Sand Dune Succession Coursework
The aim of this experiment is to discover how the pH value and the humus value of soil samples collected along a 600 metre transect change as we travel away from the shoreline. The results will indicate how succession has an affect upon the soil and the environment. Also the plant species number will be recorded and a trend will be drawn from the results of percentage cover.
Succession is a gradual directional change in an ecosystem over time. A good example of primary succession can be seen on coastal sand dunes such as Winterton sand dunes. Here many stages of succession can be seen in one place. Primary succession is when the starting point is bare ground with no living things present. In this case the starting point is bare sand.
The first species to colonise bare ground like this are called pioneer plants. These are able to survive in very difficult conditions and are more salt tolerant than other plants. The sand has very few nutrients and is also unstable. The pioneer plants, examples are sea rocket and sea holly, must be tolerant of salt pray and have xeromorphic features which aid survival in an area with a lack of fresh water in the fast-drying sand.
Over many years the environmental conditions become more suitable for a wider range of plants to live. As the number of species increases, competition increases also. This competition leads to replacement of original species by better adapted ones. It also leads to changes in abiotic factors. These changes occur as a series of seres. The first sere is the embryo dune which is the closest to the water table and shoreline. Here sand is continuously moving around by the force of the wind. To allow the sand to accumulate the wind needs to be obstructed. This could be achieved by seaweed or driftwood lying on the beach. This dune is very unstable and could easily disappear as quickly as it was formed.
The next sere is the fore dunes where sea couch grass and marram grass colonise. These plants are drought resistant xerophytes and are capable of withstanding burial by the shifting sand. As they grow up through the sand they trap more of it which results in the dunes increasing in height.
The yellow dunes begin to show a greater diversity of plants as conditions become more idealistic. As plants die they decay and a humus layer builds up and traps water and nutrients. The pH of the yellow dune is only slightly alkaline, about 7.5. This is because there is more shelter and less salt spray. Marram grass normally still dominates the vegetation in this area however other plant species can take hold of the stabilised surface such as red fescue and mosses.
Grey dunes are much more stable and mosses and lichens fill the few remaining spaces so that vegetation over may reach 100%. Marram grass becomes less common and plants such as red fescue and sea spurge begin to dominate. Small shrubs may appear for the first time. Environmental conditions at the grey dunes, around 50-100 metres from the sea, are much less hostile. There is shelter from winds and the increase in humus begins to darken the surface layers revealing soil being formed. The soil pH becomes acidic and heather grows in vast quantities. Although water content is still low, plants overcome this by developing long and spreading roots.
The dune slacks are found in between the more mature dunes where the water table reaches the surface causing seasonal or permanent water logging. Plants that are well adapted to these damp, sheltered hollows are rushes, sedges and cotton grass. If decay is slow, a peaty soil may develop.