Oxalis in Australia and southern Africa

In Australia, Oxalis is an unremarkable herbaceous plant. Perhaps its most remarkable aspect is the tendency for any given indigenous species to occur also in New Zealand, or perhaps South Africa, or even South America.

That is to say, the plants are in themselves unremarkable in their ecological nature, but what is remarkable is the blurring of the distinction between indigenous and introduced.

However, in southern Africa the same genus has evolved in a remarkable way, related to a regime of disturbance of the earth by animals. Here, many spp. of Oxalis bear true bulbs.

Bulbs (https://en.wikipedia.org/wiki/Bulb) are associated mainly with monocotyledonous plants. Oxalis is exceptional among dicotylodonous plants in having independently evolved bulbs.

Furthermore, the bulbs of southern African spp. of Oxalis have two major functions, viz.

  • being disseminated by disturbance, in which they act as 'vegetative seeds', and
  • serving as underground storage organs that allow adoption of a geophytic growth-form.

With the development of the geophytic growth-form, there has been a proliferation of species in South Africa, particularly under the mediterranean-type climate, that parallels the geophytic proliferation of various monocotyledonous families, e.g. Iridaceae.

The result is that the indigenous spp. of Oxalis in South Africa outnumber those in Australia 40-fold - and under mediterranean-type climates more than 100-fold.

The seed-like function of the 'aerial bulbils' of Oxalis have proven so efficient that they have greatly exacerbated the biogeographical blurring among landmasses. Various spp. of Oxalis, indigenous to southern Africa, have become cosmopolitan weeds, or have anthropogenically invaded similar climates in Australia in particular.

The result is that, perhaps more than any other lineage of plants, the South African spp. of Oxalis have come to represent a paradoxical duality. They combines exceptional local restriction in their original state with exceptional intercontinental spread in their anthropogenic state.

And - returning to the Australian occurrence of Oxalis - this leaves unanswered the puzzle of how those spp. lacking bulbils managed to spread 'naturally' among the southern continents in the first place.

AUSTRALIA:

https://www.castlemaineflora.org.au/pic/o/oxali/oxper.htm

https://www.utas.edu.au/dicotkey/dicotkey/OTHERS/sOxalis_perennans.htm

https://www.inaturalist.org/taxa/57865-Oxalis-exilis

https://www.inaturalist.org/taxa/323276-Oxalis-magellanica

https://www.inaturalist.org/taxa/353869-Oxalis-chnoodes

https://www.inaturalist.org/taxa/69934-Oxalis-radicosa

https://www.inaturalist.org/taxa/353867-Oxalis-rubens

https://www.inaturalist.org/taxa/404428-Oxalis-thompsoniae

SOUTH AFRICA:

https://www.inaturalist.org/taxa/53169-Oxalis-pes-caprae
geophytic

https://www.inaturalist.org/taxa/78301-Oxalis-purpurea
presumably geophytic

https://www.inaturalist.org/taxa/120434-Oxalis-compressa

https://www.inaturalist.org/taxa/119242-Oxalis-luteola

https://www.inaturalist.org/taxa/165991-Oxalis-hirta

https://www.inaturalist.org/taxa/361074-Oxalis-caprina

https://www.inaturalist.org/taxa/119764-Oxalis-flava

https://www.inaturalist.org/taxa/591296-Oxalis-truncatula

https://www.inaturalist.org/taxa/119470-Oxalis-glabra
geophytic

https://www.inaturalist.org/taxa/566867-Oxalis-ciliaris

https://www.inaturalist.org/taxa/591292-Oxalis-tenuifolia

https://www.inaturalist.org/taxa/61776-Oxalis-incarnata

https://www.inaturalist.org/taxa/528599-Oxalis-depressa

Posted by milewski milewski, December 05, 2022 08:22 PM

Comments

Posted by tonyrebelo about 2 months ago (Flag)

69-Oxalis-pes-caprae produces bulbs, but is probably not geophytic
In what way is this not geophytic? What definition of geophyticis violated?
If produces leaves, flowers and dies down, being merely a bulb for 5 months.

