By Ian Nelson, of Westminster College
The Okavanago Delta is internationally recognized for its abundance of wildlife and diversity of plant species. Among these plants are over 50 different species of trees. Many of the trees found in the Delta prefer certain habitats or soil types, which limit their distribution to specific areas. Others are less habitat specific, so they are found throughout the delta.
Our field work has been concentrated in the eastern reaches of the Delta thus far. We have visited concessions NG18, NG19, and NG41. These concessions are northeast of Maun, farther into the Delta by a distance ranging from 100 – 150 Km. The prominent rivers in the areas include the Mochaba (NG41 – Mababe) and the Khwai (NG18, and NG19 – near Khwai village).The species of trees and their abundance change drastically as we move around the landscape. The most common trees that we encounter however are the mopane, camel thorn acacia, knobthorn acacia, and rain trees.
Homogenous stand of mopane with rain tree in background.
Mopane (Calophospermum mopane) is one of the most common trees in the eastern part of the Delta. The leaves are very high in protein (13%), making it a favorite choice of food for elephants. Many of the transects that we collect data on are mopane scrubland (homogenous stands of short mopane trees), where the elephants’ browsing has made all of the trees the same height. Many of the trees in these stands have broken branches and the bark stripped from their trunk, or they are broken at the roots and tipped over all the way. Mopane trees are part of the Fabaceae family and can grow to be around 7 meters tall when undisturbed by elephants. The leaves are compound pairs meaning that each stem has two leaves that sprout from it, somewhat resembling a butterfly. The bark is dark grey and has longitudinal fissures that run up the tree. It prefers clay soils and is found in high concentrations far from water. During the dry season the mopane tree loses all of its leaves and appears to be dead but comes back as a lush green tree during the wet season. The wood is commonly used for firewood and building fences by Botswana locals.
Camel thorn acacia, also known as Acacia erioloba is one of the common acacia trees found in the areas we have been working. The camel thorn grows up to 10 meters tall and has bipinnately compound leaves. A main characteristic of acacia trees are large thorns which have evolved as a defense mechanism from browsing herbivores. The thorns can grow to be around 10-15 cm in length and can easily penetrate the sole of your shoe or the bottom of the tent (a problem which we have encountered). In addition to thorns, the camel thorn fruits with moon shaped pods that are covered in grey, velvety hair. The bark of the camel thorn is dark grey with longitudinal fissures that run along the trunk. Acacia wood is very dense, which makes it good for firewood as it burns for a long time and at a high temperature. It also is used by local people to build huts and kralls for livestock.
Another member of the Acacia family that we encounter frequently is the knob thorn acacia (Senegalia nigrescens). The knob thorn appears very similar to the camel thorn and can often be hard to distinguish. The bark is nearly identical on the two trees both in color and texture. The key difference is that the knob thorn acacia has many small protrusions along the bark (knobs). Another key difference between the two is the shape of the thorns and the pods. The knob thorn acacia has curved thorns and dark, broad, straight pods. The wood of the knob thorn acacia is very dense and has been used in the past to build railway sleepers, poles, and mine props.
The acacia trees are found most commonly in close proximity to water, however this does not mean they are only by rivers. Their root system is such that it can tap into groundwater reserves below the surface, therefore, the trees can be used to determine where there is groundwater close to the surface in the bush.
Rain tree with elephants at its base.
The rain tree (Lonchocarpus capassa) is another common tree in our study area. It is named the rain tree because the leaves often release small amounts of sap which fall to the ground around the tree. If you are seated beneath the tree you can feel this and sometimes get the sensation that it is raining. Rain trees are only found around water or where the water table is high, so local people use them as an indicator of groundwater similar to the acacia. The bark of the rain tree is grey and appears scaly similar to a snake. It has a pinnately compound leaf pattern with leaflets at the apex and the fully-grown trees can reach up to 10 meters in height. The rain tree produces soft wood, making it easy to carve out. For this reason, it has historically been used to make dugout canoes by many tribes in the area. The bark is toxic to fish, so it has also been used as a poison by fishermen looking to increase their yield.