Sand dunes are accumulations of sand stabilised by vegetation. The development of a coastal dune system requires a source of sand, and an onshore prevailing wind of at least 4.5 metres per second.
Tide levels rise and fall, inputting sand to the system, which is then dried by the wind. Individual sand grains are able to take to the air and creep along the shore. They land, and dislodge a couple more grains, which in turn take to the air and do the same. This process is called saltation. The grains stop when they strike a static object or obstruction, such as a pile of seaweed or lump of driftwood.
Sand and detritus will soon start to pile up against the obstruction, and seeds carried in by the wind or sea will do the same. The seeds are able to germinate due to the organic matter and moisture in the detritus. Wind blown sand will continue to become trapped against the growing plants, forming a small dune referred to as an embryo dune.
Embryo dunes are characterised by pioneer plant species such as sand couch (Elytrigia juncea), sea sandwort (Honckenya peploides), sea rocket (Cakile maritima), prickly saltwort (Salsola kali), sea mayweed (Tripleurospermum maritimum) and various oraches (Atriplex spp.).
Over time as the pioneer plants continue to grow, the small dunes become stabilised by root systems under the surface. With the continuous input of wind-blown sand, the dune will build up and increase in size. The early colonising plants will eventually die off and decompose, adding nutrients and organic matter to the substrate. As conditions improve over time, new plants are able to grow, taking over from those previously in the area. This is a process known as succession.
These new (and often larger) plants are able to trap more sand, and eventually larger dunes called fore dunes develop (also known as mobile or yellow dunes). Fore dunes are characteristically dominated by marram grass (Ammophila arenaria).
In time, the dunes stop growing due to a lack of wind-blown sand. They are then referred to as fixed or grey dunes. During the development of fixed dunes, younger embryo dunes are continually forming at the base of the dune system. This results in a (theoretical) dune profile as illustrated below left.
The nature of the fixed dune community is extremely variable depending on the pH of the substrate, and activities of grazing animals such as rabbits. If there is a lot of shell-sand present, the conditions will be alkaline and a diverse community of species will develop including the Creeping Willow (Salix repens), Silverweed (Potentilla anserina), Marsh Pennywort (Hydrocotyle vulgaris), Creeping Bent grass (Agrostis stolonifera) and various sedges (Carex spp.). If there is little shell-sand present, the conditions will be acidic and a smaller community of species will establish. Plants typical of acidic, low calcium soils (calcifuges) will grow here, including Heather (Calluna vulgaris), Bell Heather (Erica cineraria), Sand Sedge (Carex arenaria), and lichens and mosses.
In the development of the sand dune ecosystem, it is not uncommon for a dune slack to be created. This is when a blow-out (a damaged area of dune knocked back to bare sand by a combination of erosion and strong wind) takes the sand below the level of the water table. Dune slacks are variable in their nature depending on the frequency of inundation (which relates to the height of the blow-out floor relative to the summer/winter water table), the pH of the ground water and the substrate, the age of the slack, and the intensity of grazing. If the dune slack is old enough then the community may develop to scrub, with willow (Salix spp.), and moisture loving associates such as marsh pennywort (Hydrocotyle vulgaris), marsh orchids (Dactylorhiza praetermissa), and various rushes and sedges.
Succession is a directional change in plant and animal communities with time. There are two major types of succession – primary and secondary. Primary succession occurs on sites that have not previously been occupied by vegetation. The environmental conditions tend to be harsh and unfavourable, and the process is typically slow because of this. An example of primary succession is the development of a sand dune ecosystem. Secondary succession occurs on sites that have previously been occupied by well established communities. Conditions in these areas are much more favourable, making secondary succession a faster process. An example here is regrowth after a forest fire.
A place where succession occurs is termed a sere (f rom the Latin serere = to put in a row i.e. a sequence or series). Succession can occur in a variety of environments, and in order to distinguish which environment you are dealing with the word sere is prefixed with other words, so for example: Hydrosere refers to succession in a frashwater environment e.g. a pond or lake (from the Greek hydor = water); Xerosere refers to succession in a dry environment e.g. a desert (from the Greek xeros = dry); Psammosere refers to succession in a sandy environment e.g. a sand dune (from the Greek psammos = sand); Lithosere refers to succession in a stony environment e.g. a rock face (from the Greek lithos = stone); and Halosere refers to succession in a salty environment e.g. a saltmarsh (from the Greek hals = salt).
The events that occur in primary succession can be divided into a series of stages called seral stages. These stages are common to any primary successional sequence (i.e. the process occurs in this way in any of the habitats mentioned above).
Seeds and spores arrive on the site. These can be carried in by wind, waves, birds, animals etc. If the seeds and spores are able to germinate and grow, the community enters the next seral stage. The migration stage will continue for as long as the community continues to develop.
Seeds and spores germinate and develop. Physical conditions are unfavourable, there is lots of bare ground and the vegetation is described as “open” (lots of open space between plants). The plants that are able to grow in these harsh conditions are specialised to their environment, and are termed pioneer species.
Species become more established on the site. The physical conditions have been modified and improved by the presence of the community, so there is an increase in the variety of species. The vegetation is becoming “closed” as there if less bare ground available for vegetation.
The number of species on the site is increasing, and plants have to compete for space, light, nutrients, water and so on. Opportunistic pioneer plants often die out at this stage, to be replaced by equilibrium species which tend to be better competitors.
Few, if any, new species are added as competition resolves itself and the community becomes balanced. Each species occupies its own niche, and therefore avoids having to compete strongly with other species. The community stabilises and remains much the same over time.
No new species are added and the community remains the same over long periods of time (theoretically forever). The vegetation is said to be in equilibrium with the environment; a true state of balance has been achieved. The type of climax vegetation present is determined by the climate of the area, wind speeds and direction, animal grazing, pH, temperature and many other factors. The climatic climax for the UK as a whole is deciduous woodland (oak and ash trees), however different ecosystems will have alternative climax vegetation types (for example reeds or rushes on a saltmarsh; willow on sand dunes) as localised conditions prevent the climatic climax from developing.
Remember that succession is a temporal process; the community changes with time.
As succession proceeds there is a notable increase in biodiversity, however it will often level off, or even decrease, towards the end of the sequence as the community reaches equilibrium. Soil organic matter and nutrient levels increase throughout a successional sequence, which in turn leads to an increase in biomass as the community develops.
How does vegetation change from the seashore inland?
How do soil characteristics / microclimate change with distance from the seashore?
Comparing two area of the dune system, e.g. ungrazed/grazed sections of the fixed dunes
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