Posted by tonyrebelo about 2 months ago (Flag)

@tonyrebelo

Many thanks for the correction.

Of the spp. indigenous to the southwestern Cape, what percentage do you regard ad geophytic?

Putting the question differently, are there any indigenous spp. of Oxalis in South Africa that are not geophytic, and, is so, do they have other specialisations, e.g. perennial succulence?

Posted by milewski about 2 months ago (Flag)

I think all are geophytes: but @kenneth_oberlander would know far better than me.
(except the alien - I dont think that is geophytic.)

Posted by tonyrebelo about 2 months ago (Flag)

@milewski all indigenous sub-Saharan African Oxalis are tunicate bulbous geophytes.

Regarding non-native SA Oxalis:
Oxalis corniculata has been introduced as a garden weed, and spreads predominantly via selfing (tristyly has broken down in this species). @qgroom has done some nice work on the origins of corniculata - it seems to be of South East Asian origin (unlike all of it's closest relatives, which are New World - a nice biogeographical puzzle in itself). It is possible that some plants are IDed as O. corniculata in South Africa, but are close relatives like O. stricta or O. dillenii. They are undoubtedly introduced.

Oxalis articulata is an introduced South American stem geophyte, usually planted horticulturally. It spreads clonally via underground stems.

Oxalis latifolia and Oxalis debilis are introduced weeds of horticulture - they spread exclusively via their imbricate bulbs (tristyly is intact and they cannot self).

There are a few other species, but they are all garden plants and introduced.

I would contest the main function of bulbs as a secondary dispersal mechanism first - this is absolutely a mechanism, but I think dormancy and underground storage more important functions (one could quibble this).

Also, aerial bulbils are a distinctive trait, in several SA Oxalis, that is most likely an adaptation to clonal dispersal. With the doubtful exception of Oxalis incarnata, none of them have been introduced or become invasive in any other part of the world.

The shared southern distribution ranges - the global distribution of Oxalis outside South America is actually mostly the global distribution of section Corniculatae (with the exception of more regional lineages such as section Ionoxalis in North America, the African/Malagasy clade, Oxalis acetosella and relatives in the temperate northern Hemisphere, and Oxalis magellanica with a circum-Antarctic distribution). Corniculatae species are very often selfers and have sticky seeds, which undoubtedly helps with exozoochorous dispersal and presumably could explain their broad distribution ranges.

Regarding your Australian list - Oxalis compressa is a South African species introduced into Australia. Your other taxa in that list appear to be native Australian spp., and are mostly section Corniculatae? None of these taxa occur naturally in Africa, or have been introduced here (to my knowledge).

Refs:
https://peerj.com/articles/6384/
https://doi.org/10.3732/ajb.1100152
https://link.springer.com/article/10.1007/s00606-016-1330-6
https://doi.org/10.1002/tax.606011

Posted by kenneth_oberlander about 2 months ago (Flag)

@kenneth_oberlander

Dear Kenneth,

You have helped so much. Many, many thanks for this valuable information, which will take me some time to digest. I will revise this Post accordingly.

With kind regards from Antoni

Posted by milewski about 2 months ago (Flag)

It is possible to botanise all one's life, in South Africa or Australia, without realising that the genus Oxalis has been domesticated as a food-crop, and - even more surprising - the underground storage organs are not bulbs but stem-tubers.

I refer to Oxalis tuberosa (https://en.wikipedia.org/wiki/Oxalis_tuberosa and https://www.inaturalist.org/taxa/122836-Oxalis-tuberosa), which originates in the central Andes, from an apparently extinct ancestor.

Oxalis tuberosa was domesticated in sympatry with quinoa (Chenopodium quinoa). However, unlike quinoa, this species has not been adopted in most Western countries.

Posted by milewski about 2 months ago (Flag)

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