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Plants of the Dunes

Hundreds of plants and animals, rarely found elsewhere in a county thrive on Cornwall's sand dune systems. Agriculture, sand extraction, tourism, mining and industrial activity on the dunes have had a huge impact on the habitats and landscape.

Sand dunes develop when deep-rooting vegetation traps the windblown calcium rich sand. In the shifting sands of the smaller and more mobile dunes backing the beach, specialist plants grow in the most extreme conditions. Further inland, more plants are able to colonise the sandy soil to create flower-rich dune grassland. Plants that grow on the sand need to be able to tolerate the alkaline, calcareous soil. Consequently, many of the plants on the dunes are only found on this habitat.

Probably the most important plant which assists in the formation of sand dunes is Marram Grass or Ammophila arenaria. Marram is specifically suited to growth in sandy conditions because it is:

  • Xerophytic or drought tolerant;
  • Halophytic or salt tolerant;
  • Capable of growing very deep roots, called rhizomes, in search of water. These roots can grow up to 7cm in 10 days.
  • Able to grow when sand blows onto it. It can survive when buried by 1 metre of sand a year.
  • Has leaves which curl up during hot and dry weather to reduce water loss (transpiration) and have a corrugated surface on the inside.

These specialist adaptations allow Marram to thrive in extreme conditions where it can be found almost to the exclusion of all other plants. The Marram root network is vital in helping to trap the sand, allowing other plants to colonise.

The gradual change from bare sand at the back of the beach to flower-rich dune grassland as you move inland and eventually scrub and woodland on a large sand dune system is known as natural succession. The youngest dunes are at the back of the beach and the oldest dunes, furthest inland. The dunes are effectively growing out to sea. However, they can only grow so far, because the sea will erode away the front of the dunes.

With natural succession, the number and types of plants change as you move inland and the chemical and physical nature of the soil alters.  The number of different plant species generally increases as conditions become less harsh. As you move inland:

  • The ground becomes less salty (saline) as the influence of the sea air is reduced;
  • The ground becomes damper as the increased numbers of plants and organic matter hold more water;
  • The amount of humus or organic matter increases;
  • The pH of the soil becomes more acidic due to increased humus and leaching by rainwater of calcium carbonate in the sandy soil and;
  • The area of exposed sand decreases and conversely, plant cover increases.

Dune succession can be impacted by human activities with developments, such as golf course, agriculture or property development, squeezing and limiting the range of different habitats.

Sand dunes are important largely due to the highly variable conditions within them, forming a complex mosaic of habitats including the following:

  • Strandlines
  • Embryo dunes
  • Mobile foredunes
  • Semi-fixed dunes
  • Fixed dunes
  • Calcareous grassland
  • Dune slacks
  • Scrub
  • Woodland

Within each area of habitat there will be further subtle variations in conditions which increase the structural diversity allowing a higher number of plants and animals to flourish. Factors which can increase structural diversity include:

  • Grazing pressure
  • Aspect (angle to the sun)
  • Human pressure
  • Shelter from the wind
  • Proximity to the water table
  • Soil conditions

On many of the smaller Cornish dune systems, the whole range of dune habitats may not be present. On the smallest dunes there may only be the mobile and semi-fixed dune habitats present. 

The strandline and embryo dunes are very bare, with just a few pioneer plants with special adaptations allowing them to grow in salty, dry, moving sand. These mostly annual plants will come and go from year to year and over the seasons and the young dunes continuously change shape. The habitat is very transient and vulnerable to human disturbance because the plants are only just clinging onto the sand. 

After the pioneer plants of the embryo dunes have started trapping more sand, a few other plant species are able to grow. One of the most important and common dune building plants is Marram Grass. The tall Marram leaves slow the wind- carrying sand and the network of deep roots bind the sand together. The mobile dunes are, as the name suggests, always moving around and changing shape. There is usually quite a lot of bare sand exposed and the wind can easily blow it around.
As the Marram Grass traps yet more sand and the soil conditions start to change lots more different plants can grow and the habitat looks more like a grassland, rather than a beach with a few plants growing. As the name suggests this habitat is still fairly mobile and there is still a lot of movement of sand, but the soil will have slightly more organic matter in it and may be slightly less alkali.

The fixed grey dunes are the richest in plant species as conditions are less harsh further inland. From a distance, the dunes appear grey due to the abundance of mosses and lichens, but close-up there is a mass of different flowers creating a colourful, flower-rich grassland, especially in May, June and July. The sand is fixed by the vegetation and the habitat receives little wind blown sand. 

Rabbit or livestock grazing creates a closely cropped, short grass sward, often in sheltered hollows whilst taller grasses and flowers thrive elsewhere. Marram is more scarce as shallow-rooting plants dominate, rapidly soaking up available rainfall.

Dune slacks form when the wind scours the sand until it reaches the water table. These are wet or damp habitats and may be permanent ponds. More often they are only wet for the winter when rain is heaviest and the water table highest.Dune slacks are mostly found on the larger dune systems. Plants which thrive in wetter conditions can be found here and are often very different from the surrounding dunes.

Areas of isolated scrub are an important feature on sand dunes as rabbit warrens and shelter for birds, invertebrates and reptiles. However the frequency and extent of scrub needs to be carefully monitored and controlled.  Typical Cornish dune scrub plants include Hawthorne Bramble, Ivy Elder, Blackthorne Wild Privet and European Gorse.

Woodland is absent from most of Cornwall's dunes. It would normally be found on the landward section of the dunes, as the climax community of natural succession. In many places, dune woodland is unable to form due to developments or agriculture.