By Harry Carstairs
Trees absorb carbon dioxide from the atmosphere, helping to slow down climate change. Follow these simple steps to calculate how much carbon is stored inside a tree near you.
You will need…
- A tape measure. A flexible tailors’ one works best as you will need to wrap it around the trunk of your tree.
- A piece of paper. It doesn’t matter if there is anything written on it, so re-use an old piece of paper if possible.
- A calculator. You can use a calculator app on your phone.
- A notepad and pen. You can either write notes in your own notebook or on a printed copy of this worksheet.
Five steps to success…
- Find a tree
- Make initial observations
- Measure trunk circumference
- Measure tree height
- Calculate the carbon
It will be easiest to do this exercise with at least two people, especially for a larger trees, but you can do it on your own too. A smartphone could also come in handy.
Step 1 – Find a Tree!
You could be outside for a little while, so remember to dress for the weather. Head out into your local street, park, woodland, or wherever there are some trees near you.
As you walk, stop from time to time, crane your neck back and take a moment to gaze up into the treetops. Do you spot anything you haven’t noticed before? You will notice that different trees can have very different shapes – some have very straight trunks while others twist and turn – some have branches all the way up while others only have branches at the top. This depends on where the tree is growing and what nutrients and light are available to it, as well as its species. The different shapes of different trees means it tricky to get a precise estimate of the carbon stored in a tree using just a tape measure. However, that doesn’t stop us from calculating a ‘ball park’ figure.
Choose your favourite tree. It’s up to you which one you go for, as long as you can see the top of it, and there is a flat, safe area to walk away from it.
Step 2 – Initial Observations
Soon you will measure the circumference and height of your tree (don’t worry you won’t need to do any climbing). Before you do this, though, try answering the following questions to get to know your tree a bit better, writing down your answers if you can. If you have a camera with you, take some pictures too.
Where is your tree? (Write a description that you will remember, and you could even use your phone as a GPS to get your latitude and longitude. Are you in an urban or rural area?)
What environment does your tree live in? (E.g. street, woodland, field, river-bank, hillside… flat or steep terrain)
What species does your tree belong to? (If you don’t know, try out the Woodland Trust’s identification app which you can find here. Is it deciduous or coniferous?)
Tree form – how would you describe the shape of the tree’s trunk and branches? Are there any unusual features? Are there any signs of missing branches or other damage?
How high do you think it is? You can come back to this later and see how good your guess was after you have measured the tree’s height.
Step 3 – Measure the circumference
It is now time to give your tree a hug. Go to its base and wrap your tape measure all of the way around the trunk. Do this at about the height of your chest, at a place below the first branch and where the trunk is a relatively round, even shape.
Pull the tape measure tight, and make sure it isn’t twisted. What is the circumference of the tree? Divide the number of centimetres by 100 to get the circumference in metres.
Step 4 – Measure the height
There are a few ways to measure the height of a tree, but this one is the simplest, and doesn’t require any calculations. For more illustrations of the method, checkout the WikiHow page: https://www.wikihow.com/Measure-the-Height-of-a-Tree
First, fold your piece of paper at the corner to make a 45 degree angle as shown.
Next, face the tree and hold the paper up so that the bottom edge is horizontal, and one of your eyes is looking directly along the 45 degree edge.
If you have a friend helping you, ask them to check that you are holding the paper so that it is flat. What can you see when you look along the edge of the triangle?
If your view is between the bottom and top of the tree, take a few steps back. If your view is over the top of the tree, take a few steps forward. Keep adjusting your position until your view directly aligns with the top of the tree.
Now, use your measuring tape to measure the distance from where you are standing to the base of the tree. Write down the number.
You are nearly there – all that is left is to add on the height of your eye from the ground, and you will have the height of the tree. Was your estimate in step 2 close?
Height of Tree = Distance to tree + Height of your eye =
Step 5 – Calculate the carbon
Now that you have made your measurements, you can begin the calculations. You can do this part when you get home if you prefer.
Here is the equation you need to use, and an explanation of what each part of the equation means.
Carbon = 0.0537 x Density x Height x Circumference2
Carbon – this is how many kilograms of carbon your tree is storing
0.0537 – this factor takes into account the geometry of a tree and how much carbon is contained in wood. See the “how does it work?” section below to learn more.
Density – the wood density in kg per cubic metre. The denser the wood, the more carbon it will contain. This depends on the tree species – use the table below to find your tree’s wood density. If your tree isn’t in the table or you aren’t sure which species, use a rough value of 600.
Now you can work out…
Height – this is the tree height, as you measured it in metres.
Circumference – this is the circumference of the trunk, as you measured it in metres. Remember to square this value.
Carbon content of your tree (kg) =
How does it work?
The formula works by making some assumptions about the shape of the tree:
- Every time a branch splits off from the main trunk, the cross sectional area of the branch and the trunk above the split add up to the cross sectional area of the trunk below the split.
- All paths from the bottom of the trunk to the end of a branch are the same length as the height of the tree.
These assumptions, combined, mean that the volume of the tree’s trunk and branches is the same as a cylinder with the same height as the tree, and a circumference equal to the circumference at the base of the trunk.
Do you see any problems with these assumptions?
How would you design a more accurate method?
Laser scanning provides one way of obtaining a much more precise estimate of the volume of wood in a tree. Check out Matt Disney’s blog at disneytls.blogspot.com to learn more!
Here is an example from the blog of the high resolution models of tree structure that laser scanning can provide.
To obtain the mass of wood in a tree, the volume is multiplied by wood density. As you can see from the table, this can vary a lot depending on the species of tree. A factor is also applied to add in the mass of the roots below ground.
Finally, we know that about 50% of the mass in a tree is carbon. Applying this factor brings us to the equation you used on the previous page.
How much carbon is that?
The average person in the UK causes about 10,000 kg of CO2 to be emitted into the atmosphere each year. Because CO2 molecules contain oxygen as well as carbon, we need to multiply this by a factor of 0.27 to compare it to the carbon in your tree.
This means that the average person in the UK has a “carbon footprint” of 2,700 kg of carbon. How does this compare to the amount of carbon that your tree has absorbed from the atmosphere in its whole lifetime?
Depending on the size of the tree you chose, you might find yourself impressed or disappointed by the result! Trees do store a lot of carbon, but it takes time for it to accumulate. That is why it is so important that tree planting efforts are planned carefully, so that those trees are protected and nurtured, allowing them to grow for many decades to come.