For moving forward with our project it’s essential to calculate how much carbon dioxide a tree can consume by photosynthesis.
Of course, it depends on many factors that are hard to measure and also factors that are easy to measure.
For our project, we are going to start with rough estimates and are going to precise them during operation in the future.
Which aspects affect CO2 consumption?
For converting carbon dioxide into oxygen, trees need a lot of surfaces to capture CO2 and sunlight. Furthermore, trees must have a lot of water for the chemical reaction to happen.
These conditions apply to trees:
- old trees consume more CO2 than young trees
- solitaire trees absorb more CO2 than trees in the forest
- tropical forests store more CO2 than trees in higher latitudes
- wet trees absorb more CO2 than dry trees
What does the chemical formula look like?
The tree uses chlorophyll to perform the chemical reaction.
Sunlight and CO2 get trapped and converted into the outputs with the help of water.
The outputs of the equations are glucose, oxygen and water.
Consider that half of the water gets used by the tree and therefore it needs a lot of water for photosynthesis.
The output water usually evaporates into the atmosphere. Glucose will get used by the tree for other operations.
6 CO2 (carbon dioxide) + 12 H2O (water) + sun energy -> C6H12O6 (glucose) + 6 O2 (oxygen) + 6 H2O (water)
How to calculate the CO2 absorption
To identify the maturity of tree CO2 conversion, we measure the trunk diameter (1.3 meters over the ground) and the tree height. With these two values, we can calculate to CO2 binding ability based on the tree species.
For our example: We assume we found a spruce tree with a diameter of 50cm and a height of 35m.
Next, we have to calculate the timber volume. We include branches but exclude the roots of the tree.
For our example, the timber volume of our tree is 3.4 m3.
For calculating the mass of the timber volume, we have to look up the density for dried spruce wood.
The density for spruce wood is 0.43 g/cm3 = 430 kg/m3.
So our tree has a dry mass of 3.4 m3 * 430 kg/m3 = 1.462 tonnes.
Usually, half of the mass is made of carbon. But it also depends on the moisture of the tree.
Our example tree has 0.5 * 1.462 tonnes = 731 kg carbon
To reverse engineer the carbon dioxide input mass needed for this amount of carbon, we’re multiplying our carbon mass by 3.67.
731kg carbon * 3.67 = 2.682 tonnes of carbon dioxide.
Using this calculation, we know that our tree stored 2.682 tonnes of CO2 over its life span.
Why is the CO2 to C ratio 3.67?
The mass of carbon in the tree is less than that of the CO2 input because photosynthesis separated the oxygen atoms from the CO2 molecule.
A look in the periodic table reveals that carbon (symbol C) has a mass of 12.011 grams per mol and oxygen (symbol O) has a mass of 15.999 grams per mol.
Therefore CO2 has a total mass of 12.011 + 2* 15.999 = 44.009 grams per mol.
The CO2-to-C ratio is therefore 44/12 = 3.67.
What tree varieties store most of the carbon?
Usually, the higher the density of the wood, the more carbon it can store. The disadvantage is that it also takes longer to grow.
We sorted the species from high carbon storage to low carbon storage here:
hornbeam > beech > oak > birch > maple > larch > pine > douglas fir > spruce > fir > black poplar
Rule of thumbs:
Multiple rules of thumbs exist for trees:
- On a worldwide average, a tree stores 10kg of CO2 yearly
- One hectare of forest absorbs 6 tonnes of CO2 yearly on average
- One cubic meter of wood stores about 1 tonne CO2. (That’s an average density of 545 g/cm3.)
- You need 80 beech trees to capture 1 tonne of CO2 yearly (height 23m, diameter 30cm)
- German forests capture about 62 million tonnes of CO2 yearly. That’s about 7% of the German CO2 emissions.
- Reforestation could absorb an additional 370 to 750 billion tonnes of CO2 worldwide. (YouTube: What if there were 1 trillion more trees?)