Acacia Trees

If you were to ask South Africans what they think is the typical iconic landscape of their beloved country, it would undoubtedly include an Acacia tree, commonly known as a ‘doringboom” or thorn tree. Close your eyes and picture the African landscape … what trees do you see? The thorny Acacia tree you picture in your mind’s eye – recognisable by its classic ‘umbrella’ canopy – is an integral part of the African savannah. African thorn trees are widely but thinly scattered around, making them stand out in the landscape.

There are none more iconic than the acacias painted by Jacob Hendrik Pierneef.
Photo: Strauss & Co.

The characteristic curved crown of most African acacias is an evolutionary adaptation that enables the trees to capture the maximum amount of sunlight with the smallest of leaves.

A favourite subject for photographers, the silhouette of an Acacia tree and a red setting sun will forever be quintessentially Africa…

Products from acacia trees have been used for centuries. The wood is used for furniture and firewood, while the leaves, flowers and seeds are eaten by both wild and domestic animals. Giraffes are especially fond of acacia leaves and their mobile black tongues are adapted to plucking the nutritious leaves from among the thorns. Giraffes can eat as much as 29 kilograms of acacia leaves and twigs daily.

Herds of giraffes spend hours browsing in acacia thickets, so they pose a real threat to the survival of the trees if left to munch away undeterred. However, acacias have several defence mechanisms against browsers. The obvious one is their long, sharp thorns. Add to that the fact that stinging ants hollow out the thorns to nest inside. Any animal that dares to browse the tree is soon put to flight by the vicious ants defending their homes.

The trees have evolved a further defence to over-browsing by producing toxic tannins in their leaves as soon as they are nibbled. This makes the leaves both unpalatable to eat and poisonous. A case has been reported of hundreds of eland dying from tannin poisoning when severe drought conditions forced them to eat acacias. The tree does not produce tannins unless it is threatened – producing these chemicals is metabolically expensive, so they are only made when needed. Acacias also communicate with each other through pheromones. The African Acacia has developed an incredible early warning ‘alarm system’ to warn other trees when browsers such as antelope are in the area.

As the leaves of one tree being browsed begin to fill with poisonous tannins, they release ethylene gas, which drifts toward other acacias. It takes only a few minutes for the neighbouring down-wind trees to register the danger and step up their own tannin production. 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.

The African savannahs are burnt regularly, and any tree that wants to survive fire has to adapt cleverly. So acacias have developed thick fire-resistant bark and grow tall very quickly. Once tall enough to escape the impact of fire, they branch sideways. But despite these adaptations, very few seedlings survive, which is why acacias are so thinly spread across the veld.

Acacia is a large genus of trees with hundreds of species growing in the warm parts of the world, including Southern Africa, South America, India and Australia. In fact, the acacias evolved before the supercontinent Gondwana split into separate continents and drifted upon the tectonic plates to where they are today. This is why these trees are spread across the southern hemisphere.

The problem is that there are a lot of different species all belonging to the genus Acacia (about 1,500). So based on DNA and chemical analysis, some Australian taxonomists decided to simplify matters by claiming the genus ‘Acacia’ for the Australian species (which are in the majority) – this meant that the rest of the world would have to rename their thorn trees.

However, in taxonomy the accepted rule is that “the earliest published name has precedence”. As it happens, the first named acacia was an African tree. The tree was described in 1753, and was named Acacia scorpioides. However, rules didn’t stop the Australians and they won the day.

The genera names of African Acacia trees have now been changed from ‘Acacia’ to ‘Vachellia’ and ‘Senegalia’. While these names are now mandatory in botanical literature, they cannot stop us from using the common genus name for our iconic trees – which will probably always remain ‘Acacia’ in our minds. As it should.