THE ROLE OF DUNES IN COASTAL LANDSCAPESPosted on the 6th January 2013
(This compelling article by BirdLife Overberg member Penny Palmer on the delicate biology of coastal dunes at the Kleinmond area originally appeared in the December 2012 edition of 'Fynbos', the newsletter of the Kleinmond Nature Conservation Society. - Ed.)
The Dunes By Penny Palmer
Beaches are fascinating places and are ever-changing. Sand, which comes down in the rivers and streams, is constantly being brought on shore in each breaking wave. The dunes nearer the water line, the primary ones, are at the mercy of wind storms and high tides, and after one of our South-easterly gales that coincide with a Spring tide, one can see radical changes along a long stretch of beach. Many acid streams and rivers cross our beaches and colour the sea water over a large area, thus creating an estuarine environment in the surf. The Dawidskraal river, the outlets from Bass Lake and from Grootwitvlei, , all cross the beaches and change them radically, depending on how much water they bring down. During heavy rain, there are other streams that break through to the sea, an example being the Crystal River the flows under the western end of Porter Drive. All this freshwater has a marked effect on the land plants that usually grow in this otherwise alkaline sand. Some of this water is caught in the slacks between the rows of dunes, and here the sedges and other fresh water loving plants thrive. The courses of the lake outlets are easy to follow because of their unmistakable vegetation. Under the dunes are huge lakes of water that seep down from the mountains, on several occasions after a heavy fall of rain, there were large fresh-water lakes at the western end of Silver Sands beach. The presence of permanent underground water supports a mass of plants as well as the sedges that need a permanent supply.
The riverine plant community supports a variety of creatures from dragon flies, frogs, vlei rats to Black Crake and Gallinules – (which we now have to call African Purple Swamp Hens), and of course our Cape Clawless Otters. These animals have adapted totally to the sea and judging from the many spraints(droppings) found, live almost entirely off crayfish. This is a huge change from D J Verwoerd’s research results which yielded the following prey ; octopus – by far the favourite food, followed by bony fish, shore crabs Cyclograpsus punctatus, and lastly crayfish.
Slightly further away from the water line are the secondary dunes. These vary in shape and size and are also subject to wind carving activity, but the scattering of very tough vegetation gradually makes enough cover to stabilise the sand. The most notable plants here are the exotic Marram Grass – Ammophila arenaria., the Sea Pumpkin- Arctotheca populifolia with its large woolly leaves and beautiful yellow daisies, Sea Parsley –Dasispermum suffruticosum, and the sprawling Cineraria geifolia, with its masses of yellow flowers.
Further from the sea the dunes support thickets that are made up of an amazing collection of plants, some of which are specialists in that they thrive in alkaline nutrient poor sand, suffer the full blast of the Summer and Winter gales, sea spray, sand blasting and damage by ignorant property owners who do not know their worth. Were it not for this thick band of vegetation along our coastline, many of the valuable mansions built on coastal dunes, would have ended up in the sea, after the wind had blown away all the sand supporting the building, or worse still, smothered them under the dunes.
This vegetation binds the soil, and provides food and shelter for many creatures. Boucher describes it as complex and deserving of a study on its own.
In some places these thickets are only a few centimetres high, blown flat by the wind. These plants nevertheless, still provide protection for the sand and some reach several meters in length.
