This big question has a big answer. So hold onto your thinking hats and come with EcoTree to explore it.
We cheated and gave you the short answer first! It’s based on the estimate that a cubic metre of wood absorbs just under a ton of CO2.
But really a tree absorbs anywhere between 10 and 40kg of CO2 per year on average, depending on a whole host of factors. And it’s all those complex variables that make working out how much CO2 a tree absorbs so interesting. So let’s start breaking it down. For fun!
We’ve developed our own scientific methodology to calculate the average absorption of our trees, verified by Bureau Veritas. We’ve done it to help you quantify your contribution towards global carbon neutrality and go on a bit about this in our Carbon Manifesto. But now that’s said, let’s get to it!
The chemical composition of wood
The chemical composition of wood doesn’t vary much from tree to tree. (Hooray, one thing that doesn’t change massively in all this!)
Cellulose (C6H10O5)n is the main component of the cell walls of trees. It’s a chain of glucose molecules that the tree produces through photosynthesis. We’ll look at how in a minute. Cellulose makes up 50-80% of wood.
Plants absorb water and minerals through their roots to make sap.
The sap travels through the tree to the leaves. The leaves absorb CO2 and light.
The leaves use chlorophyll and the sun’s energy to convert CO2 & water into glucose.
Oxygen is released and the glucose nourishes the tree, transported by the sap.
Purifying the air as it grows
Amazingly, to grow by one cubic metre, a tree will purify nearly one million cubic metres of air of its CO2 (assuming 0.03 to 0.04% of air is CO2).1 Trees are the best.
Depends! Mostly on the species of the tree, but also on the humidity of the air influenced by season and location. Trees contain three types of water; bound water in fibres, free water that’s released when they dry, and water of constitution that’s part of their molecular structure.
50% of a tree’s weight is water. But the moisture content of different living tree species can range from 60% to 200%, where there’s twice as much weight from water as there are wood fibres.
So living trees are really wet, when you think about it! And it’s vital to have moist, green trees in the forest, especially to prevent forest fires. These, as we all know, are devastating. An important reason why forest management is the future.
Now we know how much of a tree is water, we need to remove that from the equation. Because to calculate the carbon content of a tree, we need to know its dry mass. That’s its mass excluding the water.
To do it, let’s use an example. We’ll imagine a 1000kg tree with 100% humidity. Now we know this tree is 500kg water and 500kg dry mass. And we know that 47.5% of that dry mass is carbon. That’s 237.5kg.
Thanks to molar mass ratios, we can break CO2 down and find that it takes 3.67kg of CO2 to create 1kg of carbon in the tree. That’s because carbon has a molar mass of 12 and oxygen 16. Combined as CO2 that’s 44. And 44/12 = 3.67.
So for our tree example, 237.5 × 3.67 = 871.63kg of CO2. Hooray! If we want to know how much it’s absorbed per year, we need to know the age. If it weighs a ton standing, we can guess it’s 30 to 40 years old. So if we assume it's 35 years old, this big boy absorbed 25kg of CO2 per year.
1,000kg of wood
500kg of dry wood
237.5kg of carbon
871.63kg of CO2 absorbed
First off, the carbon storage capacity of a tree depends on its species, as their mass varies. You can really see that in these examples: Poplar (400kg/m3), Weymouth Pine (1000kg/m3) and Ebony (1400kg/m3). So this needs to be taken into account in any calculation.
Another factor which varies is growth rate. Softwoods tend to grow much faster than most hardwoods so can absorb more CO2. But as they also live shorter lives, they ultimately store less CO2 over their lifetime.
This fact has highlighted the problems of overstocking forests with conifers to boost short-term carbon absorption.2 The carbon is released much sooner than from deciduous trees, so they can potentially become carbon sources instead of sinks.
Adam R. Martin and Sean C. Thomas came to the same conclusion in their study published in 2011 after analysing the cores of 59 different tree species from the same geographical area. Ranging from 41.9% to 51.6%, they conclude that the average is just over 47.4% carbon in the wood.
So our calculations are based on plausible estimates, averages and variations like soil, light, age and climate. But in the end, even though they absorb carbon dioxide at different rates and in different amounts, they’re all doing it!
And it just shows the importance of mixing tree species and ages in a forest specifically adapted to its environment. This is the best way to reduce our carbon footprint and limit the impact of climate change. And it’s what we do with your trees in our EcoTree forests.
Young trees or old trees? That is the question scientists are still debating. In their youth, trees grow faster so absorb CO2 quicker, but in their old age their density is much greater so they can absorb more CO2, as this 2014 study in Nature demonstrates. As for our trees, these ones below will give you the most bang for your buck when it comes to CO2 absorbed over their lifetime.
Our carbon capture calculations have been verified by world-renowned Bureau Veritas. So when you own a tree, you can track the carbon captured over its whole lifetime!
In the end, most of the figures put forward should be taken with great care, but they give an average range of 10 to 40kg of CO2 absorbed per tree per year over a lifetime.
1 Vade-mecum du forester , p. 67, XIVth edition, 2016, Forestry Society of Franche-Comté and Eastern Provinces
2 Patrick Vallet's thesis is entitled Impact of different silvicultural strategies on the “carbon sink” function of forest stands. Modeling and simulation at the plot scale