Plant succession

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Plant succession
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The Concept

• Succession is the natural, orderly change in
  plant and animal communities that occurs over
  time. If left undisturbed, an open field may
  become an "aging" forest in 150-300 years.
  However soil conditions, climate, permafrost,
  topography, and natural and unnatural forces may
  affect the pattern of succession

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• Weather, fire, flood, insects, and human activity
  can disrupt the forest, altering the pattern of
  succession

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The Stages
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Herb Stage

• In this stage it contains seed plants whose stems
  wither away to the ground each winter.
• insects and small rodents feed on the grasses,
  herbaceous plants, and seeds
• As the diversity of plants increases so does the
  variety of wildlife species.

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Shrub Stage

• usually low woody plants with several permanent
  stems instead of a single trunk
• Larger variety of wild-life attracts predators

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Young Forest Stage

• the quantity and diversity of shrubs and
  herbaceous plants decreases
• With less variety in food available, the number
  of wildlife species decreases.
• birds of prey nest in the trees of a young forest

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Mature Forest Stage

• hardwood trees begin to die, opening the canopy
  for the growth of spruce
• Shrubs return and make more food and cover
  available to songbirds, game birds, and rodents.

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Climax Forest Stage

• dead trees called snags provide homes for
  woodpeckers and other cavity-nesting birds.
  Porcupines, squirrels, spruce grouse, and several
  songbird species are typical of old stands of
  spruce trees.

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Types of Succession

• Primary
• Secondary
• Allogenic
• Autogenic
• Progressive
• Retrogressive

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Primary

• Establishment of plants on land previously not
  cultivated.

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Secondary

• Invasion by plants on land that that was
  previously vegetated.
• Possible causes include natural or human such as
  fire logging, cultivation or hurricanes

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Allogenic

• Change in Environmental conditions
• E.g.. Salt marsh to woodland.
• The environmental changes the composition of the
  plant community

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Autogenic

• Where both plant community and environment change
• Caused by the activities of plants over time
• E.g.. Eruption of Mt. St.Helens

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Progressive

• Where community becomes more complex with time
• Contains more species
• Contains more biomass over time.

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Retrogressive

• The community regresses.
• Becomes more simplistic
• Contains fewer species and less biomass
• Some retrogressive successions are allogenic in
  nature
• E.g. Introduction of grazing animals results in
  degradation of farmland.

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The Facilitation Model

• "pioneer species" establish a presence on the
  site of a disturbance.
• They modify a site, for instance, by regenerating
  the soil with organic material making the area
  more attractive for invasion by other species.
• Eventually, new species move in, edging out the
  pioneers.
• This process may repeat itself several times,
  until the ecosystem reaches the climax stage.

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The Tolerance Model

• all species involved in succession are equally
  capable of establishing themselves on a recently
  disturbed site
• but those capable of attaining a large population
  size quickly are likely to become dominant
• Unlike the facilitation model, the tolerance
  model does not depict earlier inhabitants as
  preparing the site biologically.
• this model is more akin to natural selection.

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The Inhibition Model

• all species have equal opportunity to establish
  populations after a disturbance
• some of the early species actually make the site
  less suitable for the development of other
  species
• An example of this is when plants secrete toxins
  in the soil, thus inhibiting the establishment
  and growth of other species

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Climax

• When a biological community reaches a state of
  stability and is in equilibrium with
  environmental conditions
• Dominant species in a climax community are those
  that are tolerant of the biological stresses that
  come with competition
• frequent enough events of disturbance within
  small sections of the biological community may
  prevent climax from even occurring
• Once reached change will slow down bringing an
  end to the stages of succession.
• Climax remains a theoretical notion.

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What can cause Succession

• Seismic Events earthquakes, tidal waves, or
  volcanic eruptions
• hurricanes or tornadoes.
• the movement of glaciers or even of plates in
  Earth's crust
• wildfires or sudden infestations of insects
• Humans causes such as plowing up ground, logging,
  clearance for construction even by causing
  explosions on a military reservation or
  battlefield