Carbon dioxide is a limiting factor for photosynthesis, and the amount of it in the atmosphere is rising. So an obvious question follows: will plants grow faster in the air of the future? Answering it means running the pondweed experiment on a forest.
📚 What you need to know
Rising greenhouse gas levels, carbon dioxide among them, make it important to predict how plants will respond.
Two approaches: enclosed greenhouse experiments and free air carbon dioxide enrichment (FACE).
A greenhouse allows variables to be manipulated and controlled, but only small species fit inside.
FACE pumps carbon dioxide into a natural ecosystem, so large plants and trees can be studied.
In FACE the other variables cannot be controlled, but they can be monitored.
A valid experiment changes one independent variable and controls or monitors everything else.
Preliminary research is how you find out which variables matter before you start.
Why run these experiments at all
Carbon dioxide is one of the three limiting factors from the previous page. In most natural settings it is the one in shortest supply, so raising it should raise the rate of photosynthesis and, over time, increase growth.
Whether that actually happens in a real ecosystem, and by how much, and for how long, is not something you can work out from a beaker of pondweed. It has to be measured — which is why both kinds of enrichment experiment exist.
Neither method is simply better. The greenhouse gives cleaner data about a less realistic situation; FACE gives messier data about a real one.
Enclosed greenhouse experiments
Grow the plants inside a sealed greenhouse or polytunnel and you can set the conditions yourself.
Variables that can be manipulated: carbon dioxide concentration, light intensity, wavelength of light, and temperature.
All the others can be held constant, so any change in growth can reasonably be put down to the one you changed.
The limitation is size. Only small species fit, so you cannot study a mature tree this way.
The conditions inside are also not quite natural — no wind, no grazing animals, restricted rooting space — so results may not transfer to a real ecosystem.
Free air carbon dioxide enrichment (FACE)
FACE takes the opposite approach. A ring of pipes stands in a real ecosystem and releases carbon dioxide across the plot, raising the local concentration without enclosing anything.
Because there is no structure, large plants and trees can be studied, in real soil, weather and communities.
Other variables cannot be controlled — but they can be carefully monitored, so relationships in the data can still be identified.
The results are more likely to reflect what will actually happen as atmospheric carbon dioxide rises.
The trade-off is that any effect you see could partly be down to something else, such as an unusually wet year.
Researchers often run both: a greenhouse study to establish the mechanism, then a FACE site to see whether it holds up outdoors.
Compare
Enclosed greenhouse
FACE
Where
Sealed greenhouse or polytunnel
A natural ecosystem, in the open
Size of plant studied
Small species only
Large plants and trees
Other variables
Controlled directly
Cannot be controlled, only monitored
Main strength
Clear cause and effect
Realistic, so more likely to predict the future
Main weakness
Artificial conditions, limited species
Confounding variables are unavoidable
The science behind the design
Both methods rest on the same idea: you can only draw a conclusion if you know what else was changing.
The independent variable is the one thing you deliberately change — here, carbon dioxide concentration.
The dependent variable is what you measure to see the outcome, such as growth rate or dry biomass.
Controlled or confounding variables are everything else that could affect the result. Control them if you can; monitor them if you cannot.
If an uncontrolled variable changes alongside the independent variable, the results are not valid — you cannot say which one caused the effect.
Preliminary research and small pilot studies are how you identify which variables matter, and how to keep them steady, before committing to the full experiment.
“Valid” and “reliable” are not interchangeable. Valid means you measured what you intended, with no other variable interfering. Reliable means repeats give consistent results. A study can easily be one without the other.
Worked examples
WORKED EXAMPLE
In a greenhouse trial, plants at normal carbon dioxide reached a mean dry mass of 12.0 g. At doubled carbon dioxide the mean was 15.6 g. Calculate the percentage increase.
Step 1: find the difference15.6 − 12.0 = 3.6 gStep 2: divide by the original and multiply by 100(3.6 ÷ 12.0) × 100 = 30A 30% increase in dry massalways divide by the original value, not the new one
WORKED EXAMPLE
Suggest why a FACE experiment might give a smaller growth increase than a greenhouse experiment at the same carbon dioxide concentration. [3]
Step 1: think about limiting factors outdoors
In a real ecosystem another factor may become limiting — water, minerals, light or temperature.
Step 2: think about what a greenhouse removes
Indoors those factors are kept at optimum, so carbon dioxide is the only limit.
Step 3: state the consequence
Outdoors, raising carbon dioxide alone cannot raise the rate once something else is limiting.
Another factor becomes limiting outdoors, so the extra carbon dioxide has less effectthis links straight back to the limiting factor graphs — the plateau again
WORKED EXAMPLE
A FACE plot shows greater tree growth than a nearby control plot. A student concludes that the extra carbon dioxide caused the growth. Evaluate this conclusion. [3]
Step 1: what supports the conclusion
Carbon dioxide is a limiting factor, so more of it plausibly raises the rate of photosynthesis.
Step 2: what weakens it
In an open ecosystem other variables were not controlled — soil, water and light may differ between plots.
Step 3: what would strengthen it
Monitor those variables, use several replicate plots, and repeat over more than one season.
Plausible but not proven, because confounding variables were not controlled“evaluate” wants both sides plus an improvement, not just criticism
💡 Exam tip
Know what FACE stands for: free air carbon dioxide enrichment. It is worth writing out in full once.
For greenhouse studies say controlled; for FACE say monitored. That one word difference is often the mark.
Give the size limitation of greenhouses: only small species fit.
Link back to limiting factors whenever a question asks why enrichment had a smaller effect than expected.
Use dry mass rather than fresh mass as a growth measurement, because water content varies.
For evaluate questions, give a strength, a weakness and a way to improve the study.
⚠ Common mix-up
Saying FACE controls the other variables. It cannot — it monitors them.
Assuming more carbon dioxide always means more growth. Only while carbon dioxide is the limiting factor.
Confusing valid with reliable. Valid is about the right variable; reliable is about consistent repeats.
Treating the greenhouse as the “better” method. Each answers a different question.
Forgetting a control plot. Without one, there is nothing to compare the enriched plot against.
Measuring fresh mass. A well-watered plant weighs more without having grown more.
That completes Photosynthesis. Up next: Neurones: Structure & Function — from the chemistry of energy to the wiring that carries messages around the body.
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