Acacia species East Africa
When most hear the name acacia they think of the umbrella thorn acacia – the typical picture of Africa. But the genus Acacia is the largest group of woody trees and shrubs in the subfamily Mimosoideae in the family Leguminosae (or bean family). The 132 species in Africa, 62 species in East Africa are mostly restricted to the dry savannah and semi-desert. These thorny trees play an important rule in nature. They provide food for wild animals and livestock as well as materials for local people.
All pod-bearing plants have nodules on their roots which contain nitrogen-fixing bacteria. This maintains the fertility of soil. The tiny leaflets in most acacias reduce evaporation. Sometimes you see a greenish colour on the stem and on branches. This is chlorophyll. That is how they enlarge the surface to do photosynthesis. The different species have different preferences and therefore grow in different biotopes.
When they get stressed because of draught and over browsing they produce prussic acid, which can be fatal. That is not enough, acacias communicate by chemicals when it is time to become poisonous to defend themselves.
Acacias and ants have coevolved. Enlarged and hollow thorns are shelters and breeding place for the Acacia-ants of the genus Crematogaster and Tetraponera. That is not yet enough, the acacia feeds the ants with a sugary sap and a highly nutritious snack. In return the ants defend the trees against hungry insects and herbivores by biting them and spraying a bad odor. This co-existence is called Myrmecophily and exists in the new and old world.
The scientific name Acacia has been legally declared only to be for Australian acacias. Those in Africa got the name Vachellia.
Photos by Elvira Wolfer
or Ant-galled acacia (Vachellia drepanolobium) are short shrubs in the Rift valley, Masai Mara and Serengeti. When the wind blows into the holes of these for ants built galls you hear a whistling sound and this gave them their name.
Spiders and hunting beetles feed on the ants.
(Vachellia eliatior) grows along river banks. These majestic trees are home for many bird and insect species. The flowers are small balls. Best seen in Samburu National Reserve.
(Vachellia gerrardi) grow to large trees. Thorns are quite thick and grow in pairs. The leaves have a dull green and are tiny. The flowers are whitish balls and the bark on the stem is almost black. Nice individuals stand in Nakuru and on the way to Lake Magadi as well as in Kiambu region.
(Vachellia kirkii) grows around Nairobi, Northern Uganda and Northern Tanzania. The bark is often papery and peels off.
(Senegalia mellifera) has bigger leaves than most other acacias in Kenya, which makes it easy to recognize it. The thorns are nasty when you get too close and that is why it got its other name “wait-a-bit-bush”. Buds, flowers and seeds showed on the pictures. Pods, leaves, flowers and twigs are much favoured by game and livestock. Mellifera refers to the flowers containing a lot of nectar and means “honey bearing”. Hollow tree trunks are hanged in trees as bee-hives so the locals can harvest tasty acacia honey.
or Egyptian thorn acacia (Vachellia nilotica subalata). This subspecies is endemic to Kenya and Tanzania. The pods are flat. When you break them a nice smelling sap flows out. The ripe pods turn brown.
Indian Scented-pod acacia
(Vachellia nilotica indica) have very characteristic pods. Without them they are not easy to be identified. The subspecies indica is introduced to Baringo District from India. The pods are rounder. Picture taken at Lake Bogoria.
or Sudan gum arabic (Vachellia senegal senegal) has 3 hooks below each node; outer 2 curve upward and the centre hook curves downward. It takes you time to free your cloths once they hang in the thorns…. Leaves and pods are rich in protein. Wide spread in tropical Africa.
(Vachellia seyal fistula) have conspicuous 8 cm long spines. Similar to the Whistle-thorn acacia they build galls in which ants live. The yellow flowers provide nectar for bees. In Meru National park and Lewa Downs you find the subspecies fistula and in Nakuru and Tanzania V.s.seyal without galls.
The brown colour on the stem and branches are fungi. Picture is taken in Lewa Conservancy.
The nominate form (Vachellia seyal seyal) without galls in Masai Mara.
Umbrella thorn acacia
(Vachellia tortilis) are most likely those trees creating the image of Africa with their large umbrellas providing shade to the trunk and roots. Beside small hooked thorns they also have straight 3-8 cm long, whitish thorns which even puncture shoe soles. The 2 kind of thorns should reduce the browsing by giraffes. The pods are spirals and are eaten by all kind of animals.
Umbrella thorn acacias grow in many semi-arid and arid areas in Africa.
(Vachellia xantophloea) are dominant in Naivasha and Nakuru. They need a relatively high underground water level. It is also called fever tree because mosquitoes occur where they grow and they have a substances in its bark to cure malaria. Various animals feed on the sap.
(Faidherbia albida) is one of the tallest acacias in subtropical Africa. The fruits look like dried apple rings and are eaten by baboons and other animals. The flowers are yellowish white coloured spikes. It occurs in Kenya and Tanzania.