The welwitschia, one of the strangest trees on the planet

The welwitschia, one of the strangest trees on the planet

Violet Field inPlanetJan 4, 2021 9 min read9 views

Discover, every day, an analysis of our partner The Conversation. Today, two biodiversity researchers tell us the story of one of the most original trees that can be observed on earth

Welwitschia Mirabillis - © Nhelia / Pixabay
  • The welwitschia species has a longevity of several centuries (up to 2000 years according to some authors) and its largest known individual reaches 4 meters in diameter, according to a study published by our partner The Conversation.
  • Today it grows exclusively in the Namib Desert, along a coastal strip less than 200 km wide, from Namibia to Angola.
  • This “biography” of welwitschia was carried out by Germinal Rouhan, lecturer, systematic botanist, and Serge Muller, professor (both researchers at the Institute for Systematics - Evolution - Biodiversity of the National Museum of Natural History).

Like ginkgo, Chinese redwood, and Wollemi pine, welwitschia belongs to the group of gymnosperms, the seeds of which are often protected in cones, as is the case with firs and spruces. Gymnosperms, unlike sophoras or catalpas, for example, are therefore not flowering plants (called angiosperms ).

Welwitschia mirabilis from Namibia © Thomas Schoch / Wikimedia, CC BY-NC-SA

The diversity of gymnosperms was highest in the Mesozoic (-250 to -66 million years ago) before dropping sharply. Only about a thousand species remain today, especially widespread in the boreal and mountainous regions.

Certain lineages of gymnosperms are no longer represented by one or a few species, often considered as “relics” of these ancient groups. The welwitschia, which is found in the deserts of southern Africa, is one of these “relict species”.

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How to recognize welwitschia?

A unique representative of its genus, of its family ( Welwitschiaceae ), and even of its order ( Welwitschiales ), the welwitschia ( Welwitschia mirabilis Hook.f.) has a short and thick trunk resulting from a taproot 2 to 3 m deep.

This trunk ends in a disc, on the edge of which only two opposite, thick and leathery leaves develop. They grow continuously throughout the life of the plant and can reach dimensions of 2 to 4 meters, with a wavy twisted shape, lying on the sandy or stony substrate.

The abrasive sand, the effect of which is increased tenfold by the winds, helps to divide the sheets longitudinally into numerous strips, which then gives the illusion that these two single laciniate sheets are much more numerous. The reproductive organs are cones, and since it is a dioecious species, the male and female cones are carried by different individuals.

Male cones © Conchi Revuelto, CC BY-NC-ND (via The Conversation)

Female cones © Hans Hillewaert, CC BY-NC-ND (via The Conversation)

The growth of welwitschias, like that of a tree, is visible in the concentric annual rings of the trunk, and the age of individuals can therefore be roughly estimated by counting these rings. The species has a longevity of several centuries, even up to 2000 years according to some authors. The largest known individual, "The Big Welwitschia", reaches 1.4m in height and 4m in diameter with its foliage. The Namib Desert (Southern Africa) is the only place in the world where Welwitschia mirabilis grows naturally.

In his novel Le Tour du monde d'un gamin de Paris, published at the end of the 19th century, Louis-Henri Boussenard describes it as follows:

“A veritable forest grew as far as the eye could see. We say forest, because there is no other word to designate in general an agglomeration of trees and in particular that of welwitschias […]. These trees, stocky, or rather flattened, had taken all their development in width. The trunk looked like an enormous log hard as ironwood from which escaped two single leaves, woody, thick, monstrous, two meters long and seventy-five centimeters wide. The impression produced by the sight of these vegetable legless was astonishment, almost disgust […]. It's too much. Never, since the world became world, has we seen anything like it. "

How she was "discovered"

Austrian botanist Friedrich Welwitsch © Biodiversity Library (via The Conversation)

This amazing species was discovered in 1859 by Friedrich Welwitsch, an Austrian botanist, director of the Lisbon Botanical Garden, during an expedition to Angola. By his own statements, Welwitsch fell to his knees on the scorching ground and stared at the thing unwillingly to touch it, lest it turns out to be his imagination.

Emotion overcome, Welwitsch collected samples and forwarded them to JD Hooker, director of the Royal Botanic Garden at Kew (London), who was no less intrigued by this species, as he recorded in 1863:

“It is a plant that I do not hesitate to consider from a botanical point of view as the most wonderful that has been discovered in the present century. "

After extensive discussions with other biologists, in particular Charles Darwin (who compared it to a plant platypus!), Hooker described and illustrated it in 1862 at the request of Welwitsch.

