This essay aims to explore whether the succession of a psammosere in Holme-Next-The-Sea is similar to the common theoretical succession of a psammosere.
Holme-Next-The-Sea is home to large, well-developed dunes. On the East coast of the UK, roughly two-and-a-half hours away, the psammosere in Hunstanton appeared to be an optimal choice of location to conduct our studyJ.H1.
To begin explaining specifically what this study sets out to do, it’s important to understand what a psammosere is, and what a theoretical succession looks like. I would argue that it is also important to understand why they exist in the first place and so to do this, we need to understand what sand is and how it movesJ.H2.
Sand is a naturally occurring granular material made up of material originally found in rocks, but that has been brushed off, knocked off or worn down. Sand is defined by size, and so it must be finer than gravel, but coarser than silt, specifically 0.05mm – 2.0m in diameter (1). Sand is usually found in the form of quartz; however, another common type of sand is calcium carbonate.
Due to the small size of sand, and the material that it is composed of, each individual particular has very little weight. This allows the waves on the beach to push sand around, and it also means that the wind, if strong enough, has the energy to pick up sand and move it along the beach.
Now that we understand what sand is and how it moves, we can begin to understand what a psammosere is and how it might be formed. A psammosere is an ecological succession that develops in the sands of a coastal environment (2). The typical succession of a psammosere is as follows:
Dunes first form when onshore winds carry sands along the beach and material at the top of the beach, which could have been anything from a tree branch to a small stone will have created an obstruction and caused the wind to drop the sand it was carrying. These first dunes initially are populated by robust and strong pioneer plants that are able to cope with the salty and windy conditions. This process forms the first embryo dunes in a psammosere, and are typically colonised by prickly saltwort, sea rocket, and sea sandwort.
The largest dunes are often originally formed by sand being obstructed by marram grass and then being deposited. Marram grass acts as a catalyst for the development and creation of sand dunes. The grass is highly adapted to grow up and through the blown sand that may cover it, and so this causes the height of the dunes to grow rapidly. The grass also stabilises the environment, as it acts as an anchor in holding the sand in place, because it can form such a large and strong-rooted barrier. This stage of a psammosere is referred to as the yellow dunes. As these dunes exist through time they start to increase in the organic matter that they hold, and are protected from the onshore wind and salt by new dunes forming towards the beach. They are colonised by a variety of plant species, depending on the soil in the location, and the water conditions.
As dunes grow old and lose height due to the erosion by the wind, they often form low-lying areas, known as dune slacks. Dune slacks are very large depressions in sand dune successions that are often deep enough to expose the water table. There are two ways they can form. If a foredune grows large enough it can form a dune ridge that prevents any further deposition of sediment inland by blocking the wind. The low point behind the ridge is the dune slack. Since there’s little erosion taking place behind the dune ridge, the water table generally isn’t exposed in the dune slack.
The more interesting dune slacks form as a result of blowouts. Excessive trampling and grazing of plants on a dune will reduce the stability of the dune since there are no roots to bind it together. This makes it easier to erode the dune, producing a deep depression in the succession that often reaches down to the water table. (3)
As fresh water is exposed at the surface, new species of vegetation can grow in the dune slack. A dune slack that forms behind a grey dune often contains a variety of vegetation due to the fact that humus on the grey dune is blown into the dune slack making it even more hospitable.
If the dune slack has been there for long enough then the ecosystem may develop to a dune scrub, with willow, and moisture-loving associates such as Marsh Pennywort, Marsh Orchid, and various rushes and sedges. The next stage is characterised by the colonisation of woody plants such as brambles, sea buckthorn and small trees. Over time, a thick layer of humus develops. In these dunes, the collapse of old heather bushes may provide a place for young birch trees to grow. This is known as dune scrub. (4)
The shade caused by the dune scrub should allow for taller vegetation to out-compete species from earlier stages for resources such as sunlight and water. This typically causes the overall vegetation diversity to lower. The climax of a psammosere succession is usually deciduous woodland. No new species are added and the community remains in equilibrium for long periods of time. The vegetation should be in equilibrium with the environment and a balance can be achieved.
In the image below (Fig 1.) we can visually identify the succession of a psammosere, and some of the characteristics that we would expect to find at each stage.
This is a theoretical succession of a sand dune. This is the succession I will be comparing what we actually found in Holme-Next-The-Sea to, and the characteristics such as the height of the dunes, the pH and the soil colour are all things I hope to compare against.
Aims of the investigation:
My aim is to research whether the data that we gather about the succession of the psammosere in Holme-Next-The-Sea is ordinary – i.e. it roughly follows the typical theoretical succession of a sand dune – or whether what we find is significantly different.
To measure this, I will test this by creating theoretical succession criteria that I will test against. I want to test against the physical location and physicality of the dunes, i.e. height, distance from strand line etc., I also want to test the pH level of the soil and test this against what is theoretically expected. I could also think about comparing Humus content of the soil, and the soil colour.