The Dune Lifecyle
It’s easy to look at a sand dune and just see a pile of sand, but lots of different factors and processes are involved in making a coastal sand dune system. In fact, sand dunes have a lifecycle, with young dunes forming at the beach and old dunes pushed further inland.
Nature’s process of building a sand dune is called succession. Waves push sand up onto the beach, then sand moves around the coast and forms dunes when it’s picked up or pushed around by the wind. If there are any obstacles on the coast, the larger sand grains will be dropped in front of it and pushed up it while smaller grains of sand are often deposited behind it. As this process continues, ridges of sand build up and can start to form a sand dune. The stronger the wind, the higher the dunes!
The wind-facing side of the dune often has a gentle slope, while the sheltered side is much steeper. As the wind is always changing, dunes are also always changing, growing and shifting…
Embryo dunes are the youngest sand dunes. They are the earliest stage in a dune’s life. These dunes face the beach, are made up of mostly exposed sand, and are also the smallest dune stage – reaching just up to a few metres tall. Here pioneer species like marram grass are the first to colonise the sand and stabilise it with their long roots.
Behind the embryo dune, the fore dune often stands a few metres taller, with more vegetation and less bare sand. As more plants grow in the sand, the dune is less able to move and the pH of the dune becomes less alkaline, making it easier for other secondary plant species to now grow.
With pioneer plant roots now reaching deep into the dune, semi-fixed dunes are pretty stable, but the exposed yellow sand on their surface can still move. Depending on the wind, these dunes continue to accrete sand blown up from the beach, or the sand on these dunes can be blown over the ridge of the fore dunes to grow the dunes behind them. These are also known as yellow dunes.
Sand dunes are separated by dips, which are known as dune slacks. Slacks are formed as vegetation and sand is removed from the front of one dune and the sheltered back side of another. If these slacks erode far enough to reach the water table, freshwater pools can form. These slack pools are fantastic habitats for dune wildlife such as natterjack toads, which use the pools for breeding.
Areas of sand dunes which have exposed sand can become dune blowouts. These areas allow dune migration and dynamism, as the bare sand can be picked up by the wind and blown elsewhere in the dune system.
As you move further away from the beach, the dunes become less yellow in colour and begin to turn grey. This is because these dunes are starting to build up more humus (the broken-down organic plant matter which is found in soil) from the diverse plant life and bacteria which now lives in these stable dunes. These dunes are also better at holding in water, making them able to support larger shrubs. These are also known as grey dunes.
Mature dunes are the furthest away from the beach, are the oldest dunes in sand dune system and often have very little exposed sand. Lots of broken down organic plant material in the stable sand make these inland dunes accessible to some larger plants. In in some mature dune systems you might even see small forests of pine trees, which have been planted.