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An African Mascot: The Umbrella Thorn Acacia

Umbrella Thorn (Acacia tortilis) tree in Tanzania

A particularly magnificent specimen of Umbrella Thorn photographed in Tanzania.

This is the Umbrella Thorn (Acacia tortilis); a flagship species of acacia tree in the African savannah. It has an easily recognisable shape when it’s a fully grown adult, in fact this is where the tree acquires its name; from its umbrella-like canopy. In Afrikaans, the tree is called the ‘Haak-en-Steek’ which means roughly ‘hook and stab’ which refers to the plant’s thorns; it has one pair of sharp points and one pair of hooks at intervals along its stems. Umbrella thorns can be identified quite easily by their conspicuous white thorns however, so can many other acacias so identifying the hooks can confirm the identification.

The umbrella thorn can be found predominantly in savannah grasslands, where it often stands proudly on its own in the veld. The ubiquitous tree can be found all over Africa (as well as in parts of the Middle East) and can be found in more arid regions. In fact, the most isolated tree ever recorded stood in the Sahara desert and was most probably an umbrella thorn.

The sweet leaves of the umbrella thorn are a preferred food of many herbivores, including the towering giraffe, however, even the giraffe cannot stretch its neck far enough to penetrate the wide disc canopy and so feeds upon the leaves at the edges. Poor tree you may think; its leaves ruthlessly torn away. However, the tree has sacrificed its outer leaves but has safely protected its inner leaves. When the tree is young, it appears more bush-like and rounded, and can be confused with a myriad of other similar plants. The same is true when the tree grows in more arid areas and lacks the nutrients to grow to its full, iconic shape.

The animals that feed a lot on the umbrella thorn often have a distinctive face. Many browsing animals will have long faces; with their eyes quite far away from their mouths. Animals and plants have been evolving alongside each other in Africa for millions of years and as good as the plants are at defending themselves, the animals are good at responding to the plants’ defences. Let’s take the giraffe (Giraffa camelopardalis), a browser that enjoys the leaves of the umbrella thorn. The giraffe’s distinctive long face keeps its eyes well away from its mouth; this allows the animal to feed on a thorny plant without risking thorns or spines injuring its eyes.

A male giraffe (Giraffa camelopardalis) feeding on a young umbrella thorn (acacia tortilis).

A male giraffe feeding on a young umbrella thorn.

I’m sure you agree that the umbrella thorn is an interesting tree, but it is only the start. The bush is full of fascinating plants with interesting features and uses. I’ll bring you more posts on even more exciting plants in future.

Much love,

-Nick

 

If you liked this entry, make sure you check out our Botany category.
Previous Botany entries:
Quiet Battles: How do trees defend themselves from animal ‘attack’?
Botanical Intrigue
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Quiet Battles: How do trees defend themselves from animal ‘attack’?

There is a very quiet battle going on in the bush. An army of herbivores are eating their way through the body parts of plants, however, the plants are not giving up without a fight! Herbivorous mammals make up a huge proportion of the animal life in Africa and their diet consists of (although not 100% exclusively) on vegetable matter, or plants. Today I’ll outline a few of their tactics, there are some strategies that are obvious and others that are subtle and cunning.

Plants, like animals, have complex anatomies and are composed of different body parts with different functions. A plant’s leaves are used during photosynthesis; a chemical process whereby plants turn raw materials gained from the soil and the air into sugars, using sunlight for energy. As a result, plants’ leaves are very nutritious; they contain sugars and raw vitamins and minerals. Herbivorous animals ‘prey’ on the leaves of plants, feeding on them in large quantities; they are important to the animals, but they are of course important to the plants as well.
How can a plant physically keep an animal away from its leaves? Many plants created obstacles that make it physically difficult for feeding animals to reach the leaves. This might involve growing into shapes that make it hard to reach leaves, of growing defensive weaponry to repel threats.

 

Shapes

Several groups of trees in Africa grow into a typical umbrella shape. One reason for this shape is so that a tree might maximise the surface area of its leaves that it exposes to sunlight. But this shape also has a defensive purpose; by spreading its canopy out into a flattened disc, a tree is making it more difficult for animals to reach those leaves in the centre. A tall animal might be able to feed on the leaves on the edge of this disc, but it cannot reach into the centre and thus the leaves are spared.

Umbrella shape tree in South Africa

These trees are fanning out their canopies; partly for defence, partly for food production.

 

Thorns/Spikes

Many walks through the bush result in cuts and scratches, but not from lions and leopards, but rather from the seemingly innocuous bushes and trees. These plants equip themselves with spikes and thorns; weapons designed to cause pain to those animals that try to eat their leaves. Botanically speaking, when we talk about ‘thorns’ we are referring to those sharp structures that are formed by a plant modifying the growth of its stems into sharpened points and when we talk about spines, we refer to when plants modify leaves themselves into sharpened objects. Either way, when an animal dips its face into a tree or bush it has to be willing to put up with a considerable amount of pain in order to get its food. A plant lives in the hope that it will causes enough pain that an animal will avoid the plant all together.

Close-up spikes on an Acacia tree

Some impressive weaponry on an acacia.

 

Chemicals

But what is to stop animals from slowly eating their way through a plant’s leaves until nothing’s left? Plants have defences in their very tissues; chemicals that can prevent or reduce the amount that herbivores feed upon them. There some chemicals, called tannins, that protect plants in two ways; one way involves simply making leaves bitter and unpalatable, this means that animals will find the plant unpleasant tasting and they will avoid feeding upon it. Another way, perhaps even more cunning, is that the tannins decrease the efficiency of herbivores’ digestive systems, making it hard for animals to gain the required nutrients and so it is in their best interest to avoid eating plants with the bitter tastes because it will not do them good. There is also recent evidence that plants can communicate with each other chemically, to stimulate tannin production; we’ll talk more about that in a later post.

Adult Giraffe (Giraffa camelopardalis) feeding on leaves

A giraffe quietly feeds on leaves, while the plants flood their systems with toxic chemicals.

Plants may sit quietly, but they can look after themselves; they have to of course, it’s not like they can run away. In future posts, I hope to explore plant defences in more detail; there is plenty more to talk about. Hopefully I’ve got you interested in botany, even just a little; stay tuned for more!

Much love,

-Nick

 

If you liked this entry, make sure you check out our Botany category.
Previous Botany entries:
Botanical Intrigue
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Botanical Intrigue

Of course everyone loves African wildlife, and with good reason. The African continent hosts an exceptional biodiversity and some of the most impressive animal specimens in the world. The mighty African elephants, elegant impala, mischievous baboons; all them captivate our imagination and impress our senses, however, there are other organisms, all around us, that live interesting lives.

I, of course, talk about plants. They are fascinating organisms that interact with each other and with animals and lead dynamic lives. Unfortunately, they live their lives on a timescale that we find hard to perceive and so often we pass them off as inanimate objects.

In time, we will use this page to keep you informed about the interesting properties of common African plants as well as their practical, medicinal and cultural uses for humans. From tiny grass shoots, to towering and sprawling fig trees; the floral world is diverse and full of surprises.

We’ll keep you posted!

-Nick

 

If you liked this entry, make sure you check out our Botany category.
Leave a comment »