Stores, sinks and sources at Higher Level: The same ideas as SL, with different examples: the world's land sink, British Columbia's pine-beetle forests, the Amazon's south-east and Scotland's Flow Country. At HL, expect to explain how an ecosystem changes role.
Practise this as you read
- Compare photosynthesis with respiration for every example.
- Explain what turns a store or sink into a source.
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It is all about the balance: Whether an ecosystem is a store, a sink or a source of carbon depends on the difference between the carbon coming in and the carbon going out.
The points to remember
- Carbon comes in by photosynthesis and goes out by respiration, decomposition and fire.
- In more than out: net accumulation, so the ecosystem is a sink.
- Out more than in: net release, so the ecosystem is a source.
- In and out balanced: a store in equilibrium, holding what it has.
- The same ecosystem can change role when it grows, ages, burns or is cleared.
Remember it as: More in = sink. More out = source. Level = store.
Real example: added together, the world's land ecosystems take in about 3 billion tonnes of carbon a year more than they release. This land sink soaks up nearly a third of the carbon people emit.
Say which process is bigger: Do not just say 'it takes in carbon'. Every ecosystem does. Say that photosynthesis is greater, or smaller, than respiration.
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A forest shows all three roles during its life. Learn these three: they are the most asked example.
The points to remember
- Young forest = sink: fast-growing trees absorb CO₂ and turn it into new biomass.
- Mature forest = store: it contains carbon in biomass for the trees' long lifespans; growth and decay balance.
- Destroyed forest = source: fire, slash-and-burn or clearing releases the stored carbon as CO₂.
- Dead trees that decompose also release CO₂, more slowly.
- Use the verbs: stores contain, sinks absorb, sources release.
Real example: after 2000, pine beetles killed vast areas of pine in British Columbia, Canada. As the dead trees decayed and burned, the forest turned from a small sink into a large source.
Too brief is not enough: 'Trees store carbon dioxide' is too short. Say what each role does: a store contains carbon in biomass over time, a sink absorbs CO₂ for photosynthesis, a source releases CO₂ by fire or decay.
Big idea: Forests are major carbon sinks because trees absorb CO₂ during photosynthesis and store carbon in biomass and soils. Cutting forests can turn a sink into a source.
Where carbon is stored in a tree
Trees store carbon mainly in biomass: carbon-rich tissues built from CO₂ absorbed during photosynthesis.
- Wood (trunk, branches, roots): the biggest store
- Leaves
- Bark
- Leaf litter and soil organic matter under the trees
Photosynthesis moves carbon from the atmosphere into plant biomass (CO₂ becomes organic matter).
What happens to carbon when forests are cut?
The carbon does not disappear: it is transferred to other stores. What happens depends on what happens to the wood after cutting.
- Burning (fastest release): carbon in wood is oxidised, so CO₂ is released quickly
- Decomposition (slower release): decomposers break down dead wood and respire, so CO₂ is released over time (and methane may form where there is little oxygen)
- Timber (delayed release): carbon stays in buildings or furniture, then is released later when the wood decays or burns
- Soil loss: without roots, soil is eroded and its carbon is lost; bare, warmer soil with more oxygen decomposes faster
Deforestation has a double effect: (1) it releases stored carbon, and (2) it reduces future CO₂ uptake because fewer trees remain to photosynthesise.
If a tree falls and stays on the forest floor
Stops
The tree stops photosynthesising.
Decomposers feed
Bacteria and fungi feed on the dead wood and respire.
Most returns
Most of the carbon returns to the atmosphere gradually as CO₂.
Some stays
Some becomes soil organic matter (humus), stored for years to centuries.
Conditions matter: warm, wet, oxygen-rich soils speed decomposition (more CO₂ released). Cold, dry or waterlogged soils slow decomposition (more carbon stored in the soil).
Real example: in Indonesian Borneo, forest cleared for oil palm is often burned first. The trees' carbon goes up in smoke, and the young palms store far less than the forest did.
Exam sentence: Deforestation increases atmospheric CO₂ because stored carbon is released through burning or decomposition, and fewer trees remain to absorb CO₂ by photosynthesis. Always say where the carbon was stored: in the biomass or in the soil.
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Fire turns a forest from a store into a source in hours, and it adds to the enhanced greenhouse effect.
The points to remember
- Burning organic matter releases CO₂, a greenhouse gas, in hours.
- The ground re-radiates heat; greenhouse gases trap that outgoing heat.
- Burned forest photosynthesises less, so less CO₂ is absorbed and CO₂ accumulates.
- Together: more CO₂ in the air, so an enhanced greenhouse effect and warming.
- Warmer, drier weather brings more fires: a positive feedback.
Forest burns
- Farmers clear and burn forest in the south-east Amazon for cattle pasture.
CO₂ released
- Burning and decay release the carbon stored in the trees as CO₂.
Less uptake
- Pasture and damaged forest photosynthesise far less than the old forest.
Heat trapped
- The extra CO₂ traps heat re-radiated by the ground, so the climate warms.
Real example: a 2021 study (Gatti and others) measured the air above the Amazon from aircraft. The south-east, where fires and clearing are worst, now releases more carbon than it absorbs: it has become a source.
Two marks, two links: Link the fire to the extra CO₂, then the CO₂ to trapped heat. 'It causes global warming' alone is too vague.
Forests are not the only stores. Some of the biggest carbon stores are wet, cold or under the sea, where dead matter decomposes slowly.
The points to remember
- Peat: waterlogged, so little oxygen; dead plants decompose very slowly and pile up as carbon.
- Tundra: cold and frozen ground (permafrost) slow decomposition; thawing turns it into a source.
- Mangroves, seagrass and kelp bury carbon in mud: a big store for their size.
- Draining, clearing or overgrazing these places turns a store into a source.
- Read data carefully: quote numbers with units, and give the full name of a category.
Real example: Scotland's Flow Country holds about 400 million tonnes of carbon. Parts drained for forestry in the 1980s are now being re-wetted to turn them back from sources into stores.
Reading carbon data: For a difference, subtract and give units: '180 - 14 = 166 g C per m²'. From a map key, give the whole category: 'peatlands and tundra', not 'peatlands'.
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How this comes up: Paper 2, Section B (b): explain, with examples, how ecosystems act as stores, sinks and sources of carbon [7].
Some ecosystems remove carbon from the atmosphere, while others add to it.
With reference to named examples, explain how ecosystems can act as stores, sinks and sources of carbon.
Model answer plan
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