How do the water and carbon cycles operate in the tropical rainforest, and how is human activity changing them?
The interlinked operation of the water and carbon cycles in the tropical rainforest, the diagnostic stores and flows of this ecosystem, and the impact of human activity (especially deforestation) on its water and carbon balances.
An OCR A-Level Geography case study of the water and carbon cycles in the tropical rainforest, covering how the two cycles interlink, the diagnostic stores and flows of this hot, wet ecosystem, the role of recycling and the rapid nutrient and carbon turnover, and how deforestation disrupts the water and carbon balances of the Amazon.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
OCR wants you to explain how the water and carbon cycles operate, and interlink, in a tropical rainforest, describe the diagnostic stores and flows of this ecosystem, and assess how human activity, above all deforestation, disrupts its water and carbon balances. The tropical rainforest is one of OCR's two named contrasting case studies (the other is the Arctic tundra).
The answer
Why the rainforest couples the two cycles
The defining condition is abundant energy and water, which together support the highest rates of biological activity of any terrestrial biome. High insolation and humidity drive both vigorous photosynthesis (carbon uptake) and intense evapotranspiration (water recycling), and the two are physically linked through the leaf: stomata open to admit carbon dioxide and inevitably lose water vapour. This coupling is why the rainforest is simultaneously a major moisture recycler and a vast short-term carbon store, and why disturbing one cycle disturbs the other.
The water cycle in the rainforest
The rainforest water cycle is dominated by recycling. Of the heavy rainfall, a large fraction is intercepted by the multi-layered canopy and re-evaporated, and much of what reaches the soil is taken up by roots and transpired. The combined evapotranspiration loads the atmosphere with moisture that condenses and falls again as convectional rainfall, so the forest effectively waters itself; a significant share of Amazon rainfall is recycled rather than imported from the ocean. Stores are skewed to the biosphere and atmosphere rather than the soil, and the dense vegetation keeps overland flow and erosion low despite the rainfall intensity.
The carbon cycle in the rainforest
Carbon turns over rapidly. Photosynthesis fixes large amounts of atmospheric carbon dioxide into the dense biomass (high net primary productivity), while respiration and decomposition return carbon to the atmosphere; in an undisturbed forest these are roughly balanced, with a small net sink. The largest store is the above-ground and below-ground biomass; the litter and soil store is comparatively small because decomposition is so fast. Carbon also leaves via rivers as dissolved and particulate organic matter. The system is therefore a globally important short-term carbon store and, in its intact state, a net absorber of carbon dioxide, central to the global carbon balance.
Human disruption: deforestation
Deforestation for cattle ranching, soy, logging and roads disrupts both cycles. In the water cycle, removing the canopy cuts interception and transpiration, so less moisture is recycled, regional rainfall and humidity fall, overland flow and soil erosion rise, and rivers become flashier. In the carbon cycle, clearance, especially by burning, releases stored biomass carbon directly to the atmosphere, future photosynthetic uptake is lost, and exposed soil carbon oxidises. At small scale these effects are local, but extensive clearance risks a regional tipping point: reduced rainfall stresses the remaining forest, causing further dieback (positive feedback), which has already switched parts of the Amazon from a net carbon sink to a net source in some years.
Examples in context
Example 1. The Amazon as a moisture-recycling, carbon-storing system. The Amazon basin recycles a large share of its own rainfall through evapotranspiration, generating "flying rivers" of moisture that sustain rainfall across South America, and it stores on the order of a hundred billion tonnes of carbon in its biomass. This makes it a globally significant store in both cycles and the textbook OCR case study, illustrating the coupling of water recycling and carbon uptake in a single ecosystem.
Example 2. Deforestation and the tipping-point risk. Decades of clearance for cattle and soy, with associated burning, have removed large areas of Amazon forest and, in drought years amplified by El Nino and fire, parts of the south-eastern basin have been measured as net carbon sources rather than sinks. Reduced transpiration lowers downwind rainfall, stressing the remaining forest, the positive feedback behind warnings of an Amazon tipping point. This provides the located evidence and evaluation the 16-mark questions demand and links directly to the climate-change debate.
Try this
Q1. State two reasons rainforest soils contain relatively little carbon. [2 marks]
- Cue. Rapid decomposition in hot, humid conditions returns carbon to vegetation quickly; heavy rainfall leaches nutrients; most carbon is stored in the living biomass instead.
