Vegetation Succession in the UK (AQA A Level Geography)

Succession in Ecosystems

• Ecosystems develop through distinct successions from a ‘sterile’ area to a climatic climax community

• Vegetation succession can be considered:

The evolution of plant communities at a site over time - from pioneer species to climax vegetation

• At each stage of succession the plant community alters the soil and microclimate, allowing the establishment of another group of species

• One community of plants is therefore replaced by another as the succession develops

• Eventually a climax community is reached where the vegetation is in a state of equilibrium with the environment and there is no further influx of new species
  

• The process takes hundreds of years and the climax community is dependent on the climate it forms in

Seres and climax vegetation

• The developmental stages of a community are known seral stages and the final stage as the climax community

• The entire seral communities that gives the site/area its characteristics is called a sere

• Particular species are associated with each sere, and certain species becoming dominant

Climax communities

• If natural conditions are not interrupted, then climatic climax is the final stage that seres reach

• Climatic conditions include light, soil pH and moisture determines which plants survive

• This dictates the natural vegetation that should be found in an area

  • For most of the UK this would be deciduous woodland, dominated by Beech, Birch, Ash and Oak

Plagioclimax

• This is where the resultant community has been permanently influenced by humans

• For example, by burning or grazing

Sub-climax

• If vegetation does not reach its climax as a result of interruptions by local factors, such as soil changes or differences in parent rock, the interruptions are known as arresting factors

• Not all climax communities are the same, and if physical factors such as altitude or water hinder ecosystem development a sub-climax community may result

Succession

• The route to climatic climax can take place in two ways:

  • Primary succession

  • Secondary succession

Primary succession

• Found on new, bare, land surface or in water and various seral stages are passed through before climatic climax is reached

• It is an orderly sequence of events where one community is replaced by another

• Biomass is created via decomposition and provides more nutrients to the soil

• Allowing for more and greater variety of plants and animals to exist at each successive seral stage

• There are four types of seres:

  • Lithosere - rock

  • Psammosere - sand dunes

  • Halosere - salt marshes

  • Hydrosere - lakes

• The first to arrive are known as pioneer species - the trailblazers - often herbs and lichen

• These early invaders quickly colonise new surfaces as there is no competition from other species

• They begin to adapt to their environment

• However, they are short lived and are then replaced by others that outcompete them as conditions improve

Secondary succession

• If plant succession is halted before reaching dynamic equilibrium a secondary succession occurs

• Interruptions include fire, disease, climate change and deforestation

• These events can also alter the final climax community that result

• Not all climax communities are the same, and if physical factors such as altitude or water hinder ecosystem development, a sub-climax community may result

NB. Prisere succession is the primary route

Seral Progression of a Lithosere

• As an ecosystem moves towards its climax community during succession, it progresses through various steps called a seral stage or seral progression

• These stages are dependent on the biotic and abiotic conditions available to it

• Each stage is an intermediate step, and each community within each stage, is not stable

• It therefore, has to pass through many developmental stages from simple to complex, to achieve final climax conditions

What is a lithosere?

• A lithosere is:

Plant succession on exposed rock, usually through natural processes, i.e. glacial retreat, tectonic uplift or volcanic eruption

• The driving force to seral progression, is the impact that current species have on their own environment

• Lithosere succession begins with the primary sere of bare rock (prisere), 5 subseres and a climax sere

Subsere communities

• Pioneer species on the prisere are lichens, as they can tolerate extreme conditions

  • Lichens photosynthesise to produce sugars for growth and leach organic acids (as waste) which aid to break down the rock surface, further releasing rock minerals for developed growth

  • The now uneven rock surface, begins to hold water

  • As these primary lichens die, they decompose and add humus to the uneven surface

  • Secondary lichens begin to colonise, deepen the uneven surface, allowing more water, and soil particles to accumulate

  • Decomposition to humus, mixes with the increasing soil particles which helps in building layers and improving soil moisture contents further

• Moss spores are blown or carried in and begin growing in the damp soil and humus that lichens have produced

