Colophospermum mopane

The image above shows a Mopane's root system after heavy rainfall had eroded a channel. Underground, a Mopane tree sends out a detailed network of shallow, far-reaching roots, seeking out all possible sources of moisture and out-competing most other plant life for water.

Mopane trees dominate in many areas because they tolerate a wide variety of soil types, including clayey ground that is poorly drained, sodic soils high in sodium content, or soils rich in lime. 

Mopane forest, sometimes called ‘cathedral’ Mopane, is dramatically different, with trees reaching 20 metres or more.

Magnificent Mopanes like this grow in deeper, often sandy soils, frequently above granite bedrock. The tree’s roots can probe deeply for water, allowing the Mopane to produce green leaves well before the spring rains arrive.

Note the size of this tree relative to Johna, standing at its base. This particular tree stands in a beautiful drainage line in the Makuleke.

Intense daytime heat. Cold nights. Dry seasons. Fires. Elephants.

Risks abound for Mopanes out in those wide, remote plains. A thick, tough, fissured layer of bark is necessary armour.

Insulation against extremes of heat and cold is one big advantage of thick bark. Another is that water loss must be minimised, and the bark prevents gradual loss of moisture from the critical layers of xylem and phloem.

Thick bark is a shield against fire, although it may not save trees when a hot, wind-blown fire sweeps through. Elephants are not delicate browsers, stripping bark off some tree species if moist nutrients lie beneath, but Mopane bark is mostly left untouched.

Bifoliolate leaves, resembling butterfly wings!

The leaves are alternate on the branch, and each has two sessile leaflets. Leaflets are ovate in shape with the base of each leaflet strongly asymmetric. The apex tapers to a sharply rounded point. The margin is entire.

The leaflets are multi-veined, with about 7 to 12 veins arising from the point of leaflet attachment.

Use a 10 times hand lens to see a scattering of pellucid gland dots between the reticulate veins. Crush a leaf and note the turpentine smell.

A tiny appendage lies between the two leaves, at their base. This is the vestigial remains of a terminal leaflet. At some time in the ancient past, this species had trifoliolate leaves, but evolutionary dynamics found no useful net gain from having a terminal leaflet, which gradually shrunk into today's meaningless remnant.

Mopane leaflet dimensions differ across landscapes, as the image above left shows relative to the first leaf image. But the fundamentals of leaflet design make recognition easy.

The leaflets have closed in toward one another in the image above right. Why would they do that, and how do they do that?

The tree transpires water vapour through tiny pores, called stomata, that occur on the leaf surface. Carbon dioxide is taken in through those pores, and, in the complex process called photosynthesis, is combined with hydrogen from water to form glucose.

Most trees have stomata on both the upper and lower leaf surfaces. But the Mopane is different. It has stomata only on the upper surface, thus roughly halving its exposure to water loss for any given leaf size.

And there’s more.  Little motor cells at the base of the leaflets can open or close the leaflets relative to one another, depending on water abundance or scarcity. In dry conditions the leaflets are drawn close together, with upper surfaces more-or-less facing, effectively reducing transpiration. 

For this reason, Mopanes are not great shade trees on dry, hot days!

Finally, a thin layer of oil covers the leaf surface, also helping to limit moisture loss.

Winter defence tactic!

Tannin content of leaves has a major impact on browsers. During the summer, rainy season, the trees are growing actively and the leaves are fresh and highly nutritious. Tannin content of leaves is low, and the leaves have a crude protein content of 12-13%.

As winter approaches, however, soil dryness sets in and the tree's metabolic rate slows. It cannot readily replace foliage lost to browsers. The tree therefore moves to a chemical defence, producing higher and higher levels of tannin in its leaves.

Digestive systems of many animal species do not tolerate high tannin levels in food. Kudu, for example, do not browse Mopane trees in winter, despite a severe shortage of food. Mopane veld is good elephant country, but by early winter elephants can be seen chewing bark off Mopane branches in addition to feeding on leaves with their high tannin content.

In spring, the first flush of Mopane leaves is low in tannin and high in nutrients, drawing in animals stressed by winter food shortages.

Flowers are small and insignificant, but interesting.

Flowers are hermaphrodite. They are wind-pollinated and have no petals because they do not need colour to attract pollinators.

Four green sepals are prominent. Multiple stamens hang loosely from the flower, on thin filaments, enabling pollen to be released into the air by breeze movement. The stigma is quite large to capture wind-blown pollen, and its style emerges from the side of the ovary.

Wind pollination may appear to be a 'hit-and-miss' affair, but it is highly effective in Mopane scrub with plants in close proximity.

Flowering is erratic over the summer months, and some big stands of Mopane may not produce flowers for several years. But the species has no difficulty with reproduction.

Designed for success!

Reproduction is a core objective for a tree. If the species has chosen to use wind to transfer pollen from stamens to stigma, it must have flowers designed for this purpose.

Note in the image above that stamens dangle on thread-like filaments. The anthers dehisce, releasing pollen through ruptures in the walls of the pollen sacs. Breezes shake pollen loose. On a normal summer's day pollen is in the air! Stigmas are ready and waiting! Success is assured!

Mopane fruits glow in the first sunlight of a new day.

Mopane fruit is a flattened, kidney-shaped indehiscent pod, usually between about 4 cm and 6 cm in length. Young fruit is thin and firm, and as it matures it becomes papery and lightweight. Fruit is ready for wind dispersal during autumn months, roughly March to July, but fruit can still be seen clinging to trees in November.

The pod surface is covered in tiny resin glands (click on image to enlarge). Each pod normally holds one hard, oval seed.

