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Lecture 9

BIO120H1 Lecture Notes - Lecture 9: Temperate Deciduous Forest, Pioneer Species, Secondary Succession

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James Thomson

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Lecture 9: spatial eco, plant communities, and disturbance
Community dynamics: predictable successional change in plant communities
- Pioneer species get in first(from dispersal or seed bank soil)
- Soil-building processes and shade thought to be critical
- Studied in North America, esp. “old-field” previously farm lands, succession from abandoned
land to forest
- Vegetation changes spontaneously as the vegetation itself modifies the environment
Classification and terminology
- Pioneer species first to appear in the habitat (good at dispersing, r-strategists)
- Climax stage stable equilibrium, no more change
- Primary succession new substrate created, no pre-existing vegetation; substrate new
material that will become soil afterwards
- Secondary succession more often to occur, pre-existing vegetation undergoes a disturbance,
like fire, in which it was destroyed
- Disturbance = discrete event that causes abrupt change in ecosystem, community, population;
sets back succession
Eg of primary succession: Kilauea volcano, Hawaii, seeds and spores blow in, hardy pioneer plants
can establish themselves; soil development, dead plants contribute organic matter and more
complex soil starts to develop -> more plants can establish; birds attracted by plants -> birds bring in
and disperse seeds; herbaceous plants cover ground, trees grow. Tree canopy closes in, shade
becomes important
Example of secondary succession: temperate deciduous forest biome, Joker’s Hill or Koffler Scientific
Reserve, north of Toronto: Abandoned agricultural fields and pastures. 1) Annual dormant weeds
require light for growth. In farm, farmers will get rid of these seeds, but not anymore, 2) perennial
weeds for several years, 3) woody shrubs move in; 4 tree saplings; 5 tree canopy closes in, shade
becomes main factor; either shade tolerant(can grow with low light levels), or early spring plants; 6-
shrub layers thin, shad-tolerant understory only; 7 only shade-tolerant spp remain, including
canopy tree spp that are now replacing themselves - species turnover minimal
It happens spontaneously
Drivers of terrestrial succession
- Soil development; accumulation of organic matter, N content, pH buffering, water retain
- Shading; Shade-tolerant replace shade-intolerant ones
- Stable climax; may reach it, configuration f dark shade, organic-rich soils
Succession where no climax-type equilibrium
- Boreal forest: successional stages lead to spruce-fir forest, but it does not replace itself
- Acid, sandy soils: pine-oak leaf litter slows down succession, because it make the soil more acid,
therefore, less rich
- Fire-prone ecosystems and biomes
- Systems driven by seasonality
- Cycling of dominants, A replaces B, then B replaces A
- Transient substrates
Tropical rainforest, very unlikely to burn, never get dry enough for fire; succession happens on the
individual level, gap-phase pioneer species come after major tree falls down, which creates a big
gap hen it falls. Not the whole patch gets reset, but as a mosaic.
Fire-dominated communities: pine trees make a lot of flammable compound, if a fire comes there,
everything will go up in a fire, and leaf litter is flammable too. Same for eucalyptus; fire in southern
California, chaparral 2007, every august, have to clean space for houses to protect them from fire; 3
ground fire, big trees will survive, 4- crown fire, ground fire can jump to make crown fire, nothing
will survive
Adaptations to fire: serotinous cones as an adaptation to seeding in after a fire: Fire-opened cones
of pitch pine. Open up to reproduce when trees are on fire serotony. Good time to put seeds
because there is no competition anymore.
Modern understanding of patterns
- Climax = old-growth forest
- Characteristic disturbance regimes -> quasi-equilibrium
- Gap-phase succession little patches that sum up
- Intermediate disturbance hypothesis for max species diversity: