Welwitschiaceae and Welwitschia mirabilis (tumbo) description

Conservation status 2023

Welwitschia mirabilis

Hooker 1863

Common names

Tree tumbo, welwitschia, tumboa, n'tumbo [Angolan], tweeblaarkanniedood [Afrikaans] !kharos [Nama/Damara], nyanka [Damara], khurub [Nama], onyanga [Herero] (Notten 2003).

Taxonomic notes

The sole species in order Welwitschiales Skottsburg ex Reveal 1993, family Welwitschiaceae Caruel 1879, genus Welwitschia Hooker f. 1862. Synonyms for these include Tumboaceae Wettst. 1903, Tumboa Welw. 1861, Welwitschia bainesii (Hooker f.) Carrière 1867, Tumboa bainesii Hooker f. 1861, and Tumboa strobilifera Hooker f. 1862. Some authorities even put it in a class by itself, Welwitschiopsida B. Boivin 1956.

The family seems to have arisen via a whole-genome-duplication (polyploidy) event that separated it from the clade now represented by Gnetum (Li et al. 2015, Wan et al. 2021). This plant is nearly unknown in the fossil record. The most noteworthy finds are from the lower Cretaceous (Aptian, ca. 112-114 ma) Crato Formation of northeast Brazil. These fossils include young stems with attached cotyledons, isolated leaves, and axils bearing male cones (Dilcher et al. 2005), and by inference, providing a minimum date for divergence of Welwitschia and Gnetum.

Leuenberger (2001) has described the Angolan and Namibian as two subspecies, differentiated on the basis of male cone characters. The subspecies are W. mirabilis Hook.f. subsp. mirabilis and W. mirabilis Hook.f. subsp. namibiana Leuenberger. He has offered the following key (Leuenberger 2001):

1	Male cones smooth, purplish brown (rarely green when shaded), without evident wax cover; longer peduncles usually c. 5-11 cm long, secondary branches to 2 cm long; longest male cones 30-45 mm long; bract pairs overlapping c. 2 mm; bract scales more than 3/4 connate, margin of bracts smooth; Angola	*W. mirabilis* subsp. mirabilis
-	Male cones sculptured, glaucous green to salmon, with evident wax cover; longer peduncles c. 7-15 cm long, secondary branches to 7 cm long; longest male cones 20-30 cm long; bract pairs overlapping c. 1 mm; bract scales ⅓ -⅔ connate, margin of bracts slightly erose; Namibia	*W. mirabilis* subsp. namibiana

Description

Dioecious perennial plant with short stem and taproot. Beyond that, the plant is often described as 'bizarre', 'weird', or (more explicitly) 'unlike any known plant on earth'. Its short, woody, unbranched stem is surmounted by 2 (rarely 3: see photo) strap-shaped leaves that grow from a basal meristem throughout the life of the plant, becoming twisted and frayed with the passing centuries. The leaves contain numerous subparallel veins that may anastomose or terminate blindly in the mesophyll (this character is unique in Welwitschia among the gymnosperms). Stomata occur on both leaf surfaces. The woody stem widens with age to become a concave disc up to a meter across, from which grow small ramified branch systems that serve only to bear pollen and seed cones (Gifford and Foster 1988). The branched reproductive shoots arise near the leaf bases. Pollen cones red, resembling those of Ephedra, appearing in groups of 2-3 terminally on each branch. Ovulate cones also arising from branched reproductive shoots, each red cone consisting of a single nucellus enclosed in an integument and another layer derived from two confluent primordia ('perianth') with 2 'bracts'. Normally, only one seed develops within each cone; it is dispersed by wind with 'perianth' as a wing. Seeds germinate in wet years, the 2 cotyledons photosynthesizing for 1.5 years (Gifford and Foster 1988).

Distribution and Ecology

A disjunct distribution in SW Africa. The type locality is in the vicinity of Cabo Negro on the coast of Angola (latitude 15-16° S), while more widely dispersed populations are found from the coast to ca. 200 km inland in Namibia (latitude 20-24° S). The area is extremely arid, receiving no rainfall in some years and averaging fewer than 100 mm per year. Most specimens are found within 80 km of the coast in a fog belt, suggesting that the fog is an important moisture source (Rodin 1953). See also Figure 1 in Wan et al. (2021).

Distribution, based on herbarium records downloaded from the GBIF Network. Click on individual records (tree icons) for links to the source data.

The IUCN has not assessed the conservation status of Welwitschia. A web page at the Mohamed bin Zayed Species Conservation Fund (accessed 2021.12.22) reports "Despite considerable attention to the plant, the conservation status of W. mirabilis is largely unknown. Available data indicates highly variable population densities across the species range, with uncommon to rare individuals in the Kunene subrange. A substantial reduction of climatic suitability in northern Namibia by 2050 was predicted as a consequence of climate change and increasing threats from black Aspergillus fungi, ecotourism, domestic livestock, and mining were evidenced." A publication from this work, Bombi et al. (2021a) conclude a "Very high extinction risk for Welwitschia mirabilis in the northern Namib Desert". The same authors found that under current climate change predictions, "the historically realized thermal niche of welwitschia will be almost completely unavailable in the next 30 years in northern Namibia. Expected reductions of climatic suitability in our study sites were strongly associated with indicators of negative population conditions, namely lower plant health, reduced recruitment and increased adult mortality" (Bombi et al. 2021b).