  • Mosses are taller and faster growing than the lichens, so out-compete them for available light

  • Mosses are rich in organic and inorganic compounds, which are added to the soil, upon their death, increasing soil fertility 

  • As mosses develop in patches, they catch soil particles from the air and contribute to soil depth

• Grass seeds of hardy species, begin to germinate in the mat of rich organic soil produced by the mosses

  • Their roots penetrate deeply and also secrete acids that aid the sub-surface weathering process of the parent rock

  • Leaf litter and dead grasses add humus to the developing soil, allowing species diversity (small flowering plants, herbaceous and xerophytic plants) to begin colonising

  • These climatic conditions encourage bacterial and fungal growth, which increases rates of decomposition

  • Soil pH begins to change depending on climatic conditions and parent material (chalk produces alkaline soil and moorland becomes acidic)

• Small shrubs such as ferns, bracken, brambles and small bushes such as gorse and broom begin to outcompete grasses and small flowering plants

• Large shrubs such as hawthorn and birch begin to grow

  • The soil deepens and enriches with enough nutrients to begin the slow growing of larger trees such as oak and ash

• Woodland develops as the climax stage, and with no environmental changes, the tallest trees will dominate in a state of dynamic equilibrium until circumstances change

Seral Progression of a Hydrosere

• Hydrosere is the primary succession sequence that develops in water (aquatic) environments such as lakes and ponds

• It illustrates the changes within a body of water, and its community, into a terrestrial (land) community e.g. oxbow lake

• Over time, areas of open freshwater naturally dry out and eventually become woodland by going through seral hydrosere progression

• Those stages are:

• Phytoplankton stage

  • Algae colonise open water (e.g. pond) as pioneer species

  • Algal spores are carried by air to the water

  • The algae are followed by zooplankton (small animals that consume algae)

  • After death, both settle to the bottom of the pond, and decay into humus

  • This mixes with silt and clay particles brought into the water through run-off water (or wave action if by the coast), and helps to form soil

  • As soil builds up, the pond becomes shallower

• Submerged stage

  • As the water level becomes shallower, aquatic plants become rooted and establish themselves, further increasing the depth of sediment

  • Light can penetrate further into the shallower water, allowing submerged plants to photosynthesise

  • Once submerged species colonise, the successional changes become more rapid

• Floating stage

  • The low (0.5 -1.5m) water level, allows floating species, such as water lilies, to begin outcompeting submerged species

  • Floating plants have larger and broader leaves, that shade the water's surface, making conditions unsuitable for  submerged species, which begin to disappear

  • These plants decay to form organic mud, making the water even shallower 

• Emergent stage

  • Also known as the reed, marsh or swamp stage

  • Conditions are now suitable for emergent plants such as reeds, rushes and grasses

  • These plants produce large quantities of leaf litter that resists decay and forms reed peat

  • Reed peat continues to deepen, which helps form water-saturated, marshy land

• Fringing stage

  • With each successive drop in water levels, grasses and wildflowers begin to form mats of vegetation that extend across the water

  • As numbers of plants and grasses increase, plant transpiration further lowers the water levels 

  • Leaf litter from these plants add to the submerged reed peat levels

  • Eventually, grass peat emerges above the water level and soil in no longer completely waterlogged

• Shrub stage

  • The soil is drier and becomes ideal for shrubs and wet woodland

  • Shrubs such as brambles, sea buckthorn and hawthorn, along with short trees such as willow, alders and poplars

  • These plants produce shade, lower the water table further through evapo- transpiration, build up soil levels, and the accumulation of humus

  • This type of wet woodland is also known as carr woodland

• Climax woodland stage

  • Finally, conditions are suitable for large climax tree species, such as oak, ash and beech

  • These slow-growing tress eventually dominant 

  • Depending on light levels, herbaceous plants, ferns and grasses grow on the woodland floor

  • In the UK, hydroseres have formed on kettle lakes (shallow lakes formed by retreating  glaciers) and referred to as 'meres' - Sweetmere in Shropshire and Oak Mere in Cheshire