Acacias - Thorn trees of Africa

There are few more striking symbols of Africa than a thorn tree - its gnarled branches, graceful form, jagged thorns and abundant blooms, in many ways reflecting the paradoxes of the continent.

Acacias are the dominant woody plants over much of sub-Saharan Africa, with the exception of forests, grassland and mountain heathland. There are about 700 species in Africa, South America, India and Australia. All belong to the Family Mimosaceae (part of the legume group) and some were previously known as mimosa trees. Although just about all acacias have abundant yellow or white flowers, they are not widely cultivated due to their thorny branches. Several Australian species have, however, become invasive weeds in parts of Africa, where their seeds have no parasites or predators, and can lay dormant for up to 50 years.

Adapted for the Hot and Dry

African savannahs experience hot and often dry conditions, so plants must find ways of retaining moisture. The way in which acacias have evolved to do this, is to have their leaves divided into dozens of tiny leaflets (pinnae) which can be held horizontally to capture sunlight or vertically to reduce transpiration. Leaves with a large surface area are prone to dry out in the absence of shade or moisture, so are not suitable for savannah trees. Acacia foliage is sought after by many browsing animals - from giraffe to beetles - and here too it helps for them to be divided, as the tiny leaflets left behind by the browsers can continue to perform their vital photosynthesis function.

Many acacias have exaggerated umbrella-shaped crowns which enable the tree to capture the maximum amount of sunlight, with the smallest possible leaves. Some species have deep tap root systems, allowing them to survive in semi-arid terrain along dry watercourses (they tap into the water table) while others have shallow but extensive splayed root systems to capture limited rainfall in dryer areas. The well-known Fever Tree which grows along lake fringes and on poorly-drained clay soils also has a shallow root system, in order to avoid waterlogging.

Improving the Soil

Acacias belong to the group of pod-bearing plants known as legumes and have the remarkable ability to enhance soil fertility. The activity of microbes associated with roots of acacias (and other legumes such as Lucerne) fixes nitrogen to soil particles and so improves fertility. Experiments have also shown that soil fertility is higher below the canopies of acacia trees, due to leaf-fall and decomposition, and the accumulation of droppings from roosting birds and from herbivores seeking shade.

Food for the Masses, but Varied Defences

Just about all parts of an acacia tree are edible to one kind of animal or another, and in many parts of Africa their small leaves provide the only greenery in the dry season, so are bound to be eaten. The roots may be sought after by porcupines, the tiny leaflets by macro- and micro-herbivores, the sticky sap by galagos, the flower nectar by bees and butterflies, the seeds by bruchid beetles, and the stringy bark by elephant. Along with grasses, acacias are the "building blocks" of African savannahs - propping up a colossal food pyramid with super predators at the apex.

With so many creatures making a living from their growth efforts, it is no wonder that acacias have evolved some of the most effective and cunning defences in the plant kingdom. Acacia thorns may be straight or hooked depending upon the species (only the Umbrella Thorn has both) and these are capable of keeping all but the largest browsers at bay. Interestingly, straight thorns have been shown to pose little deterrent to impala, while the much smaller hooked thorns succeed in keeping them at bay. Giraffe, however, have leathery tongues which feel no apparent pain against either hooked or straight thorns, and millions of browsing insects and their larvae can simply crawl around the thorns to feed on the leaves. To combat large mammalian browsers some acacia trees - such as the Knob Thorn - release chemicals in the form of toxic tannins which render the leaflets unpalatable and cause giraffe or kudu to move on to another tree. To reduce the impact of browsing insects, some acacia species have developed an association with biting ants - ant-plant mutualism - which is one of the most intriguing relationships in the natural world.

Ants and Acacias

Among the most important insect browsers of acacia trees are beetles, grasshoppers and the larvae (caterpillars) of butterflies and moths. In a masterful strategy to ward off these micro-herbivores, some acacias provide food and shelter, in return for defence, to cocktail ants. The ants live inside the swollen bases of thorns (Whistling Thorn is perhaps the best-known example) and feed on sweet sap exuded from pores at the tips of the acacia leaves. In order to maintain such ideal living and foraging quarters, the ants patrol the branches and will gang up and physically bite any herbivore that threatens the plant. Insects quickly retreat, and even giraffe and rhino have been seen to become irritated and move off with the unwanted attentions of aggressive ants. Amazingly, the ants also nip off the growth tips and tendrils of strangling vines which threaten to suffocate the tree - and thus their home. Acacias still rely on insects for cross-pollination, however, so it remarkable to note that the patrolling cocktail ants are least active at the hottest time of day - when browsers are seeking shade but bees are out looking for nectar; in this way, they do not disturb the pollen-gathering bees from their important task!

Valuable Fuel

The wood of many acacia trees is renowned for its excellent fuel properties and can also produce good charcoal. Since many African people still rely on wood as fuel for cooking and heating it is essential that acacia woodlands are managed effectively in order that the trees do not disappear altogether. In areas that have been overgrazed, bush-encroachment by acacia and the related Dichrostachys (as well as other species) is commonplace and restorative land management can harvest this resource as attempts are made to restore savannah conditions (i.e. trees and grass). Acacia sap or gum is another important product and the Sweet Thorn and Scented Thorn produce copious amounts which can be used as a glue. "Gum Arabic" was originally derived from the Three-hook Thorn and synthetic alternatives are now used as a binding compound in cosmetics, medicines, fast foods and paints.

Further Reading
Guide to the Acacias of South Africa by Nico Smit. (1999) Briza Publications, Pretoria.

'Ants, pollinators and acacias in Mkomazi' by Willmer, Stone and Mafunda (In: Mkomazi - the Ecology, Biodiversity and Conservation of a Tanzanian Savanna.) Edited by Coe, McWilliam, Stone & Packer (1999) published by the Royal Geographic Society, London.

Posted: Plants by CC Africa, Date: 21 November 2006