It helps to start with the overall reaction of photosynthesis: 6CO₂ + 6H₂O + Sunlight ----------> C₆H₁₂O₆ + 6O₂

The basic equation has 3 inputs: carbon dioxide, water, and Sunlight and 2 outputs: Glucose and Oxygen. This means for any enclosed system, such as a plant in some kind of controlled environment, determining the amount of Carbon Dioxide absorbed by the plant for photosynthesis can be done by measuring changes in any of the inputs, which should decrease, or outputs, which should increase proportionally according to the chemical equation.

For instance, we could measure Oxygen production by the plant, and know that for every 1 molecule of Oxygen produced, 1 molecule of Carbon Dioxide was incorporated into glucose by the plant. Oxygen production can actually be observed very easily by bubbling in some underwater plants and changes in production can be seen in real time by the naked eye if you alter the light intensity. Other approaches include measuring carbon dioxide using infrared light. Carbon dioxide absorbs infrared light, which we can detect using special instruments.

Measuring CO₂ levels coming into a chamber with a plant and CO₂ levels going out gives us a direct measurement of how much carbon dioxide the plant is capturing. Most of these methods only work in labs, and can give us theoretical ideas about how much carbon dioxide plants absorb in the wild, but rates in the wild often depend on altering environmental factors.

Measurements outside of the lab are often done by comparing dry mass of plants over time, as the mass changes are proportional to the amount of glucose stored, which, as explained above, is directly proportional to the carbon dioxide absorbed from the atmosphere by the plant.

There are actual sensors that measure carbon dioxide in the air. Researchers can measure changes in carbon dioxide under controlled conditions, like a greenhouse or a chamber around part of a plant to measure CO₂. Then they can scale this up to get an estimate of what’s happening on the level of a forest.

Another way to figure this out is to measure how much new tissue a plant is making. Most of the mass of new plant tissue (wood, cellulose, etc.) comes from CO₂ that the plant absorbs through holes in its leaves. So researchers can measure plants at the start of the growing season and at the end. They can take sample plants at the start and end, dry them out, and weigh them. That tells them how much new tissue a typical plant made in the season. Then they can calculate how much CO₂ the plant had to absorb to make that much tissue. This is not easy because they have to measure the amount of new root growth too. That isn’t too hard with a small plant, but imagine uprooting a grown tree without destroying any of the tiny roots.

Once they have equations for a particular species, they can do estimates based on measurements that are easy to make. For example, if they know that, on average, it takes X amount of CO₂ for a red osier dogwood to grow 1 cm, they can just estimate how many of those bushes there are, sample some to get the average amount they grew in a particular place and time, and estimate the amount of CO₂ absorbed.

One of the big themes here is obviously making reasonable estimates based on data, then calculating what this would mean on the level of an entire forest.

Do you think trees absorb more carbon dioxide when they are young or when they are mature?

That’s a great question! The way scientists measure carbon uptake by trees depends on a number of things. They have to take into consideration the species of tree, growth conditions in different environments, age of trees, and density of surrounding trees. If that sounds really confusing, it is! But scientists have made formulas for it. I’ll tell you about one of the formulas below, from some scientists. The problem is, those formulas have to be based on “assumptions” because we can’t go out and measure every tree in the forest accurately.

Some assumptions they make:
-35% of the green mass of a tree is water, 65% is solid dry mass
-50% of the dry mass of the tree is carbon
-20% of tree biomass is below ground level in the roots, so they multiply the mass # by 120% to account for stuff below ground
-carbon figure is multiplied by a factor of 3.67 to determine the equivalent amount of CO₂ it turns into.

For example, they said: "Example: For a 12 year old spotted gum tree weighing 600kg green, then the amount of CO₂ sequestered by the entire tree = 600kg x 65% x 50% x 3.67 x 120% = 859 kg CO₂ or 72 kg CO₂ /yr”.

But what if you don’t know the green mass of the tree? They have a formula for that too. Tree mass (kg) = Volume of the tree (m³) x density of wood (kg/m³)

After all of those calculations, scientists can begin to understand how much CO₂ our forests are absorbing. It’s a tough job - but a very important one. If anything, it demonstrates just how important even one old and big tree is to our planet’s ecosystem.

The simplest way to do that would be just by weight or volume.

Cellulose is a compound sugar, and the number of carbon atoms, along with the weight of a sugar molecule, is known, so you just plug the numbers into the formula and tell the calculator do crank out the number.

However, you can also stick a plant into an airtight enclosure and measure how quickly the CO₂ concentration goes down. Because you know the air pressure in the enclosure, and the volume of the enclosure, you can easily calculate how much CO₂ was there to begin with, and how much is left.

Not easily!!

Scientists may have overcome a major hurdle to calculating how much carbon dioxide is absorbed and released by plants, vital information for determining the amount of carbon that can be safely emitted by human activities. The problem is that ecosystems simultaneously take up and release CO₂. The key finding is that the compound carbonyl sulfide, which plants consume in tandem with CO₂, can be used to quantify gas flow into the plants during photosynthesis.

"In photosynthesis, plants 'breath' in carbon dioxide from the atmosphere and, with sunlight energy, convert it and water into food and oxygen, which they then 'exhale,'"

Previous laboratory research showed that carbonyl sulfide is taken up in step with photosynthesis. But unlike CO₂, there is no emission of carbonyl sulfide from plants.

"That key missing piece has been a thorn in the side of carbon-cycle research for years. "

"The only way to measure what is sequestered by plants is measure the biomass of the plants. This can be done through estimating allometric relationships between something that is easily measured non-destructively (basal diameter of stem, etc.) and the biomass of the plant."

Biomass sampling can give you good results, but can be time consuming. Allometric relationships can be used to avoid the destructive measurements. You will need to know (or develop) a relationship between diameter and biomass. This may require destructive sampling to build this…although you may find some of this in the literature.