Sand Dunes Develop When Deep-Rooting Marram Grass Traps the Windblown Seashell Sand, Rich

Plant Adaptations in Sand Dunes

Sand dunes develop when deep-rooting marram grass traps the windblown seashell sand, rich in calcium. In the shifting sands of the smaller and more mobile dunes backing the beach, specialist plants such as sea holly and sea spurge 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 shell 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 and are rare in the rest of the county and Britain.

Natural Succession

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 'chop' off 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;  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.

1. Strandline/Embryo dunes

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. .

2. Mobile Foredunes

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.. 3. Semi-fixed Yellow Dunes

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. There is still a lot of Marram Grass in this habitat.

4. Fixed Grey Dunes

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.

Plant adaptations for life on the dunes

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;  Able of growing when sand blows onto it and thrives in these conditions and;  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.

Marram Grass Factfile

 Scientific studies have demonstrated that Marram Grass can die if repeatedly walked over 10 times.  Marram roots can grow up to 7cm in 10 days.  Marram can survive when buried by 1 metre of sand/year.

Other adaptations for life on the seaside desert shown by Orache, Sea Holly, Sea Sandwort and Sea Rocket include:

 Waxy leaves which seal the water in the plant. The wax also allows excess salts to be secreted onto the leaf surface without harm to the plant and;  Fleshy stems and leaves to hold moisture

Source: http://www.cornwall.gov.uk/default.aspx?page=13684