Remarkable Specimens

Although Welwitschia produces no growth rings, radiocarbon dating is feasible and results have indicated maximum ages in excess of 1,500 years (Herre 1961). This result is consistent with the large size and slow growth rate observed in mature plants. Wan et al. (2021), in their detailed study of the Welwitschia genome, attribute this longevity to continuous expression of gene families regulating meristem activity and factors that are likely to regulate cell growth and differentiation, as well as expression of genes that reflect "adaptation for efficient metabolism under environmental stress, functioning to prevent the basal meristem and young portions of the leaves from dying during the long periods of adverse conditions." As such this is the first analysis that provides a clear causal genetic mechanism linking sets of genes to extreme longevity in gymnosperms, and may presage similar work in long-lived conifers. At the least, it suggests a congruence between longevity and the ability to tolerate severe environmental stress, a relationship long recognized in many of the oldest conifers.

Ethnobotany

Observations

Remarks

Discovered by Friederich Welwitsch (1806-1872) in 1860 (Rodin 1953), and described by him the following year (Welwitsch 1861). The specific epithet means "miraculous." Also, the Afrikaans name for the plant, "tweeblaarkanniedood", means "two leaves that cannot die", an appropriate name for this oldest of gnetophytes (Wan et al. 2021).

Some remarkably detailed ecophysiological studies have been performed, cited below and available online (Institute of Plant Ecology 1997). The findings are rather complex but seem to suggest that the plant requires periods of high humidity (i.e. fogs) if it is to sustain a positive carbon balance. It may survive the absence of such conditions for as much as 150 days--possibly much longer. Welwitschia is also of considerable ecophysiological interest because it is a so-called CAM or C4 plant, meaning that it uses the crassulacean acid metabolic pathway in photosynthesis (von Willert et al. 2005). This metabolic pathway is common in cacti and other desert succulents because it enables carbon dioxide absorption during the night, and then photosynthesis with closed stomata during the day; this increases plant water use efficiency. Although CAM is common among angiosperms, among the gymnosperms it has only been recorded in Welwitschia and in the cycad Dioon edule (although it will probably be found in other cycads, too).

Citations

Bombi, P., D. Salvi, T. Shuuya, L. Vignoli, and T. Wassenaar. 2021a. Very high extinction risk for Welwitschia mirabilis in the northern Namib Desert. Journal of Arid Environments, doi: https://doi.org/10.1101/2020.05.05.078253.

Bombi, P., D. Salvi, T. Shuuya, L. Vignoli, and T. Wassenaar. 2021b. Climate change effects on desert ecosystems: A case study on the keystone species of the Namib Desert Welwitschia mirabilis. PLOS ONE, https://doi.org/10.1371/journal.pone.0259767, accessed 2021.12.22.

Botanischer Garten und Botanisches Museum Berlin-Dahlem, Freie Universität Berlin. 1997.02.05. Welwitschia-Annex. http://www.bgbm.fu-berlin.de/bgbm/pr/garden/bereiche/areas/welwitsh.htm, accessed 1999.02.24, now defunct.

Carr, Gerald D. 2002. Non-Flowering Plant Families. http://www.botany.hawaii.edu/faculty/carr/nfpfamilies.htm, accessed 2021.12.22.

Dilcher, David L., Mary E. Bernardes-de-Oliveira, Denise Pons, and Terry A. Lott. 2005. Welwitschiaceae from the lower Cretaceous of northeastern Brazil. American Journal of Botany 92(8): 1294–1310.

Gifford, Ernest M. and Adriance S. Foster. 1988. Comparative morphology of vascular plants, 3rd ed. New York: W.H. Freeman (Chapter 18).

Herre, H. 1961. The age of Welwitschia bainesii (Hook. f) Cearr.: C14 research. S. Afr. J. Bot. 27:139–140).

Hooker, J. D. 1863. On Welwitschia, a new genus of Gnetaceae. Transactions of the Linnean Society of London 24:1-48. Available: Biodiversity Heritage Library, accessed 2021.12.22.

Institute of Plant Ecology. 1997.02.17. Welwitschia mirabilis. http://www.uni-muenster.de/Biologie/pflanzenoekologie/science/willert/vw_welwmira.htm, accessed 1999.02.24, now defunct.

Leuenberger, B.E. 2001. Willdenowia 31: 357-381.

Li, Zheng, Anthony E. Baniaga, Emily B. Sessa, Moira Scascitelli, Sean W. Graham, Loren H. Rieseberg, and Michael S. Barker. 2015. Early genome duplications in conifers and other seed plants. Science Advances 1(10):e1501084.

Notten, Alice. 2003. Welwitschia mirabilis Hook.f. www.plantzafrica.com/plantwxyz/welwitschia.htm, accessed 2014.12.28.

Rodin, R.J. 1953. Distribution of Welwitschia mirabilis. American Journal of Botany 40:280-285.

von Willert, D.J., N. Armbrüster, T. Drees, and M. Zaborowski. 2005. Welwitschia mirabilis: CAM or not CAM — what is the answer? Functional Plant Biology 32:389–395.

Wan, T., Liu, Z., Leitch, I.J. et al. 2021. The Welwitschia genome reveals a unique biology underpinning extreme longevity in deserts. Nature Communications 12:4247. https://doi.org/10.1038/s41467-021-24528-4 (open access).

Welwitsch, F.M.J. 1861. On the botany of Benguela, Mossamedes, etc., in western Africa. Journal of the Linnaean Society (Botany) 5:182-187.