IGCSE Geography Sand dunes SQA

Sand dunes Biosphere

Biosphere Sand dunes Strand line/Embryo dunes Plants growing at the back of the beach, together with flotsam and jetsam dropped at the high tide mark eg seaweed, dead seabirds and driftwood, cause the wind speed to drop locally. Sand being carried by the wind can then fall at these points and begins to build small sand piles. This now forms another more substantial wind break, which encourages more and more sand to be deposited. The sand dune is beginning to build. Depending on wind speed, the dune will either gradually move inland (high wind speeds), or it will remain static. If there is a good supply of sand, new dunes may form to seaward (between the dune and the sea).

Biosphere Sand dunes Strand line/Embryo dunes These embryo dunes may disappear as quickly as they form but some may eventually be colonised by plants which stabilise the sand. Conditions here are very extreme:. >highly alkaline pH values (over 8) from the calcium carbonate of the shell fragments; > rapid drainage (because sand does not hold water well) and the desiccating effect of on-shore winds ; > high salinity from frequent immersion in sea water; > no humus for nutrients; > high wind speed and lots of salt spray. Consequently dunes may not grow higher than 1m

Strand line/Embryo dunes Biosphere Sand dunes Strand line/Embryo dunes Species which are dominant in this hostile environment are Sea twitch grass and Sea rocket. Sea couch grass can also survive regular immersion in sea water. Sea couch grass. Sea rocket. These early colonisers are very tolerant of salt and can therefore grow much closer to the sea than most other grasses, and help to stabilise the embryo dune and decay to provide humus for the plants of the next stage.

Biosphere Sand dunes Fore dunes If the embryo dunes persist they can be colonised by lyme grass, sea couch grass and sea holly, developing them into fore dunes. The pioneers from the previous stage have added humus to the sand, making conditions more favourable. Marram grass These plants are drought-resistant (sand does not hold water) and capable of withstanding burial by the shifting sand, as well as still significant salinity levels. Falling levels of calcium carbonate (shells) reduce levels of alkalinity. As they grow up through the sand, they help trap more sand and so the dunes increase in height. Here the dunes may reach up to 5 metres. Importantly, these improvements lead to the marram grass taking over from sea couch grass. It thrives on the fresh supplies of sand and its long root system (rhizomes) helps to stabilise the dune still further encouraging it to build up further .

Biosphere Sand dunes Fore dunes Lyme grass Marram grass

Biosphere Sand dunes Yellow dunes As the fore dunes continue to build up we now call them yellow dunes due to the still large amount of sand content visible. These yellow dunes begin to show a greater diversity of plants as conditions become more favourable. As plants die and decay, a humus layer builds up and this traps both fresh water and nutrients. The pH is now only slightly alkaline (about 7.5), there is more shelter and less salt spray. Salinity also drops with distance from the sea. Marram usually still dominates the vegetation but other plants are taking hold on the stabilised surface. Plants may include creeping fescue, sand sedge, mosses, lichens, sea holly and sea spurge.The dunes by this stage may well have reached between 5-10 metres high. As much as 80% of the sand surface may now be vegetated. Additionally, rabbits and other mammals may add their droppings to help enrich the developing soil.

Biosphere Sand dunes Yellow dunes

Biosphere Sand dunes Grey dunes The grey dunes are much more stable and mosses and lichens fill the few remaining spaces between plants so that vegetation cover may reach 100%. Marram grass becomes less common and appears now in isolated patches due to less sand being blown onto the dune and other plants out-competing it for water and nutrients. Small shrubs (brambles, gorse, buckthorn) appear for the first time. There is shelter from the harshest winds, humus is beginning to darken the surface layers and a true soil begins to form. Soil pH is increasingly acid and heathers may take advantage of the acid conditions. Water content is still low and plants have to search for water with their spreading root systems. These large dunes are commonly 10 metres in height and wider than those dunes nearer the shore.

Biosphere Sand dunes Grey dunes

Biosphere Sand dunes Dune slacks The dune slacks are found in between the grey dunes and the more mature dunes, where the water table reaches the surface causing seasonal (and sometimes) permanent waterlogging and surface water. Plants which are well adapted to these damp, sheltered hollows include rushes, sedges, cotton grass and creeping willow. If decay is slow, a peaty soil may develop.

Biosphere Sand dunes Mature dunes The most mature dunes are found several hundred metres from the shore. High humus and water content allow these plants to succeed previous vegetation. Left undisturbed these dunes develop a soil which can support shrubs and trees including hawthorn, ash and birch. Eventually an oak climax vegetation may develop.