@arthur_chapman @tonyrebelo @jeremygilmore @botaneek @troos @benjamin_walton @sedgesrock @bushbandit @reiner @adriaan_grobler @yvettevanwijk1941 @rion_c @richardgill @ellurasanctuary @russellcumming
The Nullarbor Plain (https://en.wikipedia.org/wiki/Nullarbor_Plain) of southern Australia is named after its virtual treelessness.
This is an area about as large as England and Scotland combined.
This treelessness is particularly anomalous by comparison with the Great Western Woodlands (https://en.wikipedia.org/wiki/Great_Western_Woodlands), which border the Nullarbor on its western side. These eucalypt-dominated woodlands constitute the tallest broadscale vegetation in a semi-arid climate, anywhere on Earth.
Although a few small trees of Acacia (https://www.inaturalist.org/taxa/369486-Acacia-oswaldii), Pittosporum (https://www.inaturalist.org/taxa/349255-Pittosporum-angustifolium), and Alectryon (https://www.inaturalist.org/taxa/783975-Alectryon-oleifolius) encroach marginally on the Nullarbor Plain, eucalypts are remarkably absent.
The odd treelessness of the Nullarbor Plain is well-known. However, what has previously been overlooked is that this is also a 'desert' for an inconspicuous growth-form category that is, in a sense, the antithesis of trees, namely geophytes (https://www.merriam-webster.com/dictionary/geophyte).
The only geophytes recorded from the Nullarbor Plain are Wurmbea dioica (https://www.inaturalist.org/taxa/504086-Wurmbea-dioica) and possibly Wumbea tenella (https://www.inaturalist.org/taxa/925387-Wurmbea-tenella) (Reference: McKenzie N L and Robinson A C (1987) A biological survey of the Nullarbor region, South and Western Australia, in 1984. CALM, Australian National Parks and Wildlife Service, NP&WS, SAust. 413 pp.).
(Wurmbea, which belongs to Colchicaceae, happens to be one of the few cormous geophytes in Australia.)
This poverty of geophytes contrasts strongly with the exceptional richness of geophytes in the Little Karoo of South Africa (https://www.oneearth.org/ecoregions/succulent-karoo-xeric-shrublands/), which has a similar climate.
What makes this comparison all the more intriguing is that the sole genus of geophytes present on the Nullarbor Plain is also the most unqualified example of a floristic link between Australia and southern Africa.
Naturalists familiar with the southern continents will know that 'gondwanan' elements such as Proteaceae (https://en.wikipedia.org/wiki/Proteaceae) and Restionaceae (https://en.wikipedia.org/wiki/Restionaceae) are shared between Australia and South Africa (https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2699.2007.01757.x).
However, such links are easily exaggerated. This is because
Wurmbea is one of the few clear exceptions to the above, because
Elaborating the last point:
In Western Australia, Wurmbea tends to be associated with 'granite exposures' (https://en.wikipedia.org/wiki/Granite_outcrops_of_Western_Australia#:~:text=Granite%20outcrops%20of%20Western%20Australia%20are%20weathered%20landforms%20that%20occur,inselbergs%2C%20castle%20koppies%20and%20nubbins. and https://heartlandjourneys.com.au/about-us/ancient-wonderland/life-on-granite-outcrops/ and https://www.rswa.org.au/publications/Journal/83(3)/v83(3)withers.pdf and https://www.researchgate.net/publication/228584618_Overview_of_granite_outcrops_in_Western_Australia).
These environments are somewhat rejuvenated with nutrients, because of fresh weathering of the bedrock. Furthermore, the occurrence of bare sheets of rock provides some protection from the regime of intense wildfires that otherwise affects the entire landscape.
What this means is that, unlike most elements of the flora of southern Australia, Wurmbea is not particularly adapted to poor soils or intense fires. It is specialised for seasonal dormancy, but this can be seen as an adaptation mainly to the dry summers - which particularly desiccate the shallow soils on the aprons of granite outcrops.
In the southwestern Cape of South Africa, Wurmbea has a similar adaptive profile.
An association with granite is less noticeable, because granite is less widespread than in southwestern Australia, and does not form similar outcrops. However, Wurmbea likewise occurs mainly on soils of moderate status w.r.t. nutrients, and in vegetation not particularly subject to wildfire. Examples of its habitats include disturbed ground or seasonally wet depressions in renosterveld.
The above may help to explain the occurrence of Wurmbea on the Nullarbor Plain.
The soils here, derived from limestone, are neither rich nor particularly poor in nutrients. The vegetation is not free of wildfires, but the fire regime is mild because the dominant plants (particularly the amaranthaceous Maireana sedifolia, https://en.wikipedia.org/wiki/Maireana_sedifolia) do not have flammable foliage.
One way of explaining the floristic link of Wurmbea between Australia and South Africa, therefore, is to see this genus as 'interstitial'. It characterises moderate environments, evading the ecological hegemonies imposed by kwongan and eucalypts in Australia, and fynbos and succulent karoo in South Africa.
By virtue of this lack of specialisation, the fortuitous restriction - which is poorly-described as 'gondwanan' although we lack a better adjective - of Wurmbea to two southern landmasses has peeked through. And this includes an environment - the Nullarbor Plain - where no other geophyte has prevailed.
