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Stage 1: Before you start

Stage 1: Succession | Questions to investigate

What is this environment like?

A saltmarsh is essentially a muddy seashore with vegetation growing on it. It is a marshy area that forms a transitional zone between land and salty (or brackish) water.

Saltmarshes develop on depositional coasts, bays, and estuaries where tidal movements are gentle and erosion is light. Due to the sheltered conditions, fine particulate materials like muds, silts and clays begin to bind together, or flocculate. By doing this they become larger and heavier, and eventually fall out of suspension and start to build up (processes known as sedimentation and accretion). These processes are continuous, meaning the height of the mud increases over time and the surface gradually experiences longer periods out of the water.

Eventually the height of the muddy shore reaches a critical point when it is out of water for 2 or 3 days continuously. This is important as it becomes possible for the seeds of flowering plants, carried in by wind or water, to germinate undisturbed and attach their roots into the ground. Small halophytic (salt tolerant) pioneer plants such as glasswort (Salicornia spp.), seablite (Suaeda spp.) and cord grass (Spartina spp.) can then colonise the surface of the mud.

The presence of these first plants on the developing saltmarsh will affect the physical conditions. Roots help consolidate the mud that has already built up by binding it together and carrying oxygen into it; and stems and leaves help trap more sediment. In addition, the plants provide a source of food and places of refuge for animals. Decomposition of the plants over winter adds organic matter, nutrients and minerals to the muddy soil.

With the physical environment improving over time, conditions become suitable for additional species to be present and plants such as sea purslane (Atriplex portulacoides), sea lavenders (Limonium spp.), sea aster (Aster tripolium), plantains (Plantago spp.), scurvy grass (Cochlearia spp.) and thrift (Armeria maritima) will take over from the pioneer species. Over time, with the continual improvement of the abiotic environment, various sedges, reeds and rushes will take over. These are climax species, marking the end of the development of the saltmarsh.

Saltmarsh development can be summarised as a directional change in plant and animal communities over time, and is an example of a process known as succession.

The saltmarsh environment is an extremely harsh one, with a number of abiotic factors making conditions for growth difficult. These include high salinity (salt content) in the soil solution, low concentrations of essential nutrient ions, anaerobic soil (little or no oxygen present), sulphide toxicity, temperature shock and changes to light availability when vegetation is immersed in water, and drag and scour when water and particulates move pass the plants.

Due to these difficult environmental conditions biodiversity on a saltmarsh tends to be relatively poor when compared to other ecosystems, although it does increase with height up the marsh.

Succession

Succession is a directional change in plant and animal communities with time. There are two major types of succession – primary and secondary. Primary succession occurs on sites that have not previously been occupied by vegetation. The environmental conditions tend to be harsh and unfavourable, and the process is typically slow because of this. An example of primary succession is the development of a sand dune ecosystem. Secondary succession occurs on sites that have previously been occupied by well established communities. Conditions in these areas are much more favourable, making secondary succession a faster process. An example here is regrowth after a forest fire.

A place where succession occurs is termed a sere (f rom the Latin serere = to put in a row i.e. a sequence or series). Succession can occur in a variety of environments, and in order to distinguish which environment you are dealing with the word sere is prefixed with other words, so for example: H ydrosere refers to succession in a frashwater environment e.g. a pond or lake (from the Greek hydor = water); Xerosere refers to succession in a dry environment e.g. a desert (from the Greek xeros = dry); Psammosere refers to succession in a sandy environment e.g. a sand dune (from the Greek psammos = sand); Lithosere refers to succession in a stony environment e.g. a rock face (from the Greek lithos = stone); and Halosere refers to succession in a salty environment e.g. a saltmarsh (from the Greek hals = salt).

The events that occur in primary succession can be divided into a series of stages called seral stages. These stages are common to any primary successional sequence (i.e. the process occurs in this way in any of the habitats mentioned above).

Stage 1: Migration

Seeds and spores arrive on the site. These can be carried in by wind, waves, birds, animals etc. If the seeds and spores are able to germinate and grow, the community enters the next seral stage. The migration stage will continue for as long as the community continues to develop.

Stage 2: Colonisation

Seeds and spores germinate and develop. Physical conditions are unfavourable, there is lots of bare ground and the vegetation is described as “open” (lots of open space between plants). The plants that are able to grow in these harsh conditions are specialised to their environment, and are termed pioneer species.

Stage 3: Establishment

Species become more established on the site. The physical conditions have been modified and improved by the presence of the community, so there is an increase in the variety of species. The vegetation is becoming “closed” as there if less bare ground available for vegetation.

Stage 4: Competition

The number of species on the site is increasing, and plants have to compete for space, light, nutrients, water and so on. Opportunistic pioneer plants often die out at this stage, to be replaced by equilibrium species which tend to be better competitors.

Stage 5: Stabilisation

Few, if any, new species are added as competition resolves itself and the community becomes balanced. Each species occupies its own niche, and therefore avoids having to compete strongly with other species. The community stabilises and remains much the same over time.

 Stage 6: Climax

No new species are added and the community remains the same over long periods of time (theoretically forever). The vegetation is said to be in equilibrium with the environment; a true state of balance has been achieved. The type of climax vegetation present is determined by the climate of the area, wind speeds and direction, animal grazing, pH, temperature and many other factors. The climatic climax for the UK as a whole is deciduous woodland (oak and ash trees), however different ecosystems will have alternative climax vegetation types (for example reeds or rushes on a saltmarsh; willow on sand dunes) as localised conditions prevent the climatic climax from developing.

Remember that succession is a temporal process; the community changes with time.

As succession proceeds there is a notable increase in biodiversity, however it will often level off, or even decrease, towards the end of the sequence as the community reaches equilibrium. Soil organic matter and nutrient levels increase throughout a successional sequence, which in turn leads to an increase in biomass as the community develops.

Questions to investigate

How does vegetation change from the seashore inland?

How do soil characteristics / microclimate change with distance from the seashore?

Comparing two area of the saltmarsh, e.g. ungrazed/grazed sections of the saltmarsh

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