Biosphere Sand dunes evolution of vegetation communities as ecosystems to climax stages as demonstrated by vegetation succession on coastal dune belts, ie colonisation, the role of pioneer species and a knowledge of plant names at various stages in the succession 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.

Sand dunes Biosphere Mature dunes

Biosphere Sand dunes Pioneer stage Both species have a waxy coating on the leaves which enables them to retain water. The leaves are often rolled, which is another adaptation which reduces the leaf surface area exposed to the atmosphere. This decreases the plant transpiration rates. These will be some of the first plants to begin to stabilize new dunes with their networks of root systems. By helping to bind the sand in one place, they produce a more stable wind break, encouraging yet more sand to be deposited and building the dune still higher. However, as the amount of sand being deposited increases, these pioneer plants cannot grow quickly enough to avoid being smothered. Where the rate of sand deposition is greater than 30cm per year, they will be outcompeted by Marram Grass. Marram Grass then takes over the process of binding the dunes. This species is less tolerant of exposure to sea water and therefore occurs slightly further away from the sea than the first pioneer species. Its growth can keep pace with sand being deposited at a rate of up to 1 metre per year. Each plant can also spread sideways by up to 3 metres a year, helping to trap the sand and keep it in one place.

Biosphere Sand dunes White dune stage Marram Grass stabilizes the sand some distance below the surface because of its deep vertical root system and extensive horizontal root network. The clumps of Marram Grass are continually buried, only to regrow up through the newly deposited sand. Under dry atmospheric conditions, the leaves of Marram Grass will roll up tightly, exposing only the tough outer waxy cuticle of the leaf. This reduces transpiration and water loss. These later plant pioneers are more efficient at binding the surface sand, both because of the cover they provide and because of their near surface horizontal network of roots. ·                    Salinity decreases with increasing distance from the sea ·                   Humus content increases slightly from colonisers ·                   Soil acidity increases slightly due to less calcium carbonate ·                   Plants e.g. Sea Holly continue to stabilise the dune, allowing the key establishment of marram grass ·                   Marram grass succeeds Couch grass. It thrives on fresh supplies of sand and it’s long roots stabilise the dune further and help the dune build up

Biosphere Sand dunes Gray dune stage The dunes by now have a more or less continuous plant cover, effectively anchoring the dune sand in one place. Where a series of dunes has formed at the back of the beach, this stage will be on the dunes further inland. In colder autumn and winter months much of the available water will come from dew being deposited at night. Developing humus layers from decaying vegetation will help to retain some of the moisture. Water will still be scarce in hot summer months. As sand stops being deposited, Marram Grass will die out. Grey-green lichens (Peltigera, Cladonia) will colonize the dune extensively, combining with accumulating humus to give the dune sand a characteristic dirty grey colour. Mosses such as Bryum and Sand-dune Screw-moss, Tortula ruraliformis, also colonize the dunes. The lichens and mosses become dry, hard and dormant when water is in short supply, but as soon as it becomes available, they start to grow again. In high summer, the dried lichens and mosses give the dunes a very crunchy texture, which is almost like walking on crisps. Where there are large quantities of pulverized sea shells included in the dune sand, plants which thrive in lime-rich areas will become established.

Biosphere Sand dunes Gray dune stage SEMI-FIXED DUNE   ·                    Once yellow dune over 10m high, less sand builds up behind it. Marram grass dies out and forms thin humus layer. ·                    Other plants able to grow here e.g. dandelions

Biosphere Sand dunes Gray dune stage GREY DUNE / FIXED DUNE   ·               Damper soils with higher humus content ·               More acidic due to leaching of carbonates by rain ·               Shelter increases, so sand and salinity decreases Wider variety of plants develop e.g. heather, grasses, lichens

Biosphere Sand dunes Gray dune stage The dunes by now have a more or less continuous plant cover, effectively anchoring the dune sand in one place. Where a series of dunes has formed at the back of the beach, this stage will be on the dunes further inland. In colder autumn and winter months much of the available water will come from dew being deposited at night. Developing humus layers from decaying vegetation will help to retain some of the moisture. Water will still be scarce in hot summer months. As sand stops being deposited, Marram Grass will die out. Grey-green lichens (Peltigera, Cladonia) will colonize the dune extensively, combining with accumulating humus to give the dune sand a characteristic dirty grey colour. Mosses such as Bryum and Sand-dune Screw-moss, Tortula ruraliformis, also colonize the dunes. The lichens and mosses become dry, hard and dormant when water is in short supply, but as soon as it becomes available, they start to grow again. In high summer, the dried lichens and mosses give the dunes a very crunchy texture, which is almost like walking on crisps. Where there are large quantities of pulverized sea shells included in the dune sand, plants which thrive in lime-rich areas will become established.