The pods ripen from green to the reddish brown shown above, and on to the darker brown of maturity.

Besides having flowers that are wind pollinated, Mopanes, along with many tree species, use wind to distribute seeds. But there is no need to send seeds far and wide, for Mopane trees thrive in close proximity. The botanical term for wind dispersal is anemochory.

The image above left, taken in September, before spring rains, shows two tall Mopane trees, one leafless, the other holding some foliage. Both trees are still carrying fruit high up (click image to enlarge). That fruit will either blow loose in a wind, or fall the the ground, possibly to be eaten by animals, or to germinate where they lie.

Soon the first rains will restore life to the land, and the Mopane's seeds will germinate, fulfilling a primary goal of every tree . . . reproduction.

Split open a Mopane pod . . .

You will find within a flat, kidney-shaped seed with convoluted appearance. Glands in the seed release a turpentine smell when crushed, and crushed seeds become sticky. This means they can be transported on the hooves of animals. 

The pods are indehiscent, not splitting open to release seeds, and they germinate readily on the soil surface after a good shower of rain. 

Mopane worms are strikingly-coloured caterpillars.

They are the larval stage in the metamorphosis of the emperor moth, Gonimbrasia belina. During their lifespan of 4 to 6 weeks, the feed voraciously on Mopane leaves. Sometimes entire trees are left leafless, but they soon recover.

Female Emperor moths lay clusters of eggs on mopane leaves, typically soon after the first rains of summer. One female may lay up to 1,000 eggs. Once the caterpillars hatch, they are ravenous feeders on leaves.

On reaching maturity, the caterpillars head off en masse over land, sometimes over roads, seeking suitable places to burrow and pupate.

Despite their scary appearance, Mopane worms are edible, and are an important source of protein for millions of people. Protein content is high, and they contain iron and other micronutrients.

Worms are collected by hand, the gut content squeezed out, and often fried and added to stews and other dishes. Dried Mopane worms can be stored for many months.

An unwanted creature hosted by Mopane trees!

That white, lumpy, crusty substances on the leaf above are lerps, nicknamed Mopane Sugar or honeydew. These are like little shelters, built and occupied by the tiny Mopane psyllid Arytaina [Retroacizzia] mopani. 

A psyllid is a sap-sucking insect that looks like a miniature cicada with legs designed for jumping. It is small, from about 1.5mm to 4 mm long, and a hand lens helps to examine it.

In late winter or early spring, Mopane psyllids lays clusters of black eggs, like black spots, glued to the leaf. The new generation of psyllid hatches and sucks phloem sap from the veins of the leaf. Their excretions form the whitish, translucent, hard, insoluble lerp deposits on to the leaf surface. Psyllids live within these hollow, lumpy coverings, protected from predators, sun and rain.

For the Mopane tree, these psyllids are alien invaders. They suck nutrients from the leaf veins, causing the blackening in the image above. Both the leaf discolouration and the lerps reduce the photosynthetic area of the leaf surface. Psyllids and their lerps also affect leaf area available for Mopane words to consume and, being nutritious, encourage browsers to eat the leaves.

Lerps are high in fructose, glucose, potassium and nitrogen, and provide food for baboons, monkeys and birds. They make leaves more palatable to some animals that otherwise avoid Mopane leaves because of their turpentine taste. Even people living in harsh environments lift and eat mopane sugar off leaves.

Lerps on a Mopane leaf are shown above left. Note the locations, along veins, from which nutrients are sucked.

In the image above right, Ted used a blade to lift lerps, revealing the psyllids inside.

In a win-win deal, ants may protect psyllids from predators in return for a few weeks of delicious honeydew! But Mopane trees are the losers from psyllid activity.

Good wood!

Mopane wood and seeds burn easily because of their high resin content. Few types of wood give hotter and longer lasting braai fires. Fireworks displays of shooting exploding sparks are released from burning Mopane wood. 

And when the fire is done, the ash makes a good garden fertiliser.

The termite-resistant heartwood is dark reddish-brown with a resin-like smell.  It is difficult to work, but household products and furniture made from this wood are beautiful and strong.

Bugged by bees!

On a hot day, walking in Mopane veld, little bees, sometimes called ‘sweat bees’, can be an extraordinary nuisance, attracted to traces of water in our eyes or on our lips. They do not leave us alone.

This is a tiny gnat-like and stingless bee, known as the mopane bee (Plebina denoita). These bees have nests in Mopane trees, often in hollow cavities in trunks, with entry through a small waxy tube. Small amounts of honey can be found in the nests. 

False Mopane?

Bifoliolate leaves are rare among Kruger's trees, and Mopane leaves are easy to identify. But the Small False Mopane, or Guibourtia conjugata, also has a compound leaf with one pair of asymmetric leaflets. 

Venation is the key. False Mopane leaflets, above left, have midribs, and Mopane leaves are multi-veined from the base. Mopane leaflets have bluntly pointed tips, whereas the False Mopane leaflets have more rounded tips.

Residential hotel!

Mopane trees host many residents, some welcome and others not, ranging from Mopane bees, to psyllids, to Emperor moths and their 'Mopane worms'. There are others, too.

The trunks of larger Mopane trees tend to have cavities that are used by hornbills, barbets, woodpeckers and other birds for nesting. These cavities also make Mopanes a favourite environment for tree squirrels (Paraxerus cepapi), and where squirrels abound, owls will be found. 

With sharp eyes and patience, Arnott’s Chats can be spotted in Mopane woodland, foraging energetically for insects, ants, beetles and spiders. 

Mopane trees are both a fascinating eco-system and am important provider of food to a wide range of creatures, from humans, to elephants and even down to psyllids.