Q2. Explain one way deforestation changes the local water cycle. [4 marks]
- Cue. Removing the canopy cuts interception and transpiration, so less moisture is recycled and regional rainfall falls, while reduced infiltration and bare soil raise overland flow, erosion and river flashiness.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H481/01 (style)6 marksExplain how the water and carbon cycles are linked in a tropical rainforest.Show worked answer →
A medium-tariff Levels-of-Response question (AO1 and AO2). The link is vegetation, which couples the two cycles. In the water cycle, dense forest intercepts rainfall and returns vast quantities of water to the atmosphere by transpiration and evaporation (evapotranspiration), recycling moisture that falls again as convectional rainfall. In the carbon cycle, the same vegetation fixes carbon by photosynthesis (using that water) and returns it by respiration and decomposition.
For AO2, reward candidates who show the coupling explicitly: photosynthesis needs both water and carbon dioxide, so the high rainfall and humidity drive high carbon uptake, and transpiration both moves water and accompanies the stomatal exchange of carbon dioxide. The strongest answers note the rainforest is therefore a major short-term carbon store and a powerful moisture recycler, so disturbing one cycle (clearing trees) disrupts the other.
OCR H481/01 (style)16 marksAssess the impact of deforestation on the water and carbon cycles of the tropical rainforest.Show worked answer →
A 16-mark extended response across four Levels (AO1 and AO2). Establish the undisturbed system: a closed-canopy rainforest recycles much of its own rainfall through evapotranspiration and stores large amounts of carbon in biomass and soil. Deforestation disrupts the water cycle by cutting interception and transpiration, reducing recycled moisture and convectional rain, raising overland flow and soil erosion, and lowering humidity, potentially drying the regional climate. It disrupts the carbon cycle by releasing stored biomass carbon (especially if burned), reducing future photosynthetic uptake, and exposing and oxidising soil carbon.
A strong AO2 judgement weighs scale and feedback: local clearance has measurable effects, but at the basin scale extensive deforestation can cross a tipping point where reduced rainfall causes further forest loss (positive feedback), switching parts of the Amazon from a carbon sink to a source. Reward a supported, located conclusion (the Amazon) rather than a list of impacts.
Related dot points
- The global water cycle as a closed system of stores and flows; the drainage basin as an open sub-system with inputs, flows, stores and outputs; the water balance; and the natural and human factors that change water stores and flows across scales.
An OCR A-Level Geography answer to the water cycle in Earth's Life Support Systems, covering the global water cycle as a closed system of stores and flows, the drainage basin as an open sub-system, the water balance equation, and how natural and human factors change water stores and flows across global to local scales.
- The carbon cycle as a closed global system of stores and fluxes; the biological, geological and oceanic sub-cycles; carbon sequestration over short and long timescales; and the natural and human factors that change carbon stores and fluxes.
An OCR A-Level Geography answer to the carbon cycle in Earth's Life Support Systems, covering the carbon cycle as a closed global system of stores and fluxes, the biological, geological and oceanic sub-cycles, fast and slow carbon sequestration, and how natural and human factors change carbon stores and fluxes.
- The interlinked operation of the water and carbon cycles in the Arctic tundra, the diagnostic role of permafrost and frozen stores, and the impact of human activity and climate change (especially permafrost thaw) on its water and carbon balances.
An OCR A-Level Geography case study of the water and carbon cycles in the Arctic tundra, covering how the cycles operate in a cold, frozen environment, the diagnostic role of permafrost and the active layer, the huge soil carbon store, and how warming, permafrost thaw and resource extraction disrupt the water and carbon balances.
- The consequences of carbon-cycle change for the atmosphere, oceans and ecosystems; the links between the carbon cycle and climate; and the mitigation and adaptation strategies that manage the water and carbon cycles at different scales.
An OCR A-Level Geography answer to the consequences of carbon-cycle change and the management of the water and carbon cycles. Covers the impacts of rising carbon on the atmosphere, oceans and ecosystems, the link between the carbon cycle and climate, feedbacks, and mitigation and adaptation strategies from international agreements to local action.
- The evidence for and causes of past and present climate change; the greenhouse effect and feedbacks; the differential impacts on people and environments; and the mitigation and adaptation responses, evaluated synoptically across physical and human geography.
An OCR A-Level Geography answer to the Climate change debate in Geographical debates, covering the evidence for past and present climate change, natural and anthropogenic causes, the greenhouse effect and feedbacks, the differential impacts on people and environments, and the mitigation and adaptation responses, treated synoptically for Paper 03.
Sources & how we know this
- OCR A-Level Geography (H481) specification — OCR (2016)