The more obvious species in these thicket are four species of Rhus , Milkwood-Sideroxylon inerme,(tree number 579) Olea exasperata (619)- dune olive, Osyrys compressa,(99)- pruimbas, Olea capensis subsp capensus (618)- vals-ysterhout, Euclea racemosa(599)- Kus-ghwarrie, Pterocelastrus tricuspidatus (409) -kershout, Cassine peragua (414)- Bos-lepelhout, and Robsonodendron maritimum (was Cassine maritima)(413.1)- duin-valssybas. Regrettable rare are fascinating trees such as the Maurocenia frangula (417)- aasvoëlbessie, which is confined to this area, Its leaves are almost round , thick and leathery and male and female flowers are borne on different trees. Chionanthus foveolatus subsp tomentellus(615) -Kaapse pokysterhout, a member of the Oleaceae, has a double row of deep pits alongside the mid vein on the underside of the leaves. It is however, the smaller less visible plants that are so fascinating. Starting with a bewildering variety of sedges, that need water all year round, there are the bright yellow Cineraria geifolia with its bright yellow flowers. the dark pink Senecio elegans which in places takes an almost succulent form, a fascinating Helychrysum niveum that looks like an erica with millions of tiny white flowers that cover its round cushion-like surface. Then there is the strange Galium tomentosum which produces masses of tiny flowers with an impossibly long corolla with the stigmas and anthers all crowded at their ends.
Quite recently it was discovered that our Tarchonanthus camphoratus is actually T. littoralis, whatever it is, this is a most amazing tree, particularly at Rooiels, where it grows among the rocks almost at the sea’s edge.
Creepers play their part in this mixture. The Cynanchum africanum – bobbejaantou, Asparagus asparagoides- krulkransie and Kedrostis nana- ystervarkpatats, all add to the seemingly chaotic mix of plants.
The under story is also remarkable as the density of the bush in many places shuts out sunlight, yet plants such as Knowltonia vesicatoria- brandblaar, Satyrium carneum -ewwa-trewwa, and Ferreria crispa – spinnekopblom and the shade-loving orchid Bonatea speciosa, seem to flourish. There is a very specialised Erica- E brachialis, which is a specialist in that it can resist sea spray and develops a sturdy trunk and thick branches. It can keep its ovaries on the plant for a long time, presumably, to shed them when the environment is conducive to their survival.
Anyone taking a good look at this vegetation will realise what a tough lot these trees and bushes are. In the more sheltered areas, such as Access Road, Betty’s Bay, and near the eastern- most bridge over the Kleinmond lagoon, some of the trees grow tall and straight, while in the more exposed areas, they sprawl along the sand and rocks and intertwine to form an impenetrable thicket, giving shelter to many creatures from beetles to birds.
Most of the plants are fruit-bearing and are host to many species, the most noticeable being the Cape Bulbul and Red Winged Starling, Southern Boubou Shrikes, Cape White-eyes, Olive Thrush and Karoo Prinias are also often seen, as are the Speckled Mouse Bird and the Ramaron Pidgeon, now known as the African Olive Pigeon.
Many of the trees found in these thickets, will grow further inland, and will provide an effective wind break, and if watered during the summer will grow fast enough to become shade trees after a few years. Among the suitable species are Milkwoods, the various Rhus species and Osteospermum monilifera(736.1) - bietou, this is classified as a tree and there are several huge old specimens in our region, that have been sheltered from fire for years and have been shaped into shade trees.
As can be imagined there is an incredibly delicate balance of all the factors that make up this amazing vegetation. The ph. is vital and this is seriously disrupted when aliens such as the Australian acacias, e.g. Rooikrans and Port Jackson are allowed to spread. They also upset the delicate balance in the event of a fire in that they increase the temperature significantly. Their ability to shoot down a strong tap root to water, almost before the first leaves appear, makes them very competitive and they can easily overwhelm the slower-growing natural plants. Their worst offence however is that they pump Nitrogen into the soil which is naturally almost bereft of any nutrients as most of the local plants have a mechanism for taking nitrogen from the air in the exact amounts that they need.
Once again I am indebted to that super sleuth Caroline Joubert, who has the uncanny knack of identifying obscure plants which try to disguise themselves as something completely different.
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Trees of Southern Africa. 2002. Update: Meg Coates Palgrave. Struik. Cape Town.
D J Verwoerd. 1987 .Observations on the food status of the Cape Clawless Otter –Aonyx capensis at Betty’s Bay. SA Journal of Zoology. Vol.22, No.1 pp33-39
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viii. PPJ Herman. Bothalia 32.1 21-28 (2002)