THE BIG PICTURE

Euphorbias in the spotlight

Southern Africa’s euphorbias are hot again botanically for the first time in nearly a century. In his new book on the southern African species, PETER BRUYNS explains why they are interesting and where you can look out for them

PHOTOGRAPHS BY PETER BRUYNS

Euphorbias in the spotlight

Southern Africa’s euphorbias are hot again botanically for the first time in nearly a century. In his new book on the southern African species, PETER BRUYNS explains why they are interesting and where you can look out for them 

PHOTOGRAPHS BY PETER BRUYNS
The bushveld candelabra tree (Euphorbia cooperi) can reach up to seven metres high. Here it grows in shallow soil on this large granite dome at Gutschwa Kop, north of Nelspruit.

 

Through history, the spotlight has often shone on the euphorbia, or spurge, family of plants, which has been known to humans for millennia. Euphorbia was reputedly named after Euphorbos, said to be a physician to Juba, the ruler of the ancient kingdom of Mauretania in west Africa more than 2 000 years ago.

The euphorbia family, Euphorbiaceae, is also found far and wide across the temperate and tropical world and consists of more than 8 000 plant species.

ENDEMIC EXCITEMENT

What is exciting about euphorbias in southern Africa is the high proportion of species that are endemic. About 172 species are found in our subcontinent – defined in this study as Botswana, Lesotho, Namibia, South Africa and Swaziland. Almost three-quarters of these, or 74%, are endemic and so grow naturally only in this region.

What is more, almost all are succulent. This is not at all the case, for example, in the Americas, in Eurasia and the northern hemisphere generally.

In one of the most striking evolutions in southern African euphorbias, the plant has lost its branches and become just a single, often quite small and short stem such as this melon spurge (Euphorbia meloformis).

TAKING A LOOK

Settlers at the Cape such as soldier and traveller Colonel Robert Gordon (1743-1795) were early cultivators of euphorbia in their gardens. In the late 18th century, plant collectors at the Cape such as the Swede Carl Thunberg (1743-1828) and the Scot Francis Masson (1741-1805) sent herbarium and live specimens of South African euphorbia back to Europe. More were added the following century by James Bowie (c.1789-1869), Christian Ecklon (1795-1868), Carl Zeyher (1799-1858) and the Drège brothers, Carl (1791-1867) and Johann (1794-1881).

CLASSIFYING FINDS

At Kew, the plants attracted the attention of Nicholas Brown (1849-1934), assistant keeper of the herbarium, who carried out the last revision of the southern African euphorbia species for volume 5, part 2, of the Flora of Tropical Africa, published in 1911/1912, and for volume 5, part 2, of the Flora Capensis, published in 1915. South African botanist R. Allen Dyer (1900-1987) worked on the succulent euphorbias of the Eastern Cape and co-authored the major two-volume work on southern Africa’s succulent euphorbias in 1941 with the Americans Alain C. White & Boyd L. Sloane.

Regional accounts for Namibia and Botswana have appeared in the eight decades since then. Much more extensive exploration has also brought new species to light, mostly in formerly inaccessible areas such as northern Namibia’s Kaokoveld. This new book brings together the results of the exploration and research of the last 80 years, ensuring that the distributions of species and their variations have been clarified.

Another evolution in southern African euphorbias is this modified growth form, where short shoots are packed into a dense mound of plants, such as with this Euphorbia clavarioides at Ubala, north of Kokstad. 

RAIN SMART

Among its many species, euphorbia has adapted to a wide range of rainfall. In southern Africa, for instance, E. giessii, E. lignosa, E. phylloclada and E. virosa grow in the hyper-arid coastal Namib desert with less than 50 millimetres of rain a year.

In the Drakensberg’s moderately moist (mesic) grasslands with at least 1 000 millimetres annually, E. clavarioides, E. gueinzii, E. natalensis and E. striata occur.

Wetter still are the granite domes of northern Mozambique where annual rainfall reaches 2 000 millimetres. E. marrupana, E. mlanjeana, E. namuliensis, E. unicornis are among the locally plentiful species of euphorbia there.

