Few trees are as iconic as the flat-topped Acacia on the plains of the African Savannah. On par with the Big 5, these trees instantly evoke nostalgia in those who know them, and will forever be associated with the African Safari.
What’s in a name?
The botanical names of African Acacia trees have been changed in the last decade, from ‘Acacia’ (such as Acacia karroo) to Vachellia (Vachellia karroo), and Senegalia. Despite wide-spread unhappiness in the world of Botany, the common name for these unique trees will always remain ‘Acacia’. As it should. Australian Acacias are still called Acacia spp. Collectively, these species (Acacia, Vachellia, and Senegalia) will still be known as Acacias as a common name. There are around 1000 species of Acacia worldwide, primarily in Australia and Africa.
The iconic, flat-topped African Acacia (Nyanga Flat - Top) is called Vachellia abyssinica.
The dome-shaped Vachellia erioloba, known as the Giraffe Thorn or Camel Thorn tree, is the most recognizable tree in the Kalahari desert, and occurs widely in the drier areas in Southern Africa. It can grow up to grow up to 18 m tall and live up to 200 years. The tap root can grow up to 60 m, allowing it to access deep ground water sources and live in extremely dry climates.
A Deeper Meaning:
The Acacia is of spiritual significance in that it symbolizes regeneration, perseverance, and integrity. The evergreen nature of this tree denotes the immortality of the human spirit.
The ancient Egyptians made funeral wreathes of Acacia. They believe that Osiris was the first god to be born under the Acacia tree, and all others followed. The Hebrews planted a sprig of evergreen Acacia to mark the grave of a departed friend. Acacias are also mentioned in the Bible, with specific reference to the building of the tabernacle.
A Useful Tree:
Senegalia senegal (Gum Arabic Tree) found in Sudan and the northern Sahara, is the main source of gum arabic, which has been used for over 2,000 years in paints, watercolours, candy, medicines, calico printing, dyeing, and in the making of silk, paper, and cosmetics.
The various species of Acacia were used by early shipbuilders for its durable wood, and today many countries cultivate the trees. Along with the wood being used for furniture, flooring and even weapons and jewellery, the gum is used as an adhesive, for medicine, and even for chewing gum and desserts. Furthermore, the blossom can be used as flavouring, the seeds utilised in sauces and the tannin as a dye. And in Central America, the swollen thorns are made into beautiful necklaces.
In landscaping and gardening, Acacias are often used to control soil erosion in dry and damaged soils. They can be planted as protective hedges, creating shady areas for animals and to accommodate a variety of bird species for nest building areas. These hardy water-wise plants are perfect garden additions to areas where water may be scarce.
Sweet Thorn (Vachellia karroo): Edible gum seeping from cracks in the tree’s bark is important food for the Bushbaby’s winter diet. The tree is especially useful as food for domestic and wild animals, like goats. The flowers make a good source for honey bees, and honey from the Sweet Thorn has a pleasant taste. The Sweet Thorn tree makes excellent firewood, and the wood is also used as fencing poles in making a kraal. Traditionally the inside of the bark was used to make a tough rope with.
Masters of self-defence, employing bodyguards!
Almost all Acacia species have long, sharp thorns, which prevent (most) animals from eating their leaves. Some species grow thorns that are as long as 8-10cm, and sharp as a knife.
Stinging ants (bodyguards!) live inside hollowed-out thorns, which provide further discouragement. The trees have developed a symbiotic relationship with stinging ants who live in the thorns (which they hollow out and use as nesting sites) whilst feeding on the nectar of the tree’s flowers. If a big African mammal takes a bite of its leaves, the stinging ants see to it that the animal will certainly think twice before munching more!
Along with the production of thorns and the usual accompaniment of ants that nest in these trees, the African Acacia has developed an incredible early warning ‘alarm system’ to warn other trees when browsers such as the antelope are in the area. Wouter Van Hoven (a Zoologist from the University of Pretoria) has found that when the leaves of Acacias are nibbled, they produce high doses of tannin in their foliage, which may be lethal to browsers.
When the leaves begin to fill with poisonous tannins, they release ethylene gas, which drifts toward other acacias. In response, the nearby trees begin to manufacture poison themselves. Giraffes can eat as much as 29 kilograms of acacia leaves and twigs daily. Herds of three or more giraffes spend hours browsing in acacia thickets, so they pose a real threat to the survival of the trees if left to munch away. But all it takes is a few minutes for the neighbouring trees to step up their leaf tannin production to repel lurking browsers. The simultaneous tannin release by all nearby acacias essentially forces the giraffes to travel upwind to trees that have not yet received the panic alert.
There’s a reason they are dome-shaped or flat-topped:
Flat-topped crowns help trees to resist drying winds by allowing leaves to shelter each other, while the umbrella-dome shape of most African acacias enables the trees to capture the maximum amount of sunlight, with even the smallest of leaves. Because the African savannahs are regularly burnt, any tree that wants to survive fire has to cleverly adapt. Thick bark is a useful defense. Another sensible trait is to grow tall, quickly: trees that are above the 'fire trap' (2-3m tall) aren't burnt back to the roots by a fire and can re-sprout from the top. With limited resources and living in a place where fires are frequent, trees produce a single (or at most a few) stems and grow straight up, without branching out sideways. Once tall enough to escape the impact of fire, they’re free to branch sideways.
After branching out, even though moderately tall trees don't always escape giraffe browsing, a horizontal growth-form still protects the central branches. It's common to see Vachellia tortillis (Umbrella Thorn) that only grow large thorns on the outer tips of their branches, providing enough defence to protect inner branches. So, in a fire-dominated, browse-affected and unlimited light environment, the flat topped tree structure is perfectly designed!
It's so rare that a seedling survives both fire and being eaten, that mature trees tend to be at low density in savannahs. This explains why they are so memorable and often isolated on the sweeping landscapes and golden plains of grass. The next time you see one of these iconic trees, think back on what you’ve learnt here, how amazing Nature is, as you snap the perfect sunset photograph, with some giraffes strolling past a flat-topped Acacia!
Written by Carina Crayton BSc(Agric)HONS
I stretch my arms wide
To catch the last of the light
Tomorrow the rains may come
another dry season will be done
If not, I’ll still be here
I’ll still allow majestic giraffes near
For a minute or two and then
My sweet poison will defend
For life, I will stand strong
on the golden plains where I belong
I’ll filter the dust
I must reach deep for water
Mother Earth and I – we’re one
Together we’ll fight the burning sun
In my shelter they’ll thrive
all that I keep alive
You will always remember me
I’m that African Acacia Tree