Polygonaceae and Plumbaginaceae as biogeographical outliers in the Cape Flora of South Africa

In both South Africa and Australia, large regions of nutrient-poor substrates have distinctive floras, dominated by sclerophyllous or semi-sclerophyllous, fire-prone shrubs.

These are 'heathlands' in a loose sense (https://link.springer.com/chapter/10.1007/978-3-642-68935-2_2 and https://search.worldcat.org/title/ecosystems-of-the-world-9b-heathlands-and-related-shrublands-analytical-studies/oclc/1003129176 and https://ui.adsabs.harvard.edu/abs/1982JEcol..70..922G/abstract).

The families Polygonaceae (https://en.wikipedia.org/wiki/Polygonaceae) and Plumbaginaceae (https://en.wikipedia.org/wiki/Plumbaginaceae) would not seem to belong in these ecosystems.

This is because these two closely-related dicotyledonous families are mainly

herbaceous, and
associated with nutrient-rich - or in the case of Plumbaginaceae sodic or even saline - substrates in the Northern Hemisphere.

So, Polygonaceae and Plumbaginaceae seem remote from the 'southern heathland' floras, both geographically and ecologically.

However, these families have managed to defy the general pattern in a limited way, which tends to pass under the radar for botanists whose interest is centred on fynbos (https://en.wikipedia.org/wiki/Fynbos) in South Africa and/or kwongan (https://en.wikipedia.org/wiki/Kwongan) in Australia.

What are easy to overlook are that

both the genus Polygonum and a particular genus in the Plumbaginaceae have somehow found their way to the southernmost reaches of South Africa, where they have speciated into 'narrow endemics' in the Cape Flora (https://link.springer.com/article/10.1023/A:1018360607299), and
one genus of Polygonaceae, namely Muehlenbeckia (https://en.wikipedia.org/wiki/Muehlenbeckia), has penetrated southwestern Australia, where it grows as an unobtrusive but anomalous liane at the edges of what is the most extreme example of 'heathland' on Earth.

The species to which I refer are as follows:

Polygonum undulatum, a biogeographically remarkable but fully indigenous 'misfit' in 'renosterveld' South Africa
a set of about ten species of Limonium, all restricted to low-lying plains under a mediterranean-type climate in the southwestern Cape of South Africa
Muehlenbeckia adpressa, a small liane, widespread in southernmost, mesic Australia and present on nutrient-poor substrates the South-West Botanical Province (https://en.wikipedia.org/wiki/Botanical_Provinces_of_Western_Australia).

Polygonum undulatum is remarkable partly because it has undergone some degree of evolutionary convergence with the 'ericoid' growth-form (https://www.inaturalist.org/observations?verifiable=true&taxon_id=592411). I refer to the following features

perennial and somewhat woody,
evergreen and perhaps semi-sclerophyllous.

Most species of the same genus (https://en.wikipedia.org/wiki/Polygonum), which occur in the Northern Hemisphere, are herbaceous and deciduous (including annuals). Many are weedy/ruderal (https://www.wswa.org.au/western_weeds/polygonaceae.htm), and some are so thoroughly domesticated that they have become important in agriculture.

https://en.wikipedia.org/wiki/Muehlenbeckia_adpressa

https://www.inaturalist.org/observations/105053325 and https://exploreparks.dbca.wa.gov.au/site/hellfire-bay and https://stock.adobe.com/images/hellfire-bay-is-a-popular-beach-in-the-cape-le-grand-national-park-west-of-esperance-western-australia-australia/275286283 and https://www.alltrails.com/trail/australia/western-australia/hellfire-bay-trail

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milewski 在 2024年04月26日 22:07 所貼文

Polygonaceae and Plumbaginaceae as biogeographical outliers in the Cape Flora of South Africa

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