What is it?
A sand dune is a hill or ridge beyond the reach of the tides, formed of sand over many years. They are ever-changing structures, and fascinating for it. Dunes usually run in bands parallel to the beach, getting taller and more chaotic the further away you get – ‘blow-outs’ where craters are formed by sand being blasted away by the wind, and wet depressions called ‘slacks’ dipping below the line of the ridges.
Travelling further from the beach towards the back of the dunes, you can clearly see the vegetation type change, often seen over several hundred metres or more. Gradually, given enough time and fresh sand delivered by the winds and tides, the whole system grows while shifting seawards, leading to this succession of vegetation; each dune ridge going through the different vegetation phases in turn.
Dune systems can also shrink, eroding from the seaward edge (foredune) as a consequence of storm events, rising sea levels or an interruption in the supply of sand.
Why is it like this?
Dunes are formed from sand blown inland from the beach by onshore winds, and trapped by debris or strandline plants. Accumulating sand makes good habitat for tough beach grasses such as sand couch and lyme grass, whose strong horizontal roots stabilise the collected sand, encouraging more to settle. Soon, these grasses are completely engulfed by the sand, and are replaced by fast-growing marram grass that keeps pace with the accumulating sand.
Eventually, as new pockets of sand begin collecting, the salinity of the existing dune reduces, and the dune surface begins to stabilise, allowing finer grasses, herbs and lichens to grow. As the plants decompose they add some nutrients to the substrate, but much of this is leached out of the stabilised substrate by rain, making conditions suitable for acid-loving plants like heather.
In places, the wind can scour the sand away to form a hollow, and if this meets the water table (which can be drawn upwards by the sand), dune slacks can develop, and may support particularly rich wetland vegetation. Although usually found on flat coastlines, sand dunes can also be found “climbing” cliffs where onshore winds are strong.
Distribution in the UK
Sand dunes occur widely around the coastline of the UK, with particularly extensive areas in the Hebrides and fewer along the south coast.
What to look for
Typically, dune systems made of lime-rich shell sand promote the widest range of plants. In the summer, fixed dunes are dotted with clovers, lady’s bedstraw, pyramidal orchid, kidney vetch and carline thistle.
Dune slacks are easily picked out from a distance by the flat terrain and greener vegetation (a result of seasonal flooding) – look out for marsh orchids and helleborines, particularly in the south. More acidic systems will support wet heath communities in slacks and lichen-dominated communities on drier fixed dunes, which appear grey. Prickly saltwort, sea rocket, oraches or sea sandwort growing along the strandline are good indicators of a healthy sand dune system.
Dunes can warm up quickly and sheltered areas may be good for invertebrates – look for bumblebees and little piles of sand left by burrowing bees and digger wasps. Warm, open areas can also be good for reptiles – lizards often leave distinctive tracks meandering across looser sand.
Some individual dunes may be just a few years old, while the oldest dune systems date back 9000 years, but old or young, all dunes are naturally dynamic, presenting conservation challenges in a landscape where little space is allowed for mobile habitats. Hemmed in by golf courses, farmland, and development, and affected by hard sea defences that change the availability of new sand, dunes systems can become fossilised. The natural processes that create sand dunes are interrupted, meaning that species, particularly those dependent on early successional stages, are lost.
Climate change presents a significant challenge, as many dunes systems lack the space to roll inland as sea-levels rise. Recreational pressure can be an issue – strandline and foredune species are particularly vulnerable to trampling, and damage to these affect the future development of the dune system.