A sand dune is formed by the accumulation of sand grains shaped into moulds and ridges by the wind. The succession of a sand dune is called a psammosere. This is the long term change in a plant community as it develops from a bare inorganic surface to a climax community (a group of species best able to exploit prevailing environmental conditions).
Each stage of a succession is called a sere, the first being the Pioneer stage. This occurs at he strand line (see diagram below). Extreme high tides or storm tides may leave a zone of several metres landward of the normal high water mark, causing the sand to be dry,salty, unstable with little nutrients and alkaline conditions. This is therefore not suitable for plant growth. Seeds are blown in by the wind or washed in by the sea and pioneer plants - highly specialised, tolerant plants - may colonise such as Sea Rocket. These may form miniature dunes as the sand gradually accumulates around the plants.
The next sere is the building stage which occurs at a few sections of a sand dune. The first is at embryo dunes. Sand accumulation which persists above the high tide line maybe suitable for colonisation by the first perennial plants in dune succession which are specialised grasses, for example Sand Couch and Lyme Grass. Both of these are able to grow upwards through accumulating wind-blown sand. As a result low, hummocky dunes are formed however the substrate is still inhospitable for plant growth.
The upward growth of embryo dunes allows the surface of fore dunes or mobile dunes to be raised so that it is out of reach all but the highest storm tides. Incursion of rainwater results in less salty substrate so Marram Grass is able to colonise and become the dominant species. It is able to grow upwards through accumulating sand and rates of up to 1 metre per year. Dead leaves of the Marram Grass adds organic material to the soil, releasing plant nutrients which leads to increasing biodiversity and less bare ground.
If conditions remain stable, mosses will cover bare sand patches in yellow or white (semi-fixed) dunes between Marram grass and plants. Mosses such as Restharrow ans Sand Sedge will become common and species diversity should continue to increase.
When vegetation has developed so that is forms a cover on the substrate, the dunes are 'fixed'. There is still a low nutrient status and risk of plant desiccation. Dunes maybe influenced by grazing and trampling however an organic layer starts to form on the surface. It can be a very species rich environment with plants such as Bedstraw, Wild Thyme and Harebell. These areas are of considerable conservation importance.
Depending on height of the water table, areas between sandy hills may be damp or even contain standing water. These are called dune slacks. Receiving nutrients leached from the surrounding dunes, they may be occupied by lime-loving species and can be rich with local or national rarities. Orchid species may sometimes be prominent species of the dune-slack community.
In absence of grazing animals, succession proceeds and tall woody plants such as Birch and Hawthorn form natural invaders. These areas are called dune scrubs. They tend to be species-poor, so in many areas management has focused on the clearing of scrubs and introduction of grazing animals to maintain the open dunes.
If grazing prevents the development of scrub or woodland, then fixed grassland known as dune heath, will eventually develop. This is plagio-climax because it results from human activities. The vegetation id dominated by plants that are adapted to grasslands and heathlands, tolerating dry, acid and nutrient poor substrate. A common invader is heather.
The final sere is the climax stage. Deciduous woodland is the natural climax vegetation of a dune system. Oak or Scots Pine are able to colonise the scrub. Plants from earlier stages die out due to competition for water, light and nutrients. Unfortunately the landward margins are often managed as golf courses, agricultural land or replaced by plantations, meaning that community climax is not always reached.
Each stage of a succession is called a sere, the first being the Pioneer stage. This occurs at he strand line (see diagram below). Extreme high tides or storm tides may leave a zone of several metres landward of the normal high water mark, causing the sand to be dry,salty, unstable with little nutrients and alkaline conditions. This is therefore not suitable for plant growth. Seeds are blown in by the wind or washed in by the sea and pioneer plants - highly specialised, tolerant plants - may colonise such as Sea Rocket. These may form miniature dunes as the sand gradually accumulates around the plants.
The next sere is the building stage which occurs at a few sections of a sand dune. The first is at embryo dunes. Sand accumulation which persists above the high tide line maybe suitable for colonisation by the first perennial plants in dune succession which are specialised grasses, for example Sand Couch and Lyme Grass. Both of these are able to grow upwards through accumulating wind-blown sand. As a result low, hummocky dunes are formed however the substrate is still inhospitable for plant growth.
The upward growth of embryo dunes allows the surface of fore dunes or mobile dunes to be raised so that it is out of reach all but the highest storm tides. Incursion of rainwater results in less salty substrate so Marram Grass is able to colonise and become the dominant species. It is able to grow upwards through accumulating sand and rates of up to 1 metre per year. Dead leaves of the Marram Grass adds organic material to the soil, releasing plant nutrients which leads to increasing biodiversity and less bare ground. When vegetation has developed so that is forms a cover on the substrate, the dunes are 'fixed'. There is still a low nutrient status and risk of plant desiccation. Dunes maybe influenced by grazing and trampling however an organic layer starts to form on the surface. It can be a very species rich environment with plants such as Bedstraw, Wild Thyme and Harebell. These areas are of considerable conservation importance.
Depending on height of the water table, areas between sandy hills may be damp or even contain standing water. These are called dune slacks. Receiving nutrients leached from the surrounding dunes, they may be occupied by lime-loving species and can be rich with local or national rarities. Orchid species may sometimes be prominent species of the dune-slack community.
In absence of grazing animals, succession proceeds and tall woody plants such as Birch and Hawthorn form natural invaders. These areas are called dune scrubs. They tend to be species-poor, so in many areas management has focused on the clearing of scrubs and introduction of grazing animals to maintain the open dunes.
If grazing prevents the development of scrub or woodland, then fixed grassland known as dune heath, will eventually develop. This is plagio-climax because it results from human activities. The vegetation id dominated by plants that are adapted to grasslands and heathlands, tolerating dry, acid and nutrient poor substrate. A common invader is heather.
The final sere is the climax stage. Deciduous woodland is the natural climax vegetation of a dune system. Oak or Scots Pine are able to colonise the scrub. Plants from earlier stages die out due to competition for water, light and nutrients. Unfortunately the landward margins are often managed as golf courses, agricultural land or replaced by plantations, meaning that community climax is not always reached.
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