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Text Barbara Curtis | Photographs Barbara Curtis, Vivienne Ward
The Namib Desert appears to be a dry and dusty place where plants and animals battle against temperature extremes, strong winds and lack of moisture. Yet through this seemingly barren wilderness run lifelines of vegetation – linear oases that provide food and shelter for many organisms ranging from tiny microbes to elephants and giraffe.
A lthough the desert receives less than 100 mm of rainfall per year, higher rainfall inland causes runoff into rivers that drain from the highlands, and then meander across the desert plains to the misty Atlantic Ocean. As the floods proceed, they fill the subterranean aquifers, providing life-supporting moisture to large trees whose roots are able to reach deep into the earth to tap this precious resource. Thus bands of lush green vegetation, dominated by tall riverine trees, snake their way to the coast along the courses of major rivers draining westwards in Namibia. The largest, and possibly the most important of these trees is the Anaboom, Faidherbia albida, which typically grows on the edges of the main channel, where the raging waters batter its trunks and pile debris among its branches.
The Anaboom (Ana tree) is widespread across Africa, from Senegal in the west to Egypt and the Middle East, down eastern Africa to KwaZulu-Natal in the south, where frost determines its southern limit. It has been introduced into countries including India, Peru, Nepal and Cyprus as a fodder tree.
Although predominantly a riverine species, it has adapted to savannah conditions on various soils. In Namibia its major distribution is in the north-west, mainly along the ephemeral rivers, but also on the floodplains of the perennial Kunene and north-eastern rivers.
A tall tree of 10 to 20 metres, with old specimens reaching a diameter of 2 metres, and with a spreading crown, it is the tallest of the Namibian acacias, with individuals towering above the much shorter canopies of the camel-thorn trees. In addition to a long taproot, up to 20 to 40 metres deep, which is able to tap stored water in the underground aquifer, this species also has extensive lateral roots which are able to tap water near the surface in a shallow flood.
It was initially classified as an Acacia due to the structure of the flowers and leaves, and the presence of paired spines. The ‘flowers’ are actually elongated, tightly packed groups of very small flowers with numerous pale-yellow stamens and no petals. The distinctive broad, woody, curled orange to red-brown pods hang in clusters on the tree, and form red mats underneath in the late dry season. Owing to many slight differences in morphology and chemistry between this species and other Acacia, as well as its unusual annual cycle, it has now been placed in a genus of its own. Its closest relative is Acacia erioloba or camel-thorn, the species with which it shares its desert habitat. These two species are thought by some taxonomists to possibly be relics of the original Africa acacias.
One of the amazing features of the Anaboom is its unusual cycle of leaf and flower production. While most trees produce leaves and flowers in the summer rainy season, with ripe pods providing nutritious food towards the end of summer, this tree is dormant during the summer, shedding its leaves at the start of the rainy season, with only a few leaves during the summer. It starts to produce fresh green leaves in autumn, as other trees are losing their leaves, and is in full leaf throughout the dry winter, earning itself the common name of Winter thorn in some countries. Flowering occurs from March to September, with mature pods ripening and falling at the end of the dry season, a critical time for all herbivores. The production of leaves and flowers in the dry season makes it a vital source of browse and nectar at a time when there is very little else available. It is insect pollinated, with a myriad of different insects attracted to its pale yellow flowers. These insects attract a variety of bird species.
As a member of the legume family, many of which host symbiotic micro-organisms on their roots that help them make proteins, their leaves and seeds are highly nutritious. They are a valuable food source for both wild and domestic animals, and herbivores are the main dispersal agents of the seeds. In Namibia it is not uncommon to see these trees growing alongside the major roads, where seeds have been deposited by cattle being herded from one place to another, and runoff from the road provides the necessary water for germination and establishment. The seeds need to pass through the digestive system of an animal in order to soften the hard seed coat that protects them. Individual tree production generally varies between 50 – 300 kg per annum. These pods are collected by local people as fodder for their livestock.
Because these trees are dormant during the rainy season they can tolerate the summer floods, which saturate the ground, causing anoxic (no oxygen) conditions around their roots. Thus they are able to grow near the river channel, where other trees would not survive. Owing to its summer dormancy it is an ideal tree under which to plant crops, since it does not compete with the crops for water and nutrients in the summer, but enriches the soil with its nitrogen-fixing abilities. In areas north of Namibia it is planted and managed to increase crop and livestock production, a practice that is being introduced to northern Namibia.
Apart from food and shelter for animals, and soil fertilisation, the tree has several uses for people. The bark, boiled in water, is used as a cure for diarrhoea. The seeds are roasted and used as a coffee substitute, or boiled and eaten. The tannins in the green bark are used by Nama people to dye skins.
These trees also have an amazing ability to regenerate after adversity. A tree that looks totally dead may send out new shoots from way up in the crown, or from near the base, and trees knocked over by a flood will generally shoot from the horizontal trunk to form a row of new trees. Root suckering also occurs, where new shoots will develop at any point along a lateral root that is close to the surface, most often where the root has been damaged. Thus a single tree can produce its own private forest, with regeneration occurring up to 25 metres from the parent tree. This regeneration from fallen trees and from trees whose roots have been exposed by flooding, is particularly noticeable in the Kuiseb – the southern-most of these ephemeral rivers – since the exceptional floods of 2011. The impact of this huge flood on the riverine vegetation, and in particular the Anaboom, is currently being researched at the Gobabeb Research and Training Centre.
This article was first published in the Flamingo October 2012 issue.