This blog about acacia trees was written by Bushwise Professional Field Guide student Maddy Prior. As part of their training, each student submits a researched blog based on a topic of their choice. Opinions contained in these blogs are the student’s.
5 min read
For many, acacia trees have been synonymous with Africa, long before they were named by Philip Miller, an English botanist, in 1754. Being British, I’m sure I’m not alone when I say that the word Africa conjures up images of the sprawling plains of Kenya and Tanzania dotted with these short, umbrella-shaped trees, offering those small patches of grass shelter from the Sun and wind. The Lion King may be to blame for this romantic imagery, or Out Of Africa, or indeed the majority of nature documentaries following the Big Five.
The Iconic Acacia Trees of Africa
While I write this in South Africa, I feel for the often overlooked landscapes of the Lowveld, which hold about 29 of the acacia species – now divided into two genera, Vachellias and Senegalais. The word acacia is derived from the Greek word akis meaning thorns – which makes little sense now that the only true acacias are now found in Australia where they lack thorns. The thorny versions are plentiful in Africa, but other species are also found across Asia and America.
The Iconic Acacia Trees of Africa
Although only classified into two genera, there is a great deal of diversity in Vachellias and Senegalais. A useful trick to differentiate between the genera is to examine the thorns. Vachellia thorns are straight and grow in V-shaped pairs and produce rounded, often bright yellow flowers.
Senegalias produce hooked thorns and flowers in spikes of yellow, cream and white. These soft, inviting flowers attract a plethora of pollinators: the obvious insects one would expect, such as bees and wasps, but some larger and surprising.
Acacia Trees: Their Pollinators and Consumers
It is thought that the giraffe is in the principal pollinator of the knob thorn tree (Senegalia nigrescens) which flowers between August and November. This knobbly tree is a favourite of the long-necked browsers, who collect the flowers’ pollen on their faces as they feed on the fine leaves, and then transport this pollen to the next knob thorn they browse from.
The name of this tree comes from the characteristic knobs that the thorns stem from, and the larger woody bumps that cover the trunks and branches of older trees. Elephants are known to eat the branches as well as foliage, and several species of monkey eat the flowers. Knob thorn trees are used by cavity-nesting birds. And the quality of the wood makes the tree attractive to carpenters to make furniture and parquet flooring. This is a tree with many consumers.
Cultural Significance and Traditional Uses
Vachellias and Senegalias are just as iconic for the people of South Africa as for foreigners, and undoubtedly more important, holding great cultural significance and acting as key components of traditional practices and beliefs.
Vachellia karroo, or sweet thorn, is the most widely-distributed tree in South Africa, and is particularly dominant in the Nama Karoo and Grassland biomes. But it is also found in the Savanna, where it provides nutrition for many species. The seed pods are enjoyed by monkeys and various species of butterfly larvae, grey go-away birds feed on the flowers, and ground-up seeds have traditionally been used as a coffee substitute.
The leaves of the sweet thorn are also a valuable source of fodder for the game species and the livestock of communities in these areas, upon which many of their livelihoods depend. The evergreen nature of Vachellia and Sengalia made them a reliable source of food and shade for shepherds existing in such harsh, arid environments.
The uses of these trees are far more diverse than just sources of food. The wood of Senegalia Senegal was used by ancient Egyptians to craft boats, furniture, and even sarcophagi, with gum of the tree (taken from the sap) used to make bandages for wrapping the mummies to go into them. The ancient Egyptians also blended the gum with dates and honey to create a contraceptive paste, the lactic acid of the dissolved gum acting as a spermicide.
Medicinal Properties of Acacia Trees
Gum acacia is still used today by people in Sahara/North Africa as a food source, 6 ounces of which can sustain an adult for a day, and to treat various ailments, such as stomach ache. A tea made from the leaves is said to treat coughs, and the leaves themselves can be applied to wounds and inflamed areas on the body as a natural remedy.
The roots of Vachellia nilotica (or gum Arabic) have even been used in medical treatments to fight cancer and tuberculosis, and its leaves to treat Alzheimer’s disease. The leaves of Vachellia erioloba (Camel thorn) can also be applied to wounds to relieve pain, once burnt and ground into a powder.
Defence Mechanisms: Ants and Chemical Signalling
Because acacias are so widely used by both humans and animals, they have developed various defence mechanisms to protect themselves. Certain species of Vachellia (e.g. Vachellia karroo, or sweet thorn) have formed a mutualistic relationship with acacia ants (e.g. Crematogaster mimosae), whereby the ants inhabit the swollen thorns that cover the branches of the trees, providing them with protection. If a browsing animal begins to feed on the leaves of the tree, the ants will swarm and irritate its face, thus causing the animals to move off to a more appealing bush.
As an extra measure to protect their delicate foliage, acacia trees will release chemical signals when being browsed, which are carried by the wind to other trees of the same species in the surrounding area, which will stimulate the production of tannins in these trees and the one being browsed. High concentrations of tannins in the leaves will make them unpalatable to browsers, dissuading them from eating the foliage.
To delve deeper into the captivating world of Acacia trees and expand your knowledge of wildlife and conservation, embark on an educational journey with Bushwise.
