Plant succession and seral stages

Overview

Plant succession, sere, a seral stage and a prisere and difference between primary and secondary succession.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is plant succession?

   Model answer

   Plant succession is the sequence of changes in the plant community occupying a site over time. Each community alters the conditions of the site - building soil, adding humus, retaining moisture, providing shelter and shade and changing the pH - and in doing so makes the site more suitable for a different, usually larger and more demanding, group of species, which then colonises and displaces it. The process continues until a stable community is reached that is in balance with the environment and changes no further.

2. 2

   Distinguish between a sere, a seral stage and a prisere.

   Model answer

   A sere is the whole sequence of plant communities that occupies a site through a succession. A seral stage is one identifiable community within that sequence - a distinct step, such as the pioneer stage or the scrub stage, recognisable by its characteristic species and conditions. A prisere is a complete primary succession, running from the initial colonisation of a bare surface right through to the climatic climax community.

3. 3

   What is the difference between primary and secondary succession?

   Model answer

   Primary succession begins on a surface that has never been colonised before and carries no soil and no seed - bare rock exposed by a retreating glacier or a lava flow, a new sand dune, or open water. Everything must be built from nothing, so it is slow and may take centuries or millennia. Secondary succession begins on land that was previously vegetated but has been cleared, by fire, felling, landslide or the abandonment of farmland. Soil, nutrients, buried seeds and surviving roots are already present, so it starts from a much later point and proceeds far more quickly, often reaching woodland within a century.

4. 4

   Name the four types of primary succession and the surface each one begins on.

   Model answer

   A lithosere begins on bare rock. A psammosere begins on bare sand, typically a coastal dune system. A hydrosere begins in fresh water, such as the margin of a pond or lake. A halosere begins in salt water on a tidal mudflat or salt marsh. All four proceed towards the same climatic climax if the climate is the same, but they start from very different conditions and pass through different seral stages.

5. 5

   What are pioneer species, and what qualities must they have?

   Model answer

   Pioneer species are the first plants to colonise a bare, hostile surface at the start of a succession. They must tolerate extreme conditions - no soil or almost none, no shelter, no fresh water, high exposure to wind and sun, and often extreme salinity or pH. They therefore tend to be small, low-growing and shallow-rooted or able to grip bare rock, drought-resistant, fast to germinate and reproduce, and able to spread easily by wind-blown seed or spores or by vegetative growth. Crucially, they begin to change the site: they trap sediment, weather the surface and, on dying, add the first organic matter to it.

6. 6

   Describe the seral stages of a lithosere.

   Model answer

   The bare rock is colonised first by blue-green bacteria and by crusty lichens, which grip the surface, tolerate drought and extreme temperatures, and slowly break the rock down by secreting acids and by physical expansion and contraction, trapping wind-blown dust and adding a trace of organic matter as they die. Mosses then establish in this thin skin of material; they hold moisture and add more organic matter, so a thin soil begins to form. Grasses, herbs and ferns follow, rooting in the deepening soil, adding much more litter and further deepening and enriching it, and shading out the mosses. Shrubs such as gorse, bramble and hawthorn then invade, their deeper roots reaching further into weathered rock and their greater litter fall accelerating soil formation. Finally fast-growing trees such as birch and rowan establish, and are eventually replaced by the taller, longer-lived, shade-casting dominants such as oak, which form the climatic climax woodland.

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