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Stage 1: Before you start

Stage 1: Succession | Questions to investigate

What is this environment like?

The fringes of a pond or lake can show zonation of vegetation communities. Soil close to the water is very wet, but it becomes drier the further up the slope. Some plants are only found in the wettest ground (e.g. marsh marigold - the plant with yellow flowers), some are found in slightly drier ground (e.g. water mint and soft rush, both in the centre of the photo) while others grow only in drier soils.

A view of the edge of a small pond from the water. The ground slopes gently upwards from the edge of the water.	This is a different view of the same pond. At the edge of the pond garden there is a small area of grassland.

The pond pictured above was deliberately constructed so that there would be a shallow slope at one end. But many natural ponds and lakes have a shallow slope between 'wet' and 'dry' land. A hydrosere is the term used to describe a successional sequence (a change over time in the plant and animal communities) in a freshwater environment.

An area of deep freshwater is not initially capable of supporting rooted, submerged plants because not enough light for photosynthesis can reach the depths. There will however be species of algae, diatoms and planktonic organisms present in the water. Upon dying these organisms sink to the bottom of the water, along with silt and sediments that are transported into the lake (or pond) by rivers, streams or surface runoff. Large amounts of sediment can be deposited in this way, and the water depth will slowly decrease as a result.

Gradually, as the pond fills in with this material, the shallower water around the edges of the lake will be able to support plant material. This is because sufficient amounts of light can now penetrate for photosynthesis. Rooted submerged plants, such as starwort (Callitriche stagnalis) and pondweeds can start to grow. waterlilies, which are rooted but with floating leaves, may become established; and floating plants, such as duckweed (Lemna gibba) may also be present.

The presence of these species in the environment will slowly begin to improve the abiotic (non-living) conditions, for example by providing shelter for animals and reducing the effect of the wind on the surface of the water. Over the years these plants become well established and may produce a dense cover of vegetation over the water surface.

Each year dead organic matter accumulates at the bottom of the lake and increases the nutrient content of the water. This in turn promotes plant growth, and the additional vegetation is able to trap and hold more of the incoming sediment. This means that the water continues to become shallower. When water depth eventually becomes too shallow to support fully submerged plants, emergent swamp plants such as yellow iris (Iris pseudacorus), branched bur-reed (Sparganium erectum), and reed mace (Typha latifolia) will start to establish.

These plants tend to have tall, flexible spear-shaped leaves. This allows them to cope with large fluctuations in water level, always retaining some portion of the leaves above the water for effective photosynthesis. The leaf bases are extremely effective at retaining incoming silt, particularly in the winter when the leaves have died back. Combined with the accumulation of decomposing plant material, this causes a further decrease in water depth, and raises the level of the lakebed, making conditions drier. Swamp plants, which are adapted to grow in partially submerged conditions, will gradually die out and give way to marsh plants such as water mint (Mentha aquatica), brooklime (Veronica beccabunga) and soft rush (Juncus spp.).

Trees that favour wet soils, such as willow (Salix spp.) and alder (Alnus glutinosa) may also become established at this stage. These trees have a very high transpiration rate, transferring large quantities of water from the sediment into the atmosphere. Together with the silt-trapping effect of the marsh plants, this greatly increases the rate at which the marsh dries out. willow and alder carr produces a canopy of leaves which restrict the amount of light reaching the ground. Many of the marsh plants will become shaded out by the trees, and so in turn are replaced by a variety of woodland floor plants including sedges, rushes, ferns and small flowering herbs. If the soil continues to dry out, species of tree such as oak (Quercus rober), beech (Fagus sylvatica) or ash (Fraxinus excelsior) can grow, marking the end-point in this freshwater succession.

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 (from 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 edge of the water inland?

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

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