The Plant Community

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The Plant Community

• The plant community may be described as a working
  mechanism
• The community itself will largely determine the
  distribution, density, and gregariousness of its
  biotic component parts, whether they function as
  individuals or patches

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The Plant Community

• Gleason 1926
• Each separate community as one minute part of a
  vast and ever changing complex of vegetation
• A plant association is merely a fortuitous
  mixture of individual plants

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The Plant Community

• Clements 1936
• Saw communities as an association of organisms,
  clearly closely interlinked and interlocked
• Whittaker 1975
• Saw the community as a distinctive living system
  with its own composition, structure,
  environmental relations, and development and
  function

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The Plant Community

• On a large scale, plant communities show
  extraordinary stability and uniformity and, are
  often considered to be relatively homogenous
  biomes (i.e. forest, grasslands, etc.)

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The Plant Community

• On a smaller scale, community variation is
  accentuated
• Aggregates of individuals and of species form
  dissimilar patches of vegetation
• These patches then combine to form a mosaic and,
  together, constitute the plant community Watt
  1947

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SUCCESSION
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Plant Succession

• The orderly and progressive replacement of one
  community by another until a relatively stable
  community occupies the area - Smith 1974

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Succession

• Primary Succession
• Occurs where vegetation forms on newly formed
  substrate such as following a volcanic eruption
  or glaciation
• These areas have no remaining biological
  propagules or organic matter from preceding
  vegetation
• All colonizing propagules must be dispersed to
  the area

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Succession

• Secondary Succession
• Vegetation reestablishes on remaining soil and is
  based on the previous vegetation
• Secondary succession begins in habitats damaged
  by fire, floods, insect devastations, grazing,
  and forest clear-cutting and in disturbed areas
  such as abandoned agricultural fields, vacant
  lots, roadsides, and construction sites

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Sere

• A unit of succession, from the beginning of the
  temporal sequence to a state of minimal species
  change - Steady State
• Seral Stage - each distinct community type within
  the sere
• Initial or Early Seral Stage - Pioneer or
  colonial community

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Sere

• A community exhibiting change towards or away
  from the climax is a seral community
• Final Seral Stage - Climax Community (is this
  ever achieved?)
• Dynamic Equilibrium

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Climax Community

• This is the final seral stage, consisting of a
  predictable equilibrium community that
  perpetuates itself indefinitely without changing
  into another community

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Mechanisms of Succession

• Facilitation
• Each seral stage changes the characteristics of
  the environment making it unsuitable for itself
  (especially for its seedlings) and suitable for
  the members of the next seral stage

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Perennial Grass
Secondary Sandhills Prairie
Blowoutgrass
Blowoutgrass/ Sand Muhly
Sand Muhly/ Perennial Grass
Disturbance
Native Sandhills Prairie
Bare Sand
Perennial Grass
Annuals
Annual/ Perennial Mixture
Shrubs
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Mechanisms of Succession

• Inhibition
• Species can prevent colonization of another
• Course of succession is dependent on which
  species arrive first and the next seral stage
  will only occur following the death of those
  individuals and the successful colonization of
  the next set of species

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Mechanisms of Succession

• Tolerance
• Whatever can live there will and the best
  competitor for resources will win
• Progress of succession dependent upon the
  lifespans of the individuals and the competitive
  abilities of the colonists

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Succession

• Progressive Succession
• Increases in diversity and biomass with the
  habitat becoming more mesic
• Retrogressive Succession
• Decreases in diversity and biomass and often
  become more xeric or more hydric

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Succession and Retrogression is a Continuum
Retrogression
Complex Ecosystems
Bare rock
Simple plants
Weeds
Grasses
Succession
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Successional Models
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How is Management Involved?

• Ranch manger
• Decides
• Stocking rate
• Grazing intensity and frequency
• Time of grazing
• Builds
• Fences
• Dams
• Wells
• Reacts to
• Drought
• hail

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Extremely Powerful!

• One of a very few Free Lunches
• Driven by both
• Random events
• Fire
• Drought
• hail
• Management
• Species
• Stocking rate
• Risk Management Time Bomb