Hooker dedicated the new genus to his Austrian discoverer, after convincing the latter to give up the name he had proposed, Tumboa - a vernacular name that the local populations applied to the plant in question (which they therefore already knew), but also to other plant species.

At ease in the deserts of southern Africa

Welwitschias today grow exclusively in the Namib Desert, along a coastal strip less than 200 km wide, from Namibia to Angola.

Namib desert welwitschia, a really strange plant (video: ratdavid9 / Youtube - 2011)

Although no surface water is permanent there, the plant seems adapted to this environment: if aridity reigns here, there is also the humid freshness coming from the coast and its mists. Condensing as dew overnight, the water in the mist is picked up by the leaves, as the broad, conical root sinks deep enough into the ground to reach underground moisture at the same time as it firmly anchors the plant in the sand.

Plate published in Curtis's Botanical Magazine © Biodiversity Library (via The Conversation)

Even in deserts without any annual precipitation, welwitschias do much more than subsist, since they are also capable of reproducing there. Ripe seeds remain viable for years until a period of rain of several days arrives: it is such an exceptional event that the plants of some populations are all the same age, being the result of germinations that have taken place. during a rare and unique rainy year.

Despite very large and now regulated harvests, the species is still quite abundant although necessarily restricted to the deserts of Namibia and Angola. However, recent studies have established its vulnerability to climate change, and lead to the report of certain populations in danger of extinction.

The question of the conservation of the species is relevant across the entire range since it is in fact represented by several populations corresponding to two subspecies, mirabilis in Angola and namibiana in the southern part of the range. The namibiana subspecies was first detected in the culture at the Berlin-Dahlem botanical garden in 2001 and accounts for differences in phenology and morphology in male cones. Researchers have estimated that these differences would justify recognizing two species in their own right.

Where to see welwitschias?

Due to its ecological requirements, the species could of course not be acclimatized outdoors in parks or botanical gardens in our temperate climates. But it is cultivated and can be admired in the greenhouses of desert plants, present in many French botanical gardens, in Paris, Nantes or even Nancy, as well as in Brussels, Berlin, London, Basel ... By going a little further, at the botanical garden of Kirstenbosch (South Africa), you can even visit a greenhouse dedicated to this flagship species, the Welwitschia House.

Welwitschia exhibited in the botanical gallery of the MNHN © Serge Muller / MNHN, CC BY-NC-ND

The botanical collections of the Museum's Herbarium also present an adult plant, collected in 1937 by the botanist Henri Humbert, as well as a young seedling clearly illustrating the two opposite leaves typical of the species.

Who are its "neighbors"?

Unclassifiable since its discovery, Welwitschia has plunged scientists into perplexity, due to the difficulties in determining its nature and its relationships with other plants.

The only current species of its genus, of its family, and even of its order, it is placed in the class of Gnetopsida with two other genera of current plants, Ephedra and Gnetum. But the kinship relationship between Gnetopsida and other plants remains uncertain. Indeed, Gnetopsida has morphological affinities with angiosperms (flowering plants) as well as with current gymnosperms (conifers, ginkgos, cycads), which is why they were first considered as a transition group in the evolution of seed plants.

The work of Shu-Miaw Chaw, recently supported by that of Jin-Hua Ran in phylotranscriptomics, revolutionized this idea by showing that genetic data could favor an entirely different evolutionary hypothesis, which confirmed Welwitschia and the other Gnetopsida as gymnosperms, but in the position of closest relatives of the Pinaceae family (firs, pines…) within conifers!

This would imply that the morphological characters shared between angiosperms and Gnetopsida would have appeared twice independently during the evolution of plants. This is a hypothesis that remains controversial because the shared characters relate to complex structures, such as the reproductive organs which are similar to flowers and involve double fertilization. Thus, the placement of Welwitschia and other Gnetopsida remains debated in the phylogenetic reconstruction of the tree of life. More than 150 years after its "discovery", Welwitschia still remains a mystery ...

This analysis was written by Germinal Rouhan, lecturer, systematic botanist, and Serge Muller, professor (both researchers at the Institute of Systematics - Evolution - Biodiversity of the National Museum of Natural History).
The original article was published on The Conversation website.