By Andrew McNaughton and Julienne du Toit
Pictures by Chris MaraisIn 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
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. When the Karoo goes through drought, the trees die back towards the drainage lines again.”
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.
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.
- The Sweet Thorn is one of the Arbor Week Trees of the Year for 2021. Click here for more.
- Andrew McNaughton is a popular local historian who writes the newsletter Karoo Connections from Seeff’s Graaff-Reinet office.