In southern Africa, many of the species that are found most widely can tolerate a wide range of rainfall. E. avasmontana, for instance, grows equally well receiving between 50 to 600 millimetres a year and E. clavarioides from 250 to more than 1 000 millimetres a year.

North of Molepolole, Botswana, bekruipbos (Euphorbia spartaria) grows among scattered trees in pale pinkish Kalahari sand.

ROAD CHALLENGE

When you are travelling over the holiday, you are likely to spot euphorbia in most habitats in southern Africa, except in dense montane forests. Species inhabit sands, flat areas from stony to heavily loamy, stony slopes and shallow soils on solid granite ‘whalebacks’.

Some smaller species benefit from the presence of protective shrubs but do not wholly depend on protection from other plants for their survival. They may even start off life as a small seedling in the open, protected by some stones or larger rocks.

Unlike plants belonging to the stapelia genus, which can be difficult to find in nature and are often very scattered, plants of euphorbia are mostly easily found and they are usually plentiful where they occur. Some of the small species with spreading, usually short, branches from a thick stem (medusoid) species occur sporadically so may be more difficult to find but most species are big enough for you to spot from a passing car.

This noorsdoring (Euphorbia ferox) growing near Kommandodrift, south-east of Cradock, has its branch tips covered in cyathia, an inflorescence that looks like but is not actually a true flower.

SANDVELD

Sandy habitats are plentiful in southern Africa. The low-lying, flat, coastal sandy areas of the western Cape sandveld are fairly rich in euphorbia species.

Many euphorbia are widespread across these sands and may sometimes be dominant. Only E. stapeloides is endemic here, unlike the many species of succulent ice plants (Aizoaceae).

What are widely called Kalahari sands cover a large portion of central southern Africa, stretching from eastern Namibia and parts of the Northern Cape through Botswana. This ‘Kalahari desert’ is not intensely dry and receives about 200mm of rainfall a year in the southwest and as much as 500mm in the north.

But there are not many euphorbia species in the region – watch carefully for the very scattered occurrences of E. duseimata, E. maleolens and E. spartaria. Look on calcrete outcrops around shallow pans along sand-choked drainage systems for E. crassipes and E. juttae.

Euphorbia species become less and less diverse among the sands as you move east across southern Africa. On the region’s eastern flank, only the geophytic E. gueinzii and the herbaceous wild spurge or inkalamasane, E. natalensis, inhabit the grassy, settled dunes of the KwaZulu-Natal and southern Mozambican coast.

Slender candelabra euphorbia or boesmangifboom (Euphorbia avasmontana) dominates the vegetation at the base of the slope at Narudas Nord in the Great Karas Mountains, Namibia.

Loamy or stony ground

In the semi-arid, Karoo and Karoo-like parts of South Africa and Namibia, euphorbia species are common in hard, stony ground in flat areas. Fewer of them occur on hillsides and they are rare on steep slopes.

In stony stands with driedoring (Rhigozum trichotomum), dwarf succulent scrub dominated by Ruschia spinosa or various dwarf daisies (Asteraceae family), you could see many of southern Africa’s smaller euphorbias.

Large populations of some species, such as E. esculenta, develop in these areas. Larger shrubby euphorbia species – such as E. radyeri around Jansenville or E. avasmontana in the Great Karas Mountains – can form large stands and then become the dominant plant.

Many tree-like euphorbias, such as E. grandidens and E. triangularis, grow in dense colonies on rocky slopes. This happens more rarely in flat areas, where the euphorbia involved is likely to be E. excelsa.

Euphorbias are poorly represented in the mountains of the south-western and southern Cape, where the soils are derived from sandstone, poor in nutrients and acidic. The herbaceous E. genistoides is endemic to these mountains. Within this area and towards the east, E. heptagona and E. polygona are found in locally semi-arid patches.

The upper part of this grootnoorsdoring or tree euphorbia (Euphorbia tetragona) is bright yellow thanks to a profusion of mature cyathia, or false flowers. It is growing at Witmos, south of Cradock, in the Eastern Cape.

