Why does succession take so long? • Different plant species have different ecological requirements. A beech or live-oak needs shade as a seedling. • They also need soil moisture which means the soil must have a high organic content. • So succession is also the development of soil and colonization by soil organisms.
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Why does succession take so long? Different plant species have different ecological requirements. A beech or live- oak needs shade as a seedling. They.
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Why does succession take so long?
• Different plant species have different ecological requirements. A beech or live-oak needs shade as a seedling.
• They also need soil moisture which means the soil must have a high organic content.
• So succession is also the development of soil and colonization by soil organisms.
3 models of successionConnell and Slatyer (1979)
Facilitation
• Initially thought that all succession was due to facilitation.
Example of facilitation: shade provided by pines allows seedlings of broad-leaved trees to survive.
Or
Growth of a nitrogen-fixing plant on sandy (nutrient poor) soils such as alder enriches the soil sufficiently for other species to colonize.
Tolerance
• All species could live at all stages of the succession, but differing dispersal abilities/adaptations ensures earliest stages occupied by pioneer-type species.
• As succession proceeds fewer and fewer of the early successional species can tolerate the new conditions and so the system matures toward D.
• Dispersal distance is therefore also a big factor to be considered.
Inhibition
• Species mutually inhibit one another through competition. System can only change when an individual dies an is replaced.
• What will influence that replacement?
Soil maturity and succession
• Soil accumulates organic matter as succession proceeds.
• Increased ability to hold moisture.
• Pioneer species are shaded out.
The balance
• Now understood that facilitation, tolerance, and inhibition all combine to produce succession.
Clements vs Gleason
• We now know that species respond individualistically to change.
• Communities are not superorganisms and will not always return to a predictable equilibrium.
Colonization of new areasPrimary Succession: Colonization of new areas
Colonization of a new area
• Follows succession from pioneers to competitors…..but all have to disperse there.
• Distance from source is important…can larvae survive long enough to be transported there? Can seeds be blown there? Can mammals swim there? Can birds fly there?
Two ways to study succession
• Follow one location from disturbance to maturity, ex. Krakatau, Mt St Helens.
• Select similar habitats at different times since similar disturbance, ex. Glacier Bay, building riverbank
• Macroalgal succession over 30 months on experimental concrete blocks.
UlvaSea lettuce
Intertidal succession
• Steadily retreating glacier since 1850s.
• New land surface revealed…primary succession.
• Oldest succession where ice first retreated.
1850
1912
Primary succession in Glacier Bay, Alaska
Glacier Bay Succession
• Retreating glaciers expose new land surface of till.
• Rate of retreat ca. 65 km in 200 years
• Succession follows broadly predictable path
Nutrient changes at
Glacier Bay• The initial soil is nutrient
poor.• Alder is an N-fixer, spruce
and hemlock are not.• “forest floor” reflects N in
leaf and wood litter.• Why is there a peak in
forest floor N at the transition to spruce-hemlock.
• Why does soil N decline in the spruce-hemlock zone?
AT least that has been the accepted story..BUT!
• Fastie (1995, Ecology) shows that alder may actually slow the succession through competition.
• The succession to Sitka Spruce was much faster in the sites deglaciated in the 1780s-1840s than the later sites.
• And hemlock has not begun to grow at any site that initiated after 1840.
Simulation showing nitrogen inputs during 2ndry succession
Importance of alder (ALRU) as a nitrogen fixer and Ceanothus (CEVE), early in succession.
Nitrogen sources
Rotmoos Glacier, Italian alps 1895
Rotmoos Glacier, Italian alps 1999
Rotmoos Glacier, Italian alps
• 1895 glacial tongue evident in valley
• 1999 2km of retreat evident
• Following is work by Kauffman, Ecology (2001)
First 50 yrs
• Sparse vegetation means little local productivity.
• Surprisingly, insects are primarily predators relying on allochthonous (derived from elsewhere) sources of prey
5 yrs: Harvestman- a glacial specialist Predator.
10 yrs: 4 spp. Of ground beetle, occupying separate niches.
20 yrs: assorted spidersAbundant.
30 yr Centipedes, under rocks50 yrs: herbivorous beetles as vegetation density increases.
50-150 yrs first
appearances
• As vegetation increases in density a more normal insect spectrum is represented with food chains supported by herbivores. Detrital cycle also evident. System now autochthonous (productivity is local).
70 yr: millipedes are important decomposers.
100 yrs: 1 sp. Of ant occupies sunniest locations
140 yrs: Densest Vegetation supportsgrasshoppers
Effect of succession on adjacent waters
• Shading of margins• As succession increases soil organic content will
also increase dissolved organic carbon (DOC), e.g. humic acids and tannins.
• Leaching into waterways these chemicals color the water and reduce transparency.