How is a Sand Dune Formed?
Sand dunes are an iconic feature of the Northumberland Coast. They are truly stunning spectacles of beauty, and they play a huge role in the coastal ecosystem. Most of the dune systems in the county are protected as they are nationally and globally important for the species they support. But what is it about the dunes in Northumberland that make them so special? To answer this question, we first need to look at how sand dunes are formed – a process known as succession which can take hundreds of years.
Dune systems are incredibly dynamic due to sand constantly shifting as a result of wind and tidal action. Sand dune formation begins when blowing sand hits an object which prevents is from blowing any further. This could be a pile of seaweed, debris or rocks high up on the beach. Sand continues to build up until a small mound forms – this is known as an embryo dune.
As they increase in size, embryo dunes themselves can become obstacles which block other objects blowing in the wind – such as seeds! Growing conditions are harsh – a salty, dry environment means only plants which have adapted special features to help them cope with these conditions will grow. Sea rocket and sea sandwort are two such plants, and they have waxy leaves to trap moisture and protect them from sand blow. The growth of sea rocket has been vital to the survival of the ringed plover chicks at Embleton in 2021 as it has provided areas of cover on the embryo dunes where there previously was bare sand.
Sea rocket growing on an embryo dune at Buston Links
The two plants to next colonise the embryo dunes are an essential part of the succession process. Lyme grass begins to grow and stabilises the dune with its shoots that grow under the sand. This stabilisation then allows another grass to establish – Marram grass. This quick growing grass is possibly the most recognised feature of sand dunes and is the main dune-fixing plant in Northumberland due to its deep roots further stabilising the sand. Natural processes of plant growth and decay results in organic matter entering the system and enabling it to hold more water. With growing conditions becoming less harsh, a greater diversity of plants are able to grow. A dune at this stage of succession is called a fixed dune.
Dune erosion at Embleton showing the extensive root system of marram grass
Fixed dunes support a stunning array of wildflowers which carpet the ground in a sea of colour. The first to bloom in March are primroses and cowslips, and from then until late September, the wildflowers provide an essential source of nectar to bees, butterflies, moths and other pollinators, which in turn are vital food for birds and their young. This delicate part of the coastal ecosystem only exists for as long as that part of the dune remains in the fixed dune stage of succession.
Bloody crane’s-bill and bird’s-foot-trefoil in flower at Football Hole, Newton-by-the-Sea
Where areas of the dune system are damaged and the sand blown out, a dune slack may appear if the ground is below the water table. These moist patches of dune support a delicate community of plants such as grass of parnassus and petalwort, the latter being a tiny member of the liverwort family and can be found in the damp dune slacks on Holy Island and Bamburgh. It is a very rare and precious plant which is only present at 10 sites in the whole of the UK.
Grass of parnassus in flower
Naturally, fixed dunes are trying to progress to the next stage of succession, which is a community of scrub and eventually woodland plant species. The more organic matter entered into the system, the more moisture it holds, and the decaying leaf matter produces a soil. Dune grassland wildflowers are no longer able to grow due to the more acidic conditions and shade produced by the taller vegetation.
Dune succession is a completely natural process which occurs over many, many years. Scrubland and woodland are important habitats, but this can exist almost anywhere. Dune grasslands can only exist under a set of very specific conditions and in limited locations, and the ones we do have in Northumberland are recognised as being internationally important for the biodiversity of life they support. To prevent succession to scrubland from occurring, fixed dunes can be managed to keep the dune grasslands healthy and thriving. Careful grazing by cattle over autumn and winter keeps scrub growth under control and keeps growing conditions perfect, ready for the first flowers to bloom in the spring.
Cattle grazing the dune grasslands at Druridge Links
Another threat to the dune grasslands is pirri-pirri bur, a spectacularly successful invasive non-native species which thrives on disturbed ground and patches of bare sand, which can smother and overgrow our native wildflowers. You can read more about this plant and how to prevent its spread in a previous blog post – http://spaceforshorebirds.co.uk/blog/pirri-pirri-bur-and-dogs/
In the next instalment of our series of blog posts about our dune grasslands, we will take a closer look at fixed dunes, the flora and fauna that thrive here and why careful management is needed to prevent this vital habitat being lost.
