Secondary succession at Higher Level: The same ideas as SL, with different examples: New England's lost farms, the North Carolina old fields and the boreal forest after the Fort McMurray fire. At HL, expect a figure of a grazed landscape: name the process when grazing stops.
Practise this as you read
- Order the seral stages on abandoned farmland and after a fire.
- Contrast secondary with primary, place by place.
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Starting again, on soil: In secondary succession the community is removed but the soil stays. Life returns on that soil, so the process starts well ahead of a primary succession.
The points to remember
- Secondary succession starts on bare soil where a community lived before.
- A disturbance removes the community, but the soil stays.
- The guide's two cases: a field where farming stops and a forest after an intense fire.
- Floods, storms and logging can start it too, and so can ending grazing or mowing.
- It follows the same pattern as primary: pioneers, grasses, shrubs, trees, climax.
Remember it as: Soil stays, so life comes back faster.
Real example: by the mid-1800s farmers had cleared most of the forest of New England, USA. As farms were abandoned for better land further west, forest grew back on the old fields; old stone field walls now run through the woods.
Name the process in full: Grassland turning into forest once grazing stops is secondary succession. 'Succession' alone is not enough, and it is not primary: the soil was there all along.
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When a farmer stops ploughing, the field does not stay bare for long. The soil holds a seed bank, and each seral stage replaces the last.
When farming stops
- First year or two: annual weeds and grasses grow fast from the seed bank and from seeds blown in.
- Next: perennial grasses and herbs outcompete the short-lived annuals.
- Then shrubs (bramble, hawthorn) grow above them and shade them out.
- Trees grow up through the shrubs; their seeds come on the wind or with animals.
- Each stage changes the light and the soil, so the next stage outcompetes it.
- After decades to a century or more, the climax forest returns.
Remember it as: Weeds, grasses, shrubs, trees.
Real example: in North Carolina, ecologists compared fields abandoned at different times. Pines take over in about 5-15 years; oak and hickory, whose seedlings grow in shade, replace them over a century.
Not always more species: In flower-rich pasture, stopping grazing lets scrub and then forest shade out the grassland flowers, so biodiversity can fall for a while as secondary succession runs on.
An intense fire can kill most plants, but it leaves the soil, and often seeds and roots in it. The forest then rebuilds itself in seral stages.
Succession after a fire
- The fire opens the canopy, so sunlight reaches the forest floor.
- Ash releases nutrients, so the soil is fertile.
- Plants grow from seeds that survived, seeds released by the heat, or seeds blown in from unburnt forest.
- r-strategist pioneers (fireweed, fast annuals, aspen) cover the ground first.
- Over time K-strategists (slow-growing, shade-tolerant trees) replace them: each stage is outcompeted.
- The climax forest returns only after many years, because trees grow slowly.
Remember it as: Light, ash, seeds, pioneers, slow trees, climax.
Real example: in May 2016 a wildfire burned about 590 000 hectares of boreal forest around Fort McMurray, Canada. Aspen sends up new shoots from its surviving roots within weeks; spruce, which grows in shade, takes about a century to take over again.
Put the stages in order: Start with what the fire changed (light, ash), then where new plants come from, then pioneers, then their replacement, then the climax. One stage per sentence.
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Both successions pass through seral stages to a climax, but secondary succession starts far ahead, because the hardest step, making soil, is already done.
Why secondary is faster
- The soil is already there: no waiting for rock to weather.
- The soil holds nutrients and organic matter (and ash after a fire).
- It holds a seed bank, plus roots and stumps that resprout.
- Soil organisms (decomposers, fungi, worms) are already there.
- Seeds arrive quickly from nearby land that was not disturbed.
- So secondary succession takes decades to a century or two; primary takes hundreds to thousands of years.
Primary succession
- Starts on bare rock, sand or silt
- No soil, no seeds, no life
- Pioneers: lichens, mosses, algae
- Very slow: hundreds of years or more
- New lava, moraine, fresh dunes
Secondary succession
- Starts on bare soil
- Soil, seed bank and roots remain
- Pioneers: grasses, dandelions, fireweed
- Faster: decades to a century or two
- Abandoned farmland, a burnt forest
In one line: Primary = from scratch, on bare rock. Secondary = the soil is already there, only the community was removed.
Real example: at Glacier Bay, Alaska, bare moraine needs over 200 years to reach forest (primary). An abandoned North Carolina field carries pine forest in about 25 years (secondary).
How this comes up: Paper 2, Section B (a): outline, with named examples, how the two types of succession differ [4].
Ecologists study both new volcanic islands and abandoned farmland to learn how communities develop.
With reference to named examples, outline how secondary succession differs from primary succession.
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