Carbon dioxide enrichment at Higher Level: The same statement as SL. HL questions also link the results to limiting factors (3.3.7): extra carbon dioxide helps only while it is limiting.
Practise this as you read
- Evaluate enclosed and FACE experiments.
- Explain why predictions are uncertain.
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The big idea: Atmospheric carbon dioxide is rising. To predict how photosynthesis and plant growth will change, scientists grow plants at higher carbon dioxide and compare them with plants at today's level.
The points to remember
- Enclosed experiments: greenhouses or growth chambers with added carbon dioxide.
- FACE experiments: rings of pipes release carbon dioxide over plants growing in the open.
- Measure the rate of photosynthesis and growth (for example biomass or yield).
- Compare with a control at today's carbon dioxide concentration.
- Results help predict future crop yields and forest growth.
Remember it as: Grow the future today, then compare.
Real example: forest FACE experiments. FACE rings have also been built around stands of trees, for example in Duke Forest in North Carolina. They showed that the extra growth can be smaller than expected when another resource, such as soil nitrogen, becomes limiting.
Trap: Extra carbon dioxide raises the rate only while carbon dioxide is the limiting factor: in a real field, light, water, temperature or nutrients may limit growth instead.
Two ways to run the experiment:
| Enclosed greenhouse or chamber | FACE (free-air) experiment | |
|---|---|---|
| Conditions | Controlled: light, temperature and water can be fixed | Natural: real weather, soil and pests |
| Strength | Only carbon dioxide changes, so cause and effect is clear | Results are closer to how crops and forests will really respond |
| Weakness | Artificial: plants may respond differently outside | Expensive; other factors vary, so results are harder to interpret |
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How it is asked: Expect a question on what these experiments can predict, or an advantage or limitation of enclosed or FACE experiments.
Evaluate the use of enclosed greenhouse experiments to predict how crops will respond to higher carbon dioxide in future.
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