Trees

African Boabab tree

 

Introduction

Since the earliest of times, trees have been an integral part of human existence and survival. In addition to supplying us with food, clothing, shelter, and fuel, they also give us medication, poison, tans, and colours. Aside from their usefulness, they also awe and amaze us with their imposing constructions, immense size, and ages that some of them can reach. Trees are not only extremely valuable to humans; they also predominate several other vegetation kinds, each of which supports a wide range of species, including food and shelter for countless numbers of microbial and animal consumers of the surrounding environment.

 

There is no definitive description of what exactly classifies a tree. For some people, it might be a small tree, whereas for others it might be a large shrub. A lot of trees may be classified as shrubs in their juvenile stage of life. Though most trees have a single stem and are taller than two meters, multi-stemmed woody shrubs, enormous succulents, and even climbers may be considered trees.

 

Southern Africa is home to an estimated 2 100 indigenous tree species. Many of which are only found here. Hundreds of exotic species from throughout the world are also grown for decorative and useful purposes. Others are used in commercial forestry to create lumber, poles, and pulp for paper production, while some are cultivated for their aesthetic value or their shade, shelter, feed, or fuel. More than 100 of these foreign tree species have established themselves in the area and are currently degrading the environment by displacing native plants. Some of these alien tree species are so naturalised in certain regions that many people do not even realise they are not indigenous. Two examples of such species are pines (Pinus patula) and eucalypts (mainly Eucalyptus grandis) which are used in large artificial tree plantations.

 

Few people are aware of how crucial trees are to the preservation of a balanced ecosystem. Numerous creatures, ranging in size from the elephant, the largest land animal of them all, to the smallest insect or microorganism, may be completely reliant on trees for survival.

 

For many animals, bushveld trees serve as a significant source of food. Browsers find nourishing food among the branches, leaves, and even fallen leaves of deciduous trees. Some animals have adapted to browse instead of graze, like the black rhino which is a browser whereas the white rhino is a grazer. In general, browsers have morphologically adapted features such as pointed mobile lips and a prehensile tongue with which they can strip leaves and branches and manoeuvre around thorns. Of course, giraffes have a competitive advantage with their long necks making them able to reach the most succulent morsels high up on the trees far out of reach for other browsers. Smaller antelope rely on young, nutritious leaves, flowers and fruits closer to the ground. There are many other animals, insects and organisms in which the presence of trees in their environment plays a cardinal role in their survival.

 

Tree Identification

A phenomenon known as “plasticity” is responsible for the easily modified appearance of plants, within limits, by different environmental conditions. Although no two trees look exactly alike, many species have specific physical characteristics by which they can be identified.

 

Canopy outline

This should be an important aspect to consider when identifying a tree as considering the leaf shape and floral morphology. The density, colour, orientation, and texture of the foliage, together with minor variations in shape and branching pattern, can all be used to identify most trees from a distance. Of course, it makes identification much simpler if the tree also has distinguishing fruit, beautiful flowers, or distinctive bark. Only mature trees typically have reliable bark characteristics. Knowing a tree is evergreen makes it much easier to identify it in the winter; nevertheless, it is of little use in the summer.

 

Illustrations of the different shapes and canopy outlines of trees can be categorised and identified.
(Source: Photo Guide to Trees of Southern Africa)

 

Geographical range

Knowing where a tree comes from can be extremely useful in identifying an unknown tree species. However, this is only valid for a tree or plant found in its natural environment, trees in towns or cities can come from anywhere in the world. Identifying a tree’s geographical range is as important as considering any morphological feature. When trying to identify a tree, focus on the species which occur naturally in that area and see if it matches any of those species.

 

It may be easier to identify certain tree species if you are aware of their rather specific habitat preferences. Some of these species, which thrive in deep sandy soil, forests, along watercourses, mangroves, on termitaria, or coastal dunes, frequently display habitat specificity.

 

Leaf inspection

Closer inspection of trees and their leaves is required to confirm the identification of a species. In the case of extremely tall trees, especially old forest trees, binoculars can be used to inspect the leaf details, flowers and fruits high up in the trees. Some morphological features are constant characteristics for tree species and will not be affected by the environment. These features include leaf type, leaf arrangement, venation pattern, and the type of leaf margin.

 

Stipules

It is important to always examine a variety of leaves from a tree, specifically from the tree canopy. Inspect for the presence of stipules – small-scale or leaf-like appendages at the base of the leaf stalk. Usually, they can be found in pairs on either side of the leaf stalk at the point of insertion. This is best observed at the tips of actively growing shoots in fresh young leaves. Most commonly, stipules are deciduous or shrivelled up in mature leaves. Distinct scars on the stems are left after stipules are shed.

Left – example of stipules on a leaf branch; Right – a stipular scar

 

When there are opposite leaves, keep an eye out for stipules that cross the stem from the base of one leaf’s stalk to the base of the opposite leaf’s stalk. Similar to a stipule, stipels are seen on compound leaves close to where the stalks of the leaflets are attached.

Tree species which has an interpetiolar stipule

Latex

The presence of latex is particularly helpful in identifying trees; however, this can only be accurately seen in living plants. In this module, the term “latex” refers to any copious liquid discharge, regardless of whether it is watery and transparent, hazy, milky, or otherwise coloured. Choose a leaf that is healthy and green, preferably one that is on a shoot that is actively growing. Break the leaf off at the place where the stalk joins the stem, and then immediately see if any liquid flows out of the damaged end or the scar on the stem. The exudate must be fairly abundant, ideally producing a drop that covers the wound entirely.

A branch that produces white latex to protect the fresh wound

Lenticels

Examine immature twigs to determine whether they are smooth, hairy, lenticellate, angular, or rounded. Lenticels are miniature corky pustules that serve as “breathing” holes and are often white or brownish.

Two different types of lenticels can be found on twigs and branches

Domatia; Glands; Secretory cavities; Bacterial nodes

Leaves should be carefully inspected for glands, domatia, secretory cavities, and bacterial nodes. The main side veins and the midrib’s forks each include tiny pits or hairy tufts called domatia.

An example of tiny pits along the midrib of the leaf vein which is called domatia

Extrafloral nectaries, or external glands that may exude nectar, are frequently found where the petiole (stalk) joins the blade in simple leaves or on the petiole or rachis in compound leaves.

Extrafloral nectaries on the petiole at the base of the leaf blade

 

Hold the leaf up to the sun and scan for translucent spots to check for secretory cavities. These are evenly dispersed across the blade and are very tiny (about the size of a pinprick). Although not all aromatic leaves have secretory cavities, those that do are typically very aromatic.

 

Bacterial nodules can be found on the leaves of some Rubiaceae (coffee family) members. These are considerably larger than secretory cavities, dark in colour, and frequently restricted to a single region of the blade, especially towards the midrib.

 

Although fruits and flowers are excellent and reliable identification characteristics for different tree species, they are absent for a large part of the year. Therefore, other traits have to be distinguished when these reproductive characteristics are absent.

 

Spine vs thorn

Botanically, thorns are modified stems, whereas spines are modified leaves.

 

The difference between thorns, spines and prickles explained in a drawing

 

General

• Recommended common names are supplied for English and Afrikaans. Namibia, however, promotes the use of only one common name for a species, regardless of language. Where applicable, regional preferences are indicated as follows: N – Namibia; SA – South Africa; Z – Zimbabwe.
• Names of alien species are preceded by an asterisk (*).
• Tree numbers of alien species are preceded by an “X.”

Explanation of icons and codes which will be used throughout the book.
(Source: Photo Guide to Trees of southern Africa)