Example of Primary Succession

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Example of Primary Succession

• 200 years of glacial retreat from Glacier Bay
• Tills and moraines (deposits of stones and
  pulverized rock) left - serving as rudimentary
  soil low in nitrogen and organic matter

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• Pioneers a black crust of cyanobacteria
  lichens, liverworts, and horsetail.
• Cyanobacteria fix nitrogen modest increase in
  soil nitrogen but organic matter is minimal.
• Willow sp., Dryas, Alder sp., and spruce
  seedlings occur, but distribution is sporadic.
• Dryas drummondi (root nodules nitrogen fixation)
  dominates after about 40 years
• Progressive increase in soil nitrogen and organic
  matter.
• Dense, closed thickets of alder (root nodules
  nitrogen-fixing) at ca. 60 years.
• Nitrogen neutral litter accumulates in soil.
• Invasion of spruce seedlings

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• Spruce-needles accumulate organic acid leaching
  soils become acidic
• Spruce shading excludes alder and many original,
  understory species.
• Hemlock begins to flourish.
• Ca. 200 years
• 1) Well drained soils a climax of spruce and
  hemlock dominants results
• 2) Poorly drained soils forest invaded by
  Sphagnum mosses, which accumulate water and
  further increase soil acidity
• Muskeg bog

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Seral stage
Spruce,
Dryas 7.0 5.3 7.3 2.8
Alder 8.8 21.8 6.8 277
Spruce 15.1 53.3 3.6 261
Pioneer 5.2 3.8 7.2 1.5
Western hemlock,
Soil depth (cm) Soil N (gm ) Soil pH
-2
Litterfall
Liverworts Lichens River beauty Epilobium
(rare) Horsetail Equisetum Cyanobacteria
Mountain avens, Dryas drummondi
Alder, Alnus sinuata
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200
40
60
Time (years) since retreat of glacier
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• Dryas and alders increase soil nitrogen levels
• Facilitates the invasion of spruce
• But, alder shades out Dryas inhibition
  competitive exclusion
• Eventually spruce shades out alder inhibition
  competitive exclusion
• Facilitation, important in early stages.
• Inhibition competition, important in the later
  phases of succession

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Secondary Succession

• Piedmont Plateau, North Carolina
• Old growth forests cut and replaced by
  agricultural crops
• Fields were asynchronously abandoned
• Successional sequence revealed by various fields
  with known dates of abandonment.
• 35 native species 1 year after fields were
  abandoned
• Seedlings of colonizers have to be able to
  withstand exposure and dry soil.
• Two species dominated crabgrass and horseweed

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• 1st year self inhibition of horseweed
• 2nd year, previous species still present 26 new
  species. Aster and ragweed dominant broomsedge
  present.
• Broomsedge can outcompete aster for soil moisture
• 3rd year, Broomsedge became dominant, maintaining
  this position for several years. Species richness
  declined.
• Seeds of pines arrive by wind.

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• 5th year, pines became established, producing a
  closed canopy by year 10
• Pine seedlings do poorly under a pine canopy
  (self-inhibition), but hardwoods thrive. Gradual
  process of replacement.
• Random-colonization model chance involved in
  which hardwoods become established.
• By 100 years, there are equal numbers of
  hardwoods and pines
• By 200 years, only scattered pines remain in a
  typical oak-hickory forest climax.

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Random colonization
White oak Dog wood Hickories
Seral stage
Loblolly pine hardwood understory
Inhibition
Inhibition
Facilitation
Broomsedge Pine seedlings
Crabgrass Horseweed
Loblolly pine
White aster Ragweed
Broomsedge
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2
3
5
10
60
150
Time (years) since abandonment of tobacco fields
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• Successional tolerance
• Many species are present at the outset
• Which species germinate first, or can propagate
  from roots, have a role of determining, by
  chance, the successional pattern
• Works only for secondary succession

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Another way of documenting changes in communities
Huerfano Butte, Pima County, Arizona
1902
1969
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