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FURTHER INFORMATION ON WURMBEA:
https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=gn&name=Wurmbea
https://en.wikipedia.org/wiki/Wurmbea
https://www.inaturalist.org/observations?place_id=6986&taxon_id=120086&view=species
https://www.yarraranges.vic.gov.au/PlantDirectory/Lilies-Irises/Wurmbea-dioica-ssp.-dioica
https://florabase.dpaw.wa.gov.au/browse/profile/21225
https://www.utas.edu.au/dicotkey/dicotkey/lilies_etc/gWurmbea.htm
https://bie.ala.org.au/species/https://id.biodiversity.org.au/node/apni/2914376
Showing the equitable distribution of Wurmbea between the two continents:
https://scholar.google.com.au/scholar?hl=en&as_sdt=0%2C5&q=Wurmbea&btnG=
https://scholar.google.com.au/scholar?hl=en&as_sdt=0%2C5&q=Wurmbea&btnG=
https://scholar.google.com.au/scholar?hl=en&as_sdt=0%2C5&q=Wurmbea&btnG=
A thorough search for other geophytes on the Nullarbor Plain:
Pterostylis mutica (https://www.inaturalist.org/taxa/140380-Pterostylis-mutica) possible
Oxalis perennans occurs on the Nullarbor Plain, but is not geophytic (https://florabase.dpaw.wa.gov.au/browse/profile/4355 and http://www.bihrmann.com/caudiciforms/subs/oxa-per-sub.asp and http://www.flora.sa.gov.au/cgi-bin/speciesfacts_display.cgi?form=speciesfacts&name=Oxalis_perennans and https://www.inaturalist.org/taxa/353868-Oxalis-perennans and https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Oxalis~perennans and https://apps.lucidcentral.org/plants_se_nsw/text/entities/oxalis_perennans.htm and https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Oxalis~perennans#:~:text=Oxalis%20perennans%20Haw.&text=Description%3A%20Herb%20with%20branches%20erect,%3B%20taproot%20stout%3B%20bulbils%20absent. and https://bie.ala.org.au/species/https://id.biodiversity.org.au/node/apni/2894269).
On the treeless plain, even the widespread species, Thysanotus patersonii, is absent.
Droseraceae absent
Stylidiaceae absent
Convolvulus erubescens (https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Convolvulus~erubescens and https://bie.ala.org.au/species/https://id.biodiversity.org.au/node/apni/2887814) present on treeless plain, but this is not a geophyte.
Pycnosorus pleiocephalus (https://www.inaturalist.org/taxa/634739-Pycnosorus-pleiocephalus) present, but this is not a geophyte.
The genera Clianthus, Glycine, Psoralea, Swainsona, and Trigonella are all present.
https://www.researchgate.net/publication/325872570_A_checklist_of_the_geophytes_of_Fergana_Valley_Middle_Asia_-Monocotyledonous_Plant_and_Biogeographical_analysis
An example of a species of Wurmbea occurring in the semi-arid environment of the Little Karoo: https://www.inaturalist.org/taxa/120335-Wurmbea-variabilis
odd outlier in Kenya:
https://www.inaturalist.org/taxa/120080-Wurmbea-spicata
one of the few spp. with fynbos affinity:
https://www.inaturalist.org/observations/137969587
Wurmbea densiflora:
https://florabase.dpaw.wa.gov.au/browse/profile/1391
@tonyrebelo
In your view, is any species of Convolvulus in South Africa geophytic?
None of the Cucurbits I know of, but I dont know the tropical or desert species.
The family Colchicaceae is not 'gondwanan', because it is absent from South America (https://www.jstor.org/stable/24546152).
https://en.wikipedia.org/wiki/Colchicaceae
https://www.pacificbulbsociety.org/pbswiki/index.php/Androcymbium?setskin=pbs_greenish
https://www.researchgate.net/publication/227723674_Historical_biogeography_of_Androcymbium_Willd_Colchicaceae_in_Africa_Evidence_from_cpDNA_RFLPs
ALSTROEMERIACEAE:
The family most closely related to Colchicaceae, in South America, is Alstroemeriaceae. Instead of the bulb-like corms typical of Colchicaceae, the underground storage organs of Alstroemeriaceae are rhizomes and root-tubers.
The following spp. (which are not necessarily geophytes) occur under the mediterranean-type climate in Chile:
Alstroemeria:
https://www.inaturalist.org/taxa/563537-Alstroemeria-ligtu
https://www.inaturalist.org/taxa/202296-Alstroemeria-pulchra
https://www.inaturalist.org/taxa/636300-Alstroemeria-hookeri
https://www.inaturalist.org/taxa/549116-Alstroemeria-pelegrina
https://www.inaturalist.org/taxa/559934-Alstroemeria-magnifica
https://www.inaturalist.org/taxa/716663-Alstroemeria-revoluta
https://www.inaturalist.org/taxa/764824-Alstroemeria-marticorenae
https://www.inaturalist.org/taxa/481553-Alstroemeria-diluta
https://www.inaturalist.org/taxa/565066-Alstroemeria-angustifolia
https://www.inaturalist.org/taxa/911169-Alstroemeria-recumbens
https://www.inaturalist.org/taxa/122404-Alstroemeria-leporina
https://www.inaturalist.org/taxa/911115-Alstroemeria-garaventae
https://www.inaturalist.org/taxa/705613-Alstroemeria-zoellneri
Bomarea:
https://www.inaturalist.org/taxa/626441-Bomarea-salsilla
https://www.inaturalist.org/taxa/437713-Bomarea-ovallei
Luzuriaga:
https://www.inaturalist.org/taxa/855424-Luzuriaga-polyphylla
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