Acacia karroo: A Sweetly Thorny Matter

Acacia karroo will spring up on any disturbed soil, protecting it with fierce thorns for decades.

Acacia karroo will spring up on any disturbed soil, protecting it with fierce thorns for decades.

By Andrew McNaughton and Julienne du Toit

Pictures by Chris Marais

In the eastern Karoo, the tree most likely to be piercing your flesh and plucking at your clothes as you walk along dry watercourses is Acacia karroo.

You’ll know it by its distinctive white thorns, so sharp that have been used for sewing needles and for pinning insect specimens. You’ll also know it by its lacy leaves, dark bark, and sweet-smelling yellow flower-balls in early summer.

In the Karoo, many will tell you that this thorn tree is invading the veld.

They say it like it was a bad thing.

Enriching the Earth

Acacia karroo thorn

Sharp thorns and fine mimosa-like leaves.

In fact, the Karoo Thorn (one of its many common names) is busy doing a splendid job at casting thorny protection over damaged land and immeasurably enriching the soil over the next few decades.

Like most pioneer plants, Acacia karroo is short-lived, usually dying after 30 or 40 years.

Unlike most pioneer plants, Acacia karroo has nitrogen-fixing fungi attached to its roots, which mean that the soil around every tree increases in fertility.

It’s well documented (among others by the Dohne Research Centre) that plants in and around Acacia karroos become more palatable and more productive.

Prince Albert Karoo botanist Dr Sue Milton (from Renu-Karoo) notes that their expansion in the Karoo is due in part to increased rainfall, and higher levels of atmospheric carbon dioxide.

“Rising temperatures and possibly fewer frost days may also contribute to their expansion. If/when the Karoo goes through the next below-average rain decade, the trees will no doubt die back towards the drainage lines again.”

Sweet Thorn

Kudu, Acacia karroo

Kudus have expanded their range into the Karoo is thanks to this tree.

While it lives, Acacia karroo is an all-in-one supermarket for browsers and grazers.

The leaves are nutritious. Grasses and other plants thrive in its shade, covering the bare earth. In winter, it drops its protein-rich pods.

Vervet monkeys, which are excellent seed dispersers, can live entirely from Acacia karroo alone. They eat its leaves, flowers, bark, and especially the delicious gum that gives the tree its other popular common name – Sweet Thorn.

Vervet monkeys thrive around these trees which provide all their food needs.

Vervet monkeys thrive around these trees which provide all their food needs.

Then, in a few decades, the Acacia karroo dies, and its rotting timber attracts thousands of insects, which in turn feed birds and mammals. Its fallen branches trap seeds and create small protective microclimates for new plants. It stops soil erosion along drainage lines and dry riverbeds.

Living or dead, these trees provide shelter for animals: protection from predators, shade in summer and windbreaks in winter.

Acacia karroo leaves the earth a richer place.

No wonder Kew Gardens in London has made it one of the ‘star plants’ in their South African Landscape section.

POSTSCRIPT: In 2005, the Acacia karroo was renamed Vachellia karroo in a quiet but controversial taxonomical coup. The Australians have claimed the name Acacia for their wattles. This seems all the more vexing because ‘Acacia’ is derived from a Greek word meaning ‘thorny tree’, and the Australian wattles don’t have thorns.

  • Andrew McNaughton is a popular local historian who writes the newsletter Karoo Connections from Seeff’s Graaff-Reinet office.

 

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3 Responses to Acacia karroo: A Sweetly Thorny Matter

  1. Kasha Potgieter December 1, 2013 at 7:45 pm #

    Hoe bly is ek toe ek bloot per toeval op die webwerf afkom. Was op soek na meer inligting en fotos van die Acacia Karroo i.s. tannin vergiftiging by kudus. Skryf ‘n roman wat aanvanklik in die Noorsveld afspeel maar daar is nie veel inligting oor die plantegroei en name nie.
    Het dekades terug vir 2 jaar in die Noorsveld gebly maar kan net ‘n paar spesies bome onthou – en dit is net pruim, klapper en peperbome! En spekbome.

    Miskien weet u waar ek meer inligting kan kry oor die Noorsveld se plantegroei met beskrywings by, asook enige ander inligting, soos klipformasies.

    By voorbaat dank
    Kasha Potgieter.

    • Julienne du Toit December 2, 2013 at 5:26 am #

      Hello Kasha

      Ek gaan sommer in Engels antwoord, want my Afrikaans is nie van die boonste rakke nie.

      You could perhaps order a book called Karoo Veld by Sue Milton and Richard Dean, published by Briza. Either visit http://www.briza.co.za or http://www.renu-karoo.co.za to order.

      In terms of geology, I can recommend two books by Nick Norman: Geological Journeys, and the one that’s just come out, Geology off the Beaten Track. Both very good and very accessible to ordinary people like us.

      Hope that helps?

  2. Alan Short August 7, 2015 at 3:32 pm #

    Foir points: firstly, Acacia karroo does increase grass production – but only up to a certain point. Once the density of trees increases beyond that point, the grass production declines. Secondly, the acacia can provide fodder for animals, but nowhere near the quantity provided for grazers by grass. Also, dense, impenetrable thickets of Acacia don’t provide shelter for larger animals. Finally, you’re assuming that the Acacias will die back without producing seedlings to replace themselves.

    So, many of the benefits of Acacia that you mentioned are true, but only up to a certain threshold. Beyond that point, the negative effects of dense Acacia invasion can outweigh positive benefits.

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