Glasgow Science Festival - Measure a Tree Activity

 How much carbon is in a tree?

Scientists measure trees using lasers, to estimate how much carbon they have absorbed from the atmosphere. Here is how you can measure a tree without any fancy equipment.

You will need:

• a tape measure,
• pen and paper,
• and a calculator

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

Remember to dress for the weather! Head out into your local street, park, woodland, or wherever there are some trees near you. Along your way, stop from time to time and take a moment to gaze up into the treetops or listen to the sounds around you. Do you find anything you haven’t noticed before?

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 or wheel around it.

Step 2 - Get to know your tree

• Where is your tree? Note a description that you will remember, or try a GPS app such as What3Words on your phone.
• 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 aren’t sure, try out the Woodland Trust’s identification app https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/british-trees/tree-id-app/
• How would you describe the shape of the tree? Are there any unusual features? Are there any signs of missing branches or other damage?
• How tall would you guess the tree is?
  

Step 3 - 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 1.3 m above the ground, 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 in metres?

Step 4 - Measure the height

• Fold your piece of paper at the corner to make a 45 degree angle.
•  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. 
• Adjust your position until your view 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.
• Finally, 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?

For more illustrations of this method, checkout the WikiHow page: https://www.wikihow.com/Measure-the-Height-of-a-Tree 

Step 5 - Calculate the Carbon

Carbon = 0.0537 x Density x Height x Circumference x Circumference

• Carbon – kilograms of carbon in your tree.
• 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 to find your tree’s wood density, or use a rough value of 600.
• Height – as you measured it in metres.
• Circumference – as you measured it in metres. Remember to square this value.

Species Densities in kilogram per cubic metre

Alder | 550

Ash | 710

Aspen | 420

Beech | 800

Birch | 670

