SEASON UNTO THE GRASSES
European seafarers in search of fresh water, meat, greens, grain and firewood in the 17th and 18th centuries thought the Namib coast to be a place of sand and stones, so barren that its equal was not to be found anywhere else "except in the Desarts (sic) of Arabia".
Namib translates literally from the Nama language as "a bare plain". On the whole it is indeed bare, but certainly not devoid of vegetation. Not only do large trees form linear oases along ephemeral rivers, but grasses and low shrubs are scattered over the desert.
The major part of the Namib constitutes the Desert Biome, the only part of southern Africa designated as such, with annuals -- mostly grasses -- as the dominant form of plant life. Their common adaptation to the desert is that they produce seed to survive unfavourable seasons when little or no rain falls.
The plants wither and die, whereas their seeds are able to withstand aridity and heat for years on end, until rains come once again.
Exclusive of perennial plants that grow along rivercourses and drainage lines, annuals provide an estimated 96% of the canopy cover. They qualify as the dominant form of plant life on the basis of episodic spurts of growth as they do not grow all over the desert all the time. The cover grows and shrinks in tandem with fluctuations in rainfall.
Annuals grow from seed to fruition in less than a year. Notwithstanding their name, they do not appear every year but only when sufficient rain falls in a short time, typically 20 mm or more. It rarely happens, perhaps once in a decade, in any given locality.
In favourable conditions grasslands appear almost overnight. Dune crests are unstable and thus always bare, but grasses grow on the slopes, in depressions between dunes and on the gravel plains. Grasses in the genus Stipagrostis, collectively known as "Bushman" grasses, are common in the desert.
Not all of them are annuals. A perennial species that is endemic to the Namib, dune grass (Stipagrostis sabulicola), grows only in the dune fields. Reed-like and about 2m in height, it is rooted in sand hummocks, often where nothing else will grow. Its roots spread out in all directions, as much as 20m from the hummock and close to the surface, so as to absorb every bit of precipitation.
Despite the prevalence of grasses and dwarf shrubs, rare and quite different plants epitomise Namib vegetation, no doubt because of their bizarre characteristics.
Perennials like the welwitschia and the nara grow in dry washes, riverbeds and pans where their taproots are able to reach groundwater. It is said that they "survive in the desert because they do not grow in the desert". Nonetheless they grow nowhere else except in the Namib and on its fringes.
The same is not true of trees that form woodlands along ephemeral rivers in the Namib. While the ana (Faidherbia albida), camel thorn (Acacia erioloba), ebony (Euclea pseudebenus), leadwood (Combretum imberbe), mustard tree (Salvadora persica), wild tamarisk (Tamarix usneoides) and umbrella thorn (Acacia tortilis) also tap groundwater, they are common in other parts of Namibia with higher rainfall.
In brokenlands in the inner Namib, rocky inselbergs sustain a richer vegetation than the plains, much like that of the escarpment. Their nooks and crannies not only trap moisture from rain, dew and fog, but provide shelter from the sun and wind. The larger species are the African moringa (Moringa ovalifolia), halfmens, milkbush (Euphorbia virosa), Namib aloe (A. namibensis) and quiver tree (Aloe dichotoma).
The southernmost quarter of the Namib forms part of the Succulent Karoo Biome, one of 25 biodiversity hotspots in the world, where dwarf succulents are the dominant form of plant life. Succulents are adapted to store moisture in fleshy roots, stems and leaves in order to survive adverse seasons.
In addition to succulents, the southern Namib supports shrubs, bulbs, herbs and grasses. Over 1 000 species of plant, nearly a tenth of them endemic, grow in the southern Namib. Overall about a third of Namibian flora is represented.
Most of the Succulent Karoo inside Namibia is protected and closed to visitors at present. Plans are afoot to open parts of it to controlled tourism .
A half-human tree
The succulent tree known as the halfmens (Pachypodium namaquanum) is cast as a descendant of humankind in Khoekhoe legend. Fugitives from war were changed into trees, so it is told, to relieve their suffering in a hot waterless land.
Seen against the skyline from a distance, clumps of them do look somewhat like people frozen in motion, their spiny trunks forever inclined northwards, with leaves on top like mops of crinkly hair. Their common name, the Afrikaans word for half-human, is used in English as well.
Endemic to a small part of the Namib, rocky desert on both sides of the Orange River, the halfmens grows on steep mountainsides. It is a stem succulent, slightly bottle-shaped and devoid of branches, that reaches a height of 1-2m.
