3.3.6Inquiry Process
Carbon Dioxide Enrichment Experiments — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 121 of 160
Part 1 of 5Why Do These Experiments?
Enrichment experiments test how plants respond to higher carbon dioxide concentrations.
Rising carbon dioxide
- The concentration of carbon dioxide in the atmosphere is increasing,
- so scientists want to know how plant growth and photosynthesis will respond.
Predicting the future
- Experiments that raise carbon dioxide concentration give evidence for predictions about crops, forests and carbon storage.
Food security
- The results help to predict future crop yields, which matter to the global food supply.
Ecosystem effects
- Changes in plant growth can alter food webs, water use and the amount of carbon stored in biomass.
Part 2 of 5Two Experimental Designs
Enclosed greenhouse
- Plants grow in a greenhouse or polytunnel, so light, temperature, carbon dioxide and wavelength can be altered and controlled.
Greenhouse limitation
- Only small species that fit in the enclosure can be studied, and a greenhouse differs from a natural ecosystem.
FACE experiments
- Free air carbon dioxide enrichment releases carbon dioxide from pipes into an area of natural ecosystem or crop field.
FACE advantage
- Large plants and trees can be studied in realistic conditions, with their normal soil, pests and neighbours.
FACE limitation
- Other variables such as rainfall and temperature cannot be controlled, only monitored, so effects may be harder to separate.
| Feature | Enclosed greenhouse | FACE |
|---|---|---|
| Variable control | High | Low |
| Plant size | Small | Large, including trees |
| Realism | Lower | Higher |
| Cost | Lower | Higher |
Part 3 of 5Experimental Design
Experimental Design
Independent variable
- The one factor changed deliberately, such as carbon dioxide concentration.
Dependent variable
- The measured outcome, such as photosynthetic rate, dry mass or height of the plants.
Controlled variables
- Other factors that could affect results, such as light, temperature and water, must be controlled or at least monitored.
Preliminary research
- Early studies help to identify which variables matter and how to control them effectively.
Part 4 of 5Evaluating the Evidence
1
Compare plants grown at ambient carbon dioxide concentration with plants grown at raised concentration, with every other condition kept the same where possible.
2
Measure dependent variables such as rate of photosynthesis, dry mass, leaf area or crop yield over a full growing season.
3
Repeat on several plant species, because different species may respond differently to extra carbon dioxide.
4
Consider whether another factor, such as water or nutrient supply, limits growth, so that extra carbon dioxide has less effect than expected.
5
Treat the conclusions as provisional, since experiments last only a few years and may not reflect long-term responses.
Part 5 of 5Exam Checklist & Tips
3.3.6 – Key Points for the Exam
- FACE Free air carbon dioxide enrichment: an experiment that raises carbon dioxide in an open ecosystem.
- Independent variable The variable deliberately changed in an experiment.
- Dependent variable The variable measured as the outcome.
- Confounding variable An uncontrolled variable that may affect the result.
Common Mistakes
- Assuming a greenhouse result applies exactly to a forest; the conditions are very different.
- Failing to monitor variables that cannot be controlled in a FACE experiment.
- Describing FACE as an enclosed experiment, when it takes place in open air.
- Changing carbon dioxide and temperature together, which makes it impossible to see the effect of either alone.
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