The welwitschia is a broadleaf evergreen tree occurring only in the hyperarid Namib Desert of Namibia and Angola. Joh Henschel reveals more about the plant’s survival in an area where rainfall is scarce, groundwater deep, and fog rare…
• 32: The locations of natural populations of welwitschia in the Namib Desert.
• Welwitsch’s miracle plant: Welwitschia mirabilis is named after Friedrich Welwitsch, who discovered it for science.
• Longevity: Living individuals have been dated at 920 years; larger plants could be as old as 2 000 years.
• One litre: The daily water requirement of a medium-sized welwitschia plant.
• Roots: These are 22–62m long and extend less than 2 metres deep but 9–15 metres sideways from the
stem. Dense networks of fine roots occur across this layer, with near-surface fine roots extending 1.5 metres around the canopy.
The welwitschia is a rather unusual plant occurring in the desert landscapes of Namibia and Angola. It is the only member of the family Welwitschiaceae. It is considered to be a gymnosperm (produces cone-like seeds), but it also has many characteristics of angiosperms, such as male flowers, pollen transfer by insects and a xylem vascular system to transport water to the leaves. These and many other unusual features render this plant iconic.
Its stunted tree stem gives rise to two evergreen leaves, the longest-lived leaves in the world, which split and fray to look like many leaves that spread across the ground. Leaves grow by about 14cm per year and reach areas of 1–21m². Its canopy constitutes an important microcosm for communities of small animals that might otherwise not occur in an area.
Pollination is performed by small wasps, bees and flies, and not by the commonly seen red or yellow spotted welwitschia bugs. Female plants produce thousands of winged seeds, dispersed by wind, but these are prone to infection by a fungus, rendering them inviable.
The biggest puzzle is where welwitschias find sufficient water in the desert. Rainfall is rare, the average being 30mm, and groundwater is deeper than 50 metres. Fog occurs over much of this species’ range during 50–90 nights per year, but deposits very little water.
Surprisingly, the main source of water comes from rainfall that infiltrates the top two metres of soil and is blocked by a calcrete layer at that depth. At half a metre depth, a layer of gypsum salts may remobilise moisture and make it available to the plants through their fine root system. A dense network of very shallow fine roots may supplement this source with fog water. These multiple sources of water sustain this evergreen plant and enable it to be resilient against dehydration in extreme desert conditions.
Recruitment events of welwitschia are very rare, and the germination of a few seedlings in a population may occur only once in every few decades. Most welwitschia populations contain older plants, and there is growing concern for this species’ future without sufficient recruitment.
This may be further exacerbated by the effects of mining, off-road driving and industrial or tourist activities that disturb the soil. This affects the survival of old plants before they have managed to generate a cohort of young plants, sufficient to sustain the populations into the future.
The most convenient place to see these plants is at the famous population of the Welwitschia Plain, which can be reached from Swakopmund, driving into the Namib-Naukluft Park, past the Moon Landscape, through the Swakop River Canyon to the Big Welwitschia.
Written by Joh Henschel
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