Succession on coastal dunes

As a dune forms, plant succession occurs. The conditions on an embryo dune are harsh, with salt spray from the sea carried on strong winds. The dune is well drained and often dry. Rotting seaweed brought in by storm waves adds enough nutrients to allow pioneer species to colonise the dune. These pioneer species are typically sandwort, lyme grass and marram grass. These plants are well adapted to the harsh conditions of the fore dune, typically having deep roots which reach the water table, root nodules that produce nitrogen compounds, and protected stoma (leaf pores), reducing transpiration. The deep roots also bind the sand together, and the dune grows into a fore dune as more sand is blown onto the grasses. The grasses shelter, stabilise and add humus to the soil, meaning other, less hardy plants can then colonise the dunes. Typically these are grasses such as red fescue or herbs such as cat's ear. These too are adapted to the low soil water content and have leaf adaptations which reduce transpiration. These plants gradually add humus to the soil, increasing its water-holding capacity.

Young dunes are sometimes called "yellow dunes", dunes which have high humus content are called "grey dunes". Leaching occurs on the dunes, washing humus into the slacks, and the slacks may be much more developed than the exposed tops of the dunes. Leaching also removed lime from the upper horizons of the soil leading to gradual acidification and reflected in a consequential change in the flora and fauna of these areas. Eventually, this may enable heather to grow and lead to the establishment of dune heath – a rare habitat in Wales. Wind erosion can remobilise the lime from lower levels and return it to the upper soil horizon.

As the dune soil develops, opportunities may arise for scrub such as hawthorn, willow or birch to grow and eventually for the development of native dune wooodland. This is now a rare habitat in Wales, having generally been reclaimed for agriculture, golf courses or other development. In contrast, the planting of conifer plantations on dunes been extensive. Once the conifer canopy closes, the native ground flora is shaded out and in many areas only a thick carpet of acidic pine needles is apparent.

Succession is a process in time, but since the coastal dunes are younger than those inland a walk from the coast back through the dunes is a surrogate for time. This illustration of succession is an important scientific feature of dunes, extensively used in teaching and research.