Under threat from illegal collectors, the species is internationally protected. It is classified as highly endangered under the Convention on the International Trade in Endangered Species of Flora and Fauna (Cites).
The horrid nara
A wild relative of gourds such as cucumbers, melons, pumpkins and squashes, the nara or butternut (Acanthosicyos horridus) is an endemic cucurbit that grows in Namib sand, usually in or near riverbeds where its long taproot is able to reach groundwater.
It grows stems instead of branches and sharp thorns instead of leaves, an adaptation that reduces water loss through trans-piration. In the absence of leaves its thorns carry out photo-synthesis to produce food for the plant from light and water.
The nara is perennial. Its numerous stems grow like tentacles from a woody rootstock and form a tangled thicket that stands about 1,5 m high. When windblown sand collects in a hump around the rootstock, the taproot simply grows ever longer, so as to keep the stems on top of the ground.
The bush bears an edible fruit, about the size of a melon, that is covered with knobby thorns. In the Kuiseb valley and delta, where the nara is plentiful, the Aonin or Topnaar tribe harvests the fruit every summer for its flesh and pips, eaten raw or boiled or dried. Archaeological evidence suggests the practice is at least 8 000 years old in the Namib.
What is a lichen ?
Lichens grow in great profusion in the coastal Namib, so much so that they seem almost common or garden, even though endemic species abound.
Thought to be long-lived, the lichen as such is actually not a plant, but a symbiosis or composite of two organisms, namely a fungus and an alga. The fungus provides the thallus or body and absorbs water direct from fog for the lichen as a whole, while the alga produces all the food required through the process of photosynthesis.
Small and inconspicuous, lichens grow in crust-like, ring-like, leaf-like and branch-like forms, generally attached to soil or rock, but sometimes vagrant or windblown. They lack roots, stems, leaves and flowers. Unable to produce seed, they grow from spores, or multiply through subdivision.
When the sun shines or wind blows, lichens shrivel up and appear to be dead. They can survive for a long time without moisture, but are put to no such test in the coastal desert. As fog creeps inland the lichens take moisture from it and fill out again.
Lichens come in a wide range of colours, among them green, yellow, orange, red, white and blue.
Ordinarily fungi and algae would not survive separately in desert conditions, but some algae overcome the odds and grow on their own in filtered sunlight under translucent pebbles, where water condenses and sustains a moist microclimate within a hyper-arid habitat. In German they are called Fensteralgen or "window algae".
ONE OF A KIND
With the family, genus and species all rolled into one, Welwitschia mirabilis truly is one of a kind. A rarity among rarities, it is a dwarf tree found only in the Namib Desert and on its fringes, northwards from the Kuiseb River. It grows mainly on gravel plains in the fog belt.
While welwitschias are classified with cycads and pines as cone-bearing plants, they are at the same time thought to represent a link to flowering plants. They live to a great age, as yet not known for certain, but possibly some 2 000 years. The largest trees are only 2m tall with a trunk diameter of 1,5m. Half of the trunk, or more, remains below ground.
A large colony occupies the Welwitschia Plain at the confluence of the Swakop and Khan rivercourses in the northern part of the Namib-Naukluft Park. One of the trees known as the Giant Welwitschia is estimated to be 1 500 years old.
A single pair of leaves is the sum total of the foliage that welwitschias produce in their lifetime. The leaves are actually the largest part of the tree visible above ground. They grow from opposite sides of the trunk, straight out of a flattish, woody and dead-looking crown. Long and broad out of all proportion to the size of the trunk, they rather resemble floppy conveyor belts at first. In time desert winds tear them to shreds and they come to look like a lot of long narrow leaves lying in a tangle on the ground.
Very tough and leathery, the leaves never really stop growing out of the crown for any length of time, not even in the driest years. Wear and tear at the ends keeps them more or less in check as they grow very slowly. They reach a length of about 3 m. If they are browsed down to the trunk, or broken off, they immediately start to grow again.
Welwitschias absorb fog water through minute pores on their large leaves -- 250 per square millimetre -- as well as through lateral roots just below the soil. With taproots that are 3 m long, they also depend on groundwater.
Male cones and female cones are borne on different trees. They first develop at about the age of 20, the female ones larger, the male ones more numerous. Like the leaves, they grow from the crown of the stem, but on stalks.
While pollen and seeds are produced in abundance, the odds against propagation are truly formidable. It is estimated that only one seed in a thousand reaches maturity. Thereafter at least 25mm of rain must fall within a few days for germination to take place. Which is a pretty tall order in the Namib.
Fog for drink ........