Dunes - natures coastal defense
Most beaches are backed by vegetated sand ridges called dunes, built up by dry beach sand blown inland and trapped by plants and other obstructions. As sand accumulates, the dunes become higher and wider.
Plants play a vital role in this process, acting as a windbreak and trapping the deposited sand particles. A characteristic of these plants is their ability to grow up through the sand and continually produce new stems and roots as more sand is trapped and the dune grows.
Stable sand dunes play an important part in protecting the coastline. They act as a buffer against wave damage during storms, protecting the land behind from salt water intrusion. This sand barrier allows the development of more complex plant communities in areas protected from salt water inundation, sea spray and strong winds.
The dunes also act as a reservoir of sand to replenish and maintain the beach at times of erosion.
The sensitive side of the beach
Frontal sand dunes are vulnerable. The vegetation can be destroyed by natural causes such as storms, cyclones, droughts or fire, or by human interference such as clearing, grazing, vehicles or excessive foot traffic. If the vegetation cover is damaged strong winds may cause 'blowouts' or gaps in the dune ridge. Unless repaired, these increase in size; the whole dune system sometimes migrates inland covering everything in its path. Meanwhile, with a diminished reservoir of sand, erosion of the beach may lead to coastal recession.
To avoid this, protecting the vegetation is vital. The beach between high and low tides is resilient but the sensitive dunes that we cross to reach it must be protected also. For this reason damaged and sensitive dunes might need to be fenced and access tracks for vehicles and people provided.
For the sake of our coast we must care for the dunes.
Plants on the beach
Vegetation on the beach and dunes tends to occur in zones, according to the degree of exposure to harsh coastal conditions.
Closest to the sea is the pioneer zone, extending landward from the debris line at the top of the beach in an area called the foredune or frontal dune.
Only specialised pioneer plants can colonise areas exposed to salt spray, sand blast, strong winds and flooding by the sea. They are often protected by waxy or hairy coverings on stems and leaves and grow low to the ground, offering little resistance to the wind. They have strong root systems and spread rapidly. They create a mesh of creeping stems so if one part is buried in shifting sand or uprooted another part can continue growing; and so they serve to stabilise the sand, forming and building dunes.
Beach spinifex grass (Spinifex sericeus) and goat's foot convolvulus (Ipomoea pes-caprae) are two of the most common of these herbland plants in the pioneer zone.
Close behind these plants on the frontal sand dunes, coastal she-oaks (Casuarina equisetifolia) are commonly found. The composition of this zone can reveal the condition of the beach. Extensive areas of herbland plants suggest that sand is accumulating; but where they are absent, especially if she-oaks are perched close to the high tide zone, erosion is likely to be occurring. Other trees of this zone are Pandanus species, coastal banksia (Banksia integrifolia) wattles (Acacia spp.), beach almond (Terminalia spp.) and beach calophyllum (Calophyllum inophyllum). The actual species found will depend on the climate. Fruits and seeds from all these species are often found in the flotsam.
Behind the frontal dunes, in areas protected from windy and salty conditions, vegetation depends on local circumstances. Freshwater swamps are usually dominated by paperbark teatrees (Melaleuca spp.) whereas, on higher better-drained ridges, woodlands of Eucalyptus and Acacia species or low rainforest (beach scrub) develop.
These zones are not fixed. As plants grow taller and humus such as dropped leaves accumulates, exposure to sun and soil conditions change. The soil becomes richer and holds more water. This enables scrub and woodland plants to move in, changing the type of vegetation type by a process called sucession.
More detailed information is available in Coastal Dune Management a series of technical notes about coastal sand dunes and their management.