.com), the Purslane family, which contains about 20 genera and has a cosmopolitan distribution, with the highest diversity in semi-arid regions of the Southern Hemisphere in Africa, Australia, and South America. The species is commonly known as Porkbush or Elephants Food (English), Spekboom (Afrikaans), iNtelezi, isiDondwane, isAmbilane, iNdibili, isiCococo (Zulu) and iGqwanitsha (Xhosa).
Spekboom grows under warm conditions on rocky slopes, in succulent karoo scrub, in thicket and bushveld, as well as in dry river valleys. The species favours north-facing slopes (locations with lots of sun).
The shrubs are able to cope with huge conditional variation. They are capable of withstanding great fluctuations in temperature and have the ability to provide steady forage through periods of drought. Spekboom is a facultative 'curssulacean acid metabolism' (CAM) plant, or in other terms it may 'change its gears' within the photosynthetic machinery under certain conditions (Zijlemans 2003). Under wet conditions, it uses the same system as rainforest plants and under dry conditions it switches to a system used by desert plants like cacti. Besides water availability, other conditions such as temperature and water salinity, force spekboom to shift its metabolism strategy (Rossler et al. 2010). This switching enables it to grow and capture carbon much faster than other plants that occur naturally in semi-arid environments (see the section on CO2-fixation).
When isolated, the plant employs a skirt of low branches. These branches eventually take root, which largely contributes to the plant's reputation as a soil binder (Powell 2009). The species reproduces asexually and sexually, but the success of sexual reproduction is limited, as recruitment via seedlings is rare. Although the plants have prolific seed rain, the seed bank is negligible and seed dormancy is limited. Flowering usually takes place from October to November after fairly good rain showers (Vlok et al. 2003).
Portulacaria afra is found in the eastern parts of South Africa, from the Eastern and (parts of) the Western Cape northwards into KwaZulu-Natal, Swaziland, Mpumalanga and the Limpopo Province as well as Mozambique (www.plantzafrica.com).
Although Portucalaria afra is considered the dominant species within the Subtropical Thicket biome, accounting for almost 70 % of the total canopy cover (Vlok 2003; Powell 2010), in heavy degraded area’s the species has vanished, leaving the soil bare and vulnerable to the elements. Subtropical thicket cannot easily grow back on places where it was once dominant. Human interference is needed, but once recovered, the creation of new microclimates supports the regeneration and development of others plants species as well (Powell 2010).
Spekboom is the key-species for the restoration of the Subtropical Thicket biome as the plants easily grow on heavily degraded slopes and most important, sequester carbon. The shrubs are capable of sequestering great amounts of carbon. Debate is still going on, but figures show that Spekboom sequesters approximately 2.4 tons (range 1.8–4.2) of carbon per hectare per year. Sequestration and storage capacity of carbon by spekboom allows the PRESENCE network to access the carbon market. This allows farmers to farm carbon as a sustainable alternative to stock farming. The carbon market therefore might enhance restoration efforts within the Baviaanskloof , not only by the reforestation of the hill slopes with spekboom but in the near future also with the protection of intact thickets and wetlands.
Authors: Bart van Eck, John Janssen & Mike Powel(2012)