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3.3.6Additional lessons

Carbon Dioxide Enrichment Experiments — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 61 of 142

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Part 1 of 6Purpose and Design

3.3.6 Carbon Dioxide Enrichment Experiments
Aim
  • Enrichment experiments test how plant growth responds to higher carbon dioxide concentrations.
Greenhouses
  • Growers can raise carbon dioxide in enclosed greenhouses, which increases photosynthesis and crop yield.
Field experiments
  • Free-air carbon dioxide enrichment rigs release the gas around plants in open fields without enclosing them.
Controls
  • Control plots are grown at normal atmospheric concentration under the same conditions.
Measurements
  • Growth is measured as height, leaf area, biomass or yield, using dry mass for accuracy.

Part 2 of 6Findings

1
Many plants show faster growth at higher carbon dioxide, because carbon dioxide can limit photosynthesis.
2
The effect is often smaller than predicted, because other factors such as water and minerals then limit the rate.
3
Over a longer time plants may acclimatise, and the extra growth may decrease.
4
Stomata of some plants partly close at high carbon dioxide, which may also reduce water loss.
5
Results are used to predict how crops and ecosystems may respond to rising atmospheric carbon dioxide.

Part 3 of 6Evaluating the Evidence

Evaluating the Evidence
Replication
  • Several plots and repeated measurements show whether differences are likely to be real.
Limitations
  • An enclosed experiment may differ from natural conditions, and a short study may not show long-term effects.
Application
  • Higher yield may be offset by lower nutritional quality or by climate change effects such as drought.
Statistics
  • A statistical test can show whether differences in mean biomass are significant.

Part 4 of 6Link to Climate

Link to Climate
Atmosphere
  • Atmospheric carbon dioxide concentration has risen since the industrial revolution, mainly from burning fossil fuels.
Carbon sink
  • Plants take up carbon dioxide in photosynthesis, so growth responses affect how much the atmosphere retains.
Prediction
  • Results from enrichment experiments help to model food production under future climates.

Part 5 of 6Worked Example

Question

  • In a field trial the control plot yields 6.0 tonnes per hectare and the enriched plot yields 6.9.
  • Calculate the percentage increase.

  1. Increase = 6.9 − 6.0 = 0.9 tonnes per hectare.
  2. Percentage = 0.9 ÷ 6.0 × 100 = 15 %.

Answer A 15 % increase in yield (example data).

Part 6 of 6Exam Checklist & Tips

3.3.6 – Key Points for the Exam

  • Carbon dioxide enrichment Raising carbon dioxide concentration around plants to study or increase growth.
  • Biomass The dry mass of organic matter in an organism or area.
  • Acclimatisation Adjustment of an organism to a change in its conditions.
  • Control plot An area grown under normal conditions for comparison.

Common Mistakes

  • Assuming that more carbon dioxide always means proportionally more growth.
  • Measuring fresh mass, which changes with water content, instead of dry mass.
  • Forgetting that the control and treatment must have the same temperature, light and water.
  • Stating that the result applies to all plants without evidence.

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