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​A rare habitat for rare plants: The Cape’s coastal dunes 

 

FEB 15, 2021 | Written & photographed by B. Adriaan Grobler. 

The Cape’s coastal dunes

 

 

Above: Near Stilbaai in the Cape floristic region,  the fynbos is dominant and largely made up of blombos (Metalasia muricata) and garlic buchu (Agathosma apiculata). The subtropical thicket is far sparser and mostly only stunted candlewood (Pterocelastrus tricuspidatus).

 

If you find yourself along the Cape coast, enjoying a day on our reopened beaches, take a moment to pause and admire some of the botanical treasures in the coastal dunes.

In the Cape floristic region, you will find coastal dunes mostly as small patches between Elands Bay in the west and Port Elizabeth in the east. These dunes cover a total area of just 2 200 km2 – making up less than 3% of the whole Cape floristic region.

 

Above: At Cape St Francis in the Cape floristic region, the coastal dunes are typically harsh environments for people and plants alike.

 

They are very different from towering, sandy deserts such as the Namib or Sahara. Instead, the Cape coast’s dunes have a wider range of habitats than most of us expect, including:

  • Mobile sand sheets of bypass dunefields
  • Semi-mobile hummock dunes above sandy beaches
  • Semi-succulent herblands and dwarf shrublands above rocky shores
  • Various types of wetlands
  • Mosaics of fynbos and subtropical thickets – the most extensive of all, particularly on stable backdunes the seemingly oxymoronic mosaics of fynbos and subtropical thicket

Which of these two thicket types occurs depends on the conditions. Fynbos thickets dominate in drier, fire-exposed sites. Subtropical thickets prevail in moister, fire-protected spots. In areas where there is plenty of moisture and no fire for long periods, small pockets of coastal forest can develop in sheltered dune areas.

Coastal dunes can be particularly harsh environments. Throughout the year, they are exposed to blistering sun and strong, salt-laden winds that also carry highly abrasive sand grains.

 

Above: This candelabra lily (Brunsvigia orientalis; Amaryllidaceae) is flowering in recently burnt dune fynbos at Goukamma Nature Reserve in the Cape floristic region.

 

Plants also have to cope with growing in dune sands that are highly alkaline, typically infertile and very poor at holding water. Remarkably, despite such an inhospitable environment, the Cape coastal dunes are rich in plant species – many of them found nowhere else in the world:

  • Cape dune flora totals about 1 000 different species
  • About 40% of these occur only in the Cape floristic region
  • 226 species occur exclusively on dunes
  • Another 104 species occur also in the region’s limestone-derived soils, but not elsewhere

This makes about a third of all plant species found in dunes chalk-dwellers, or calcicoles, growing only in highly alkaline, chalky (calcareous) soils.

 

Above: Satyrium princeps is a threatened orchid species restricted to the Cape floristic region’s coastal dunes.

 

To understand why Cape dune plants are so remarkably rich in different, highly localised species, we must look back to the Pleistocene, from 2.58 million to as recently as 11,700 years ago. During this period, there were repeated ice ages (glaciations) and corresponding lower sea levels – so along the Cape coast, for instance, shorelines were much lower than they are today.

This exposed a vast, level landscape, the palaeo-Agulhas plain, off the south coast of what we now know as the Cape floristic region, and allowed large tracts of dune sand to accumulate. It provided a huge coastal dune habitat – orders of magnitude larger than we see today – where a rich dune flora could develop.

Coastal dune ecosystems are under threat throughout the world and, sadly, the case is no different in the Cape. Large areas of dune habitat have been transformed by urban and industrial development, especially around the metropolitan areas of Cape Town and Nelson Mandela Bay.

Other areas have been invaded extensively by alien plants, especially rooikrans (Acacia cyclops) and Port Jackson wattle (Acacia saligna).

 

Above: The dwarf Cape beech (Rapanea gilliana; Myrsinaceae) is a threatened species restricted to dune thicket in the Cape floristic region.

 

Most dune plant species are naturally confined to small dune areas and combined with the impact of these widespread, transforming forces, this results in coastal dunes packed full of threatened indigenous plant species.

So on your next trip down to the beach, keep an eye out for some of these botanical rarities to help our Red List scientists better understand and protect some of South Africa’s most vulnerable plant species.

 

Left: Aspalathus cliffortiifolia. Right: Aspalathus recurvispina. Both Fabaceae are examples of very localised dune-endemic species that have suffered extensive habitat loss and are now both listed as critically endangered.

 

WHAT TO LOOK FOR IN DUNE FYNBOS

  • Daisy (Asteraceae), pea (Fabaceae), grass (Poaceae) and sutera (Scrophulariaceae) families make up the greater part of dune fynbos
  • Plant families that we typically associate with fynbos such as restio (Restionaceae) and heath (Ericaceae) families have relatively few members in dune flora – but most of them, such as Elegia microcarpa, Restio eleocharis (both Restionaceae), Erica chloroloma and Erica glumiflora (both Ericaceae), are localised to dunes (dune endemics)
  • The buchu family (Rutaceae), especially the genus Agathosma, has a lot of different species in dune fynbos
  • The protea family (Proteaceae), so characteristic of fynbos elsewhere, is conspicuously absent in dune fynbos
  • There are plenty of bulbs, especially those that bloom after fire, including members of the amaryllis (Amaryllidaceae), iris (Iridaceae) and orchid (Orchidaceae) families

Left: Erica chloroloma. Right: Erica glumiflora. Both Ericaceae are threatened species found only in or near dune fynbos.

 

Above: A member of the buchu family (Rutaceae), Agathosma stenopetala is a threatened species restricted to dune fynbos in the southeastern Cape floristic region.

 

TROPICAL RIDDLE IN CAPE DUNES 

A peculiarity of dune vegetation in the Cape floristic region is how many dune-thicket species belong to families that originated in the tropics and are generally not well represented elsewhere in the Cape. They include:

  • Asparagus (Asparagaceae), milkweed (Apocynaceae), spike-thorn (Celastraceae) and wild-currant (Anacardiaceae) families
  • Dune sourberry (Dovyalis rotundifolia), dune koko-tree (Maytenus procumbens), dwarf Cape beech (Rapanea gilliana) and dune silky-bark (Robsonodendron maritimum) are a few examples of the many dune-thicket species found only in the Cape floristic region’s coastal dunes.

Above: The dune silky-bark (Robsonodendron maritimum; Celastraceae) is an example of a species restricted to the Cape floristic region’s dune thicket.

 

MORE READING

Field guide

Field Guide to the Eastern and Southern Cape Coasts, (eds) R. Lubke & I.J. de Moor (Cape Town, 1998).

Conservation research policy

Coastal Dunes of South Africa, K.L. Tinley, (South African National Scientific Programmes Report, No 109, 1985; Committee for Nature Conservation Research, National Programme for Ecosystem Research)

Open-access research

Taxonomic, biological and geographical traits of species in a coastal dune flora in the southeastern Cape Floristic Region: regional and global comparisons’, R.M. Cowling, C. Logie, J. Brady, M. Middleton and B.A. Grobler, PeerJ 7: e7336 (2019)

 

Above: This unusual member of the Cape coastal dune flora is Hyobanche robusta (Orobanchaceae), a threatened root parasite restricted to dunes in the southeastern Cape floristic region.

 

1 Comment

  1. I am interested to know if you have done an analysis of plant available phosphorus on thicket versus fynbos heathland shrublands?

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