Sand dunes
The information presented here has been written by Ted Brambleby, Adventure Education,
Marine Environments Field Study and Resource Centre, Hastings Point Beach, NSW

Sand is made from 2 sources 1) Quartz particles from eroded bedrock and discharged from rivers or eroded Rocky Shores where there is great river out puts such as in SE Australia quartz particles tend to pre dominate. 2) Calcium carbonate derived from shells of molluscs, Forameniferans and Bryozoans which form them at sea.

Sand magnified clearly showing quartz grains Shells which eventually will be broken down

Calcareous sediments predominate along south western and south Australian coasts where there are less river outputs. Our north coast beaches are gently sloping cause of low wave energy. The tiny sand grains they comprise retain water and maintain oxygenation through circulating a slow capillary action between them. These conditions make our beaches ideal for occupation of a diversity of animals.

Bacteria, which thrive between these grains of sand, use up oxygen quickly so that only the surface regions of beach sand is sufficiently aerated to support life and it is here that animals congregate ie Polychaete worms, Swimming crabs, Moon snails, Pipis, and Sand bubbler crabs.

Deep layers lack oxygen and are dark due to the predominance of sulfur producing bacteria that release hydrogen sulfide ("rotten egg gas"). Because of it's oxygen availability and abundance of surface organisms Ghost crabs search for food on the surface at night but remain deep in burrows during the day to avoid dehydration. Bivalves such as pipi live permanently in the diatom (their food) surface rich layers and avoid dehydration by moving with their wedge shaped muscular foot up and down the beach with the tide.

Pigface

Tuckeroo seeds

Insectivorous plant
from Wallum heathland

Key members of nature's green dune squad and their significance

Higher up on the dunes it is salt winds that is the invasive force. Aerial parts of vegetation block the wind energy and cause sand to deposit around the vegetation. A characteristic of dune vegetation particularly grasses like spinnifex, pineapple sedge and dune carex is their ability to produce up right stems and sand trapping rhizomes that can grow firm roots in response to sand coverage. This sand deposition around plants results in increase height and width of the dune, a process known as plant induced dune expansion.

The ability of pioneer plants such as Spinifex and Pigface to hold wind blown sand on the frontal dunes helps create conditions which encourage the establishment of other communities such as Woodland Scrub (Acacias, Tuckeroos) Heathland (Banksias, Xerophytes such as Pea plants, Heath and Boronias and Melaleucas and Coastal Forest (Eucalypts and Angophora). All plants (Herbs, shrubs ,reeds, grasses, trees) are of equal importance in developing vegetation. Like the interplay between diverse cell types within the connective tissue meshwork of our skin underlay (dermis) that stops the outer skin from splitting and falling apart these plants enable dune stabilisation to continue indefinitely.

The whole process of dune formation and succession depends primarily on the pioneer plants and of all of these Spinifex is the chief. Spinifex hirsutus (The Captain Cook of the dunes). This is the most successful sand trapping dune coloniser and pioneer along our northern beaches. It provides the basis for a dune and establishes the first of a new set of environmental conditions conducive to the radiation of a more diverse vegetative cover.

These conditions include increased shade, reduced sand temperature, reduced wind movement, and lowered evaporation rates from the sand surface and a catchment for wind blown seeds. This inturn further lowers rates of water loss from leaves. The increase plant diversity it promotes enriches leaf litter and the accumulation of humus. This is the key to a better water holding capacity of dune soils, thus paving the way towards the expansion of bio diversity (Plant and Animal) and stabilisation of successive communities.

This succession conforms to the following set pattern as long as intervening factors don't interrupt the process.

What might some of these intervening factors be?

The set pattern

1. Pioneer Zone Primary stabilising plants eg grasses and herbaceous plants establishes the basis for
2. A Woodland or Scrub Zone with secondary stabilising plants consisting of Acacias, vines stunted trees, banksias and a few stunted herbs. These establish conditions conducive to the development of
3. Tertiary Stabilising Plant Zone of a) Heathland (low shrubs) where soil drainage is poor or relatively unprotected from sea wind); b) Forests (trees, high drainage and a history of protection from the sea)
4. Wetlands Mangroves, Melaleuca t tree swamps, Marsh plants such as sedges, succulent salt plants and salt couch occur where estuaries interrupt a dry succession.