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Tuesday, April 23, 2013

Porkbush/Vetplant (Portulacaria afra)

Family Portulaceae
Portulacaria afra or porkbush is a popular succulent garden plant in use around the world and is often used for bonsai. It has now been shown to be effective in carbon sequestration (binding atmospheric carbon which is responsible for climate change), in semi-arid landscapes and thicket vegetation it is also being used for restoration purposes.
 Derivation of name and historical aspects

The name Portulacaria is composed of Portulaca + aria suggesting a similarity to the genus Portulaca. The word afra is in reference to the fact that the plant occurs in Africa.

The porkbush belongs to a large and widespread family (Portulacaceae) which includes the popular Portulaca and is often sold in garden centres and grown in domestic gardens as an annual for summer colour, although this is not a South African species. Other members of this genus include Portulacaria armiana and Portulacaria pygmaea the former has larger grey green leaves and is native to Namibia although it is not often cultivated, whereas the latter is a dwarf succulent shrublet with small, thickly fleshy, grey green leaves and occurs on rocky hillsides in Namaqualand, South Africa.

 Description

The porkbush is an attractive, evergreen succulent shrub or small tree that can reach 2 - 5 m in height, although usually around 1.5 - 2 m in a garden situation. It has small round succulent leaves and red stems. Small star-shaped pink flowers are borne en masse from late winter to spring although flowering in cultivation is often erratic. They are a rich source of nectar for many insects, which in-turn attracts insectivorous birds.

Distribution

It is found in warm situations on rocky slopes in succulent karoo scrub, thicket, bushveld and dry river valleys in the eastern parts of South Africa from the Eastern Cape northwards into KwaZulu-Natal, Swaziland, Mpumalanga and the Limpopo Province as well as Mozambique.

Conservation status
Portlulacaria afra is listed in the National Red Data List as Least Concern which indicates that it is not threatened in its natural habitat.

 Ecology

Interesting ecology has been observed with this plant in the Eastern Cape where it forms part of the diet of the Addo elephants in the Addo National Park. Elephants eat the plant from the top downwards allowing the plant to spread itself vegetatively by spreading horizontal branches at ground level. Outside the park the plants are eaten by goats who eat the plant from ground level upwards preventing the plant from spreading vegetatively. Consequently these plants must rely solely on seed to proliferate the species which often proves difficult in such a dry climate. As a result it was observed that inside the park where the plant is subjected to browsing by elephants the pork bush survives and spreads successfully whereas outside the park the plant is becoming sparse as a result of overgrazing and poor regeneration. A very interesting mistletoe, Viscum crassulae, parasitizes this plant. The foliage of this mistletoe is hard and rough in contrast to the soft and smooth leaves of the porkbush. The small red fruits of the mistletoe are enjoyed by birds who eat the sweet outer layer and discard the sticky seed which is normally wiped onto a branch by the bird. There it germinates directly through the bark of the porkbush with specialized roots called haustoria which extract moisture and soil nutrients from the host, while the green leaves allow the semi-parasitic mistletoe to produce its own food.

Uses & cultural aspects

The leaves of the porkbush can be eaten and have a sour or tart flavour. It is heavily browsed by game and domestic stock and highly favoured by tortoises. The porkbush has also been indicated as a soil binder for preventing soil erosion. Traditional uses also include the increasing of breast milk by lactating mothers. The leaves are used to quench thirst, sucking a leaf is used to treat exhaustion, dehydration and heat stroke. Crushed leaves can be rubbed on blisters and corns on the feet to provide relief. The leaves are chewed as a treatment for sore throat and mouth infections while the astringent juice is used for soothing ailments of the skin such as pimples, rashes and insect stings. The juice is also used as an antiseptic and as a treatment for sunburn. It is also recorded that a small sprig of porkbush steamed with a tomato bredie (stew) imparts a delicious flavour. The honey made from the flowers of porkbush is said to be " unsurpassable in flavour and texture" by one reference (Roberts 1990).

The Porkbush on Climate Change

Recent research has shown the Porkbush to be an excellent 'carbon sponge' as it has the ability to sequestrate (absorb) free carbon from the atmosphere which is used to make plant tissue. Carbon is one of the major greenhouse gases which are responsible for the warming of the earth's atmosphere; it is produced in excess by burning of fossil fuels. Currently, humans are producing atmospheric carbon faster than the environment can absorb it, causing a deficit which remains in the atmosphere and causes heat from the sun to be trapped instead of being radiated back out into space. The porkbush has the unique ability to absorb more carbon from the atmosphere than most other plants and it does so particularly efficiently. A stand of Pork bush consequently has the ability to remove more carbon from the atmosphere than an equal amount of deciduous forest.

How does it manage this? The porkbush has the ability to make use of two different photosynthetic pathways, when conditions are favourable it manufactures its food to sustain growth by using the same method (pathway) that most other plants use. However, when conditions are not favourable and other plants have to shutdown and wait for sufficient rain, the porkbush can switch to a different pathway called CAM (Crassulacean Acid Metabolism) whereby it can continue to grow and slurp up huge amounts of carbon despite adverse climatic conditions. This allows the plant to excel in the arid or semi-arid conditions that it is native to.

Further to its carbon habit, the large spreading shrub covers and shades the soil from the harmful rays of the sun creating a favourable environment under the bush for insects and other wildlife to inhabit, while the dead organic matter which accumulates under the bushes has an enriching effect on the soil. This further enrichment of the soil improves its water-holding capacity which further benefits the porkbush as well as other plants and animals including micro-organisms, which occur in the area.

Projects now active in the areas where the porkbush occurs seek to utilize it as a rehabilitation aid to restore over-utilized natural habitats to their formerly productive state. At the same time these sites act as carbon sinks (kind of carbon bank) where carbon can be collected and used where it belongs and is productive to both humans and the environment. Potential earnings through carbon credits could be translated into social upliftment in the areas where this plant is being utilized.