Arbor Week 2021: a time to plant trees that feed black rhinos, butterflies, and bees.

An account of the generosity of the Sweet Thorn Tree

Story and Pictures: Andrea Abbott

Choose your tree

To celebrate Arbor Week 2021 between 30^th August and 5^th September, you might want to plant your favourite species. And certainly, any indigenous tree that occurs in your region would be ideal.

If you’re spoilt for choice though (which is likely, seeing as South Africa is endowed with about 750 indigenous tree species, plus a lot of sub-species), you might want to consider the Common Tree for 2021: the Sweet Thorn Tree.  One of its claims to fame is that it’s the most widespread and commonly occurring tree in our country, thriving in a variety of habitats.

It also mounts a spectacular display of golden-yellow magnificence when in full flower, which can be up to four times a year.  In Afrikaans, it’s called Soetdoring; in Zulu or Xhosa, umuNga; in North Sotho, mookana; in Tswana, mooka. In scientific circles it’s referred to as Vachellia karroo.  

Taxonomy that’s taxing

“Vachellia!” I hear some cry. “Surely you mean Acacia?” Well, yes and no. The Sweet Thorn was an Acacia until some years ago when, for reasons best known to taxonomists and based on their work with DNA (oops – a controversial term in this era of mRNA vaccines…), our beloved Acacias were split into two genuses: Vachellia and Senegalia. The differences between those two are somewhat technical, and thus a bit taxing for non-taxonomists like me, so I’ll mention only where those names originate.

Authoritative sites like Tree SA , indicate that Vachellia is named after the Rev. John Harvey Vachell who, in the 19^th century collected plants in China.  Yes, like you, I can’t see the link between the reverend’s flora forays in China and his name being applied to trees from southern Africa.  (On second thoughts, perhaps I can. Consider China’s huge interest in our region of the world. Of course, that interest isn’t limited to our flora, but that’s a topic for another time.)  

Senegalia refers, as you might have assumed, to Senegal.  Again, I can’t ascertain why this was chosen for trees in southern Africa.  Maybe some of the Senegalia species – e.g. nigrescens, or burkei, or caffra occur in Senegal? That’s just my guess, though. It’s all rather puzzling.

A prickly question

Getting back to Acacia – what was wrong with that?  Nothing that I can see. A dictionary check will tell you that the term acacia derives from the Greek akakia meaning a thorn or prickle, which evolved from akis (also Greek) meaning sharp point. Which is on point when you think about the merciless thorns, or spines as they’re properly called, that arm all of our… er, vachellia and senegalia species. 

Acacia as a name hasn’t been banished in arboreal circles. It still applies to some trees. But here’s where the puzzle becomes bizarre.  Acacia is now confined to Australian wattle species that have no thorns. Get it? No? Nor do I.  It’s a point a tree-loving friend of mine must ponder daily. She named her daughter Acacia.  I wonder if she’d have named her Vachellia? It just doesn’t have the same ring as Acacia. And, combined with my friend’s surname, which begins with Van, it sounds almost comical.

Karroo or karoo?

Something that might be needling you is the spelling of karroo. It refers, of course, to the Karoo, a region flush with Soetdoring trees, but it was (mis)spelled that way when the tree was first described.  Whether it was a typo or not, the spelling still stands. I’ll go out on a limb here and suggest that instead of axing the genus name that truly referenced the thorny nature of our vachellias and senegalias, the taxonomists might have used the opportunity to simply correct karroo to karoo. Apparently though, they’re not allowed to do that because of laws governing the assigning of botanical names. Were there no laws to protect Acacia?

What’s in a name?

Anyway, sticks and stones and all that, and what’s in a name?  To borrow and adapt Juliet’s famous words to Romeo, a Sweet Thorn flower will smell as sweet and be just as beautiful, and nutritious, by any other name.

The monkeys that savour those alluring pompom-like flowers know V. karroo as a pantry. So too, the multitudes of insects – bees especially – that also feed on the flowers while paying for their meal with their pollinator credit cards.

Birds that dine on the visiting insects, or nest among the thorns that provide a security fence against predators, don’t give two hoots for the tree’s name either. Likewise the mammals that browse the leaves and survive on the nutritious pods when all other food sources are in limited supply.

Honey, chewing gum, and coffee

The way I like to describe Vachellia karroo is as a tree of abundance and one that keeps on giving. For example, it’s a host to something like a dozen butterfly species; Sweet Thorn honey is delicious; and the gum that seeps out of wounds in the trunk or branches is a favourite of monkeys, bushbabies, and even people. I’ve never sampled it myself, but it must be tastier and less likely to cause tooth decay than manufactured chewing gum. It was once even exported as Cape Gum for making sweets.  

I’m reliably told too, that the seeds can be roasted as a coffee substitute, which is worth bearing in mind next time your local supermarket is looted and you’re out of coffee. That alone should make coffee addicts celebrate arbor week 2021 by planting a sweet thorn tree or three.

Muthi, shade, and fertiliser

This famous, fast-growing tree ticks several other boxes: it’s considered an indicator of good grazing and of water; has medicinal uses; and provides shade, firewood, and even building material.

As a member of the Fabaceae (bean/pea) family, it fixes nitrogen in the soil thus creating a natural fertiliser superior to synthetic versions that, when washed into waterways, create nutrient increases. This process  – eutrophication – can unfortunately have a disastrous impact on biodiversity, such as triggering toxic algal blooms, or rampant growth of invasive alien aquatic plants.

It must be said though, that the Sweet Thorn isn’t always ‘sweet’. For one thing, the pollen can be bothersome, causing hay fever for some people. Furthermore, V. karroo isn’t at all fussy about where it puts down its not inconsiderable roots, (except for on the Cape Peninsula, which it avoids like the plague). And so, being tolerant of most conditions, it’s a bush encroacher that invades grasslands.  With grasslands becoming as rare as red diamonds, we cannot have V. karroo  (or any other tree species) staking a major claim in them. So, nip that in the bud by using invading individual Sweet Thorns for that other ubiquitous South African phenomenon that’s also being celebrated in September – the braai.

Setting aside its land-claiming propensity, Vachellia karroo is a gem of a tree and an ecological asset of note. Oh, and black rhinos love them – reason enough to plant one or several (trees are social beings) in your garden. Be cautious, though, and keep an eye out for your visiting rhino when you’re sitting in the shade of your Sweet Thorn, or harvesting the seed pods to make coffee. You really don’t want to have to climb that thorny tree.

Footnote:

The other two noteworthy trees of 2021 are:

Uncommon Tree: Portulacaria afra – Spekboom, which is indigenous to the Eastern Cape but occurs too in parts of KZN and Mpumalanga. Much vaunted as a carbon sequester (it’s debateable if it does a better job than other trees), it’s resilient and highly nutritious (the leaves are delicious and high in Vitamin C) and considered a delicacy by elephants. So, plant them for the elephants in your neighbourhood.

Tree for appreciation: Warburgia salutaris  – Pepper-bark Tree. Red-listed as endangered, W salutaris is nearly extinct in the wild because it’s highly sought-after as a medicinal plant. It occurs in forest in bushveld in Zululand, eSwatini and through to tropical Africa. The leaves and bark taste peppery, but that doesn’t deter black rhino and antelope from browsing on it.

Thank you to Peter Ucko for editing this story.