WHALEBACKS & ROCKY OUTCROPS

Rounded granite domes, which form characteristic steep to gently sloping ‘whalebacks’, are an important habitat for succulents in the wetter parts of south tropical Africa and can also be seen in parts of South Africa, as shown in the opening photograph of the bushveld candelabra tree near Nelspruit. 

This occurs particularly in Angola, Mozambique and Zimbabwe, as well as further north in Congo and Kenya as well as in eastern Madagascar, but only occasionally in South Africa.

These outcrops are subjected to extreme climatic conditions, such as wide fluctuations in temperatures and light intensity. On this solid rock, isolated patches of vegetation develop in accumulations of weathered grit and leaf litter of varying thickness. Roots combine in a dense mat to give the ground an impenetrable, peat-like consistency.

These patches of skeletal soil are often dominated by various desiccation-tolerant resurrection plants, such as the tussock-forming sedge (Coleochloa), opstandingsplant or uvukakwabafile (Myrothamnus) and various species of black stick (Xerophyta).

Where the patches are sufficiently isolated and not too regularly burnt, a wealth of succulents may develop, especially aloe, bobbejaantou (Cynanchum), euphorbia, kalanchoe, plectranthus and several members of the periplocoideae, a subfamily of the dogbane (Apocynaceae).

Many species belonging to the section within the euphorbia genus that bear pairs of spines at the base of leaves, along the stem and branches, are found only in these habitats, especially in Angola and Mozambique.

In those species which become large, such as the naboom (E. ingens) once the branches are out of reach of browsers, the spines disappear.

On the left, this noorsdoring (Euphorbia ferox) from Steytlerville in the Eastern Cape is densely armed with spines, one appearing in the axil between each leaf and shoot. On the right, the spines are more densely clustered on this bobbejaannoors (Euphorbia polygona) from Toorwaterpoort in the Eastern Cape.

 

In South Africa, such domes are present on the eastern and western sides. Those on the eastern side may be inhabited by species such as E. clavigera, E. cooperi, E. schinzii and E. vandermerwei but only the last is restricted to them.

In western South Africa in the winter rainfall parts of the country, the much drier domes of gneissic rock do not have any distinctive euphorbias on them, although they may also be rich in succulents.

This edited extract is adapted from the introductory chapters of the two-volume work Euphorbia in Southern Africa by Peter V. Bruyns (Springer Nature Switzerland).
Peter Bruyns (peter.bruyns@uct.ac.za) retired in 2022 as a professor in the Department of Mathematics at the University of Cape Town. He has published many peer-reviewed papers on various groups of succulents, especially the stapeliads and ceropegia of the Apocynaceae, euphorbia, Crassulaceae, Didieriaceae and, as co-author with Cornelia Klak, on the vygies (Aizoaceae). His two-volume, sole-authored work
Stapeliads of Southern Africa and Madagascar was published in 2005.
Gommelkbos (Euphorbia gummifera) grows in arid conditions such as near Halenberg, east of Lüderitz, in the Namib desert.

EUPHORBIAS:

Need to know

  • Africa has two main centres of diversity for euphorbia – the tropical north east (Ethiopia, Kenya and Somalia); and the tropical and temperate south (southern Africa)
  • DNA dating suggests that euphorbia began to evolve 47 to 29 million years ago
  • Many euphorbia species appear to be protected from predation only by their sap, which is so unpleasant as to deter attack
  • In arid areas, it is possible for the larger euphorbia species to establish populations where practically no other plants grow. Seedlings just need a few better-than-average years of rainfall to establish themselves and to reach a size at which they are able to withstand more prolonged dry periods
  • The area with the highest concentration of species of euphorbia lies in the Eastern Cape, south west and south east of Grahamstown, where more than 22 species have been recorded
  • Smaller localised centres of diversity are: Kaokoveld and Great Karas mountains, Namibia; Richtersveld-Rosh Pinah district, Namibia/South Africa; Klein Karoo, Knersvlakte; dry parts of Eastern Cape and southern KwaZulu-Natal; Pofadder district, Griqualand West between Prieska and Kimberley; Soutpansberg and Waterberg; upper mountainous reaches of Olifants river of Sekhukuniland, South Africa

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