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Coastal dunes of the Cape: a forgotten treasure trove of plant riches

Wednesday, 10 June 2020
by B. Adriaan Grobler

My hometown, Gqeberha, lies toward the southeastern end of the Cape Floristic Region (CFR) – an area of exceptional plant diversity. The CFR is home to nearly 10,000 plant species – almost half of all species found in southern Africa (about 20,300) – with nearly 70% of these being restricted to the region. Most of the species are associated with fynbos shrublands that typically occur in mountainous areas along the Cape Fold Belt and on sandy soils of the coastal lowlands. This extraordinary diversity of the Cape is, however, not uniformly distributed throughout the region, with some areas harbouring many more species than others. The general pattern that has emerged is as follows: the number of species in a given area increases as one moves from east to west. The fynbos of the southeastern CFR, for example, is overshadowed by that of the hyperdiverse southwestern CFR (centred on the Kogelberg), where one can find twice as many species in an equal-sized area.

But there is an unusual and poorly studied type of vegetation in the Cape – one associated with coastal dunes – where this pattern does not hold. This coastal dune vegetation, occurring as fragmented patches between Saldanha Bay and Port Elizabeth, comprises a mixture of fynbos and subtropical thicket, the latter forming either dense, solid stands or smaller bushclumps surrounded by fynbos. Recent research has shown that, in coastal landscapes dominated by these dune fynbos–thicket mosaics, the southeastern CFR supports just as many species as the southwest. In the southwest, for example, the dunes of Grootbos Nature Reserve cover about 1,250 ha and house nearly 300 species, while in the southeast, about 360 plant species are found in the 1,200 ha of coastal dunes in the Cape Recife Conservancy.

Coastal dune fynbos–thicket near Still Bay in the Cape Floristic Region. Here, the fynbos is dominant and largely composed of blombos (Metalasia muricata) and garlic boegoe (Agathosma apiculata), while the subtropical thicket is far sparser, represented mostly by stunted candlewood (Pterocelastrus tricuspidatus). Photo © 2020 B. Adriaan Grobler.

Those of us who frequent beaches, especially around Gqeberha, will know that coastal dunes can be exceptionally harsh environments (it is not called the Windy City for nothing). They are exposed to high solar radiation and strong, salt-laden winds throughout the year, which also blow around highly abrasive sand grains. In addition to these factors, plants growing in dunes are also affected by the soil environment: dune sands are highly alkaline, typically infertile and very poor at holding water. In such a challenging environment, we would expect the flora to be species-poor, yet coastal dune floras of the CFR are remarkably rich in species. On average, dune habitats house only about 30% less species than habitats of the interior coastal lowlands – an impressive number given the small and fragmented areas they occupy compared to the expansive and contiguous habitats of the interior.

Coastal dunes at Cape St Francis in the Cape Floristic Region. These can be harsh environments for people and plants alike, with high solar radiation, strong, salt-laden winds year-round, and highly abrasive wind-blown sand. For the plants that manage to grow here, the highly alkaline, infertile and droughty dune sands present further challenges. Photo © 2020 B. Adriaan Grobler.

Another remarkable feature of the CFR coastal dune flora – estimated at about 1,000 species in total – is that over 30% of the species are endemic to dunes. Most of these species are typical of the Cape, being associated with dune fynbos, and include species of boegoe (Agathosma), Cape gorse (Aspalathus), heather (Erica), hardebos (Phylica) and the Cape-reed family (Restionaceae). There are, however, also several species of tropical origin that are dune-endemics; these are associated with dune thicket and include coastal cabbage tree (Cussonia thyrsiflora), seegwarrie (Euclea racemosa), dune olive (Olea exasperata), duine-kokoboom (Maytenus procumbens) and duine-sybas (Robsonodendron maritimum).

A very rare and highly threatened dune-endemic species of Cape gorse, Aspalathus cliffortiifolia, growing in coastal dunes at Schoenmakerskop in the Cape Floristic Region. This is just one of more than 320 plant species from the region that are restricted to dune habitats. Photo © 2020 B. Adriaan Grobler.

Further reading:

  • Cowling, R.M., Logie, C., Brady, J., Middleton, M. and Grobler, B.A., 2019. Taxonomic, biological and geographical traits of species in a coastal dune flora in the southeastern Cape Floristic Region: regional and global comparisons. PeerJ 7: e7336. https://doi.org/10.7717/peerj.7336

  • Grobler, B.A., Cawthra, H.C., Potts, A.J. and Cowling, R.M., 2020. Plant diversity of Holocene dune landscapes in the Cape Floristic Region: The legacy of Pleistocene sea-level dynamics. Quaternary Science Reviews 235: 106058. https://authors.elsevier.com/a/1b2H3-4PRtzDg (free access until 28 June 2020).

  • Lubke, R. and De Moor, I.J. (eds.), 1998. Field Guide to the Eastern and Southern Cape Coasts. University of Cape Town Press, Rondebosch.

  • Tinley, K.L., 1985. Coastal Dunes of South Africa. South African National Scientific Programmes Report, No 109. Committee for Nature Conservation Research, National Programme for Ecosystem Research. https://researchspace.csir.co.za/dspace/handle/10204/2353