The Australian Nullarbor, bare alike of trees and geophytes - save Wurmbea, a pure floristic link with South Africa

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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

• some of the families and virtually all of the genera differ between continents, in the most closely-matched types of fire-prone vegetation (kwongan vs fynbos),
• Myrtaceae are dominant in Australia but virtually absent in relevant environments in South Africa,
• the degrees of sclerophylly and other biological specialisations to poor soils and intense fires are greater in Australia than in South Africa, even within Proteaceae,
• Ericaceae are surprisingly different in the form of the foliage and fruits,
• under semi-arid climates, there is a nearly categorical difference in the incidence of non-halophytic succulents, and
• geophytes show remarkably little similarity in floristic composition or growth-form (emphasising Orchidaceae in Australia vs Iridaceae in South Africa).

Wurmbea is one of the few clear exceptions to the above, because

• it is nearly restricted to southern Australia (https://www.inaturalist.org/observations?place_id=6744&taxon_id=120086&view=species) and southern Africa (https://www.inaturalist.org/observations?place_id=6986&taxon_id=120086&view=species),
• it is equally speciose on the two landmasses (despite the lack of any shared species), and
• there is no major difference in the adaptive character of the plants, intercontinentally.

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.

Posted on Δεκέμβριος 05, 2022 0552 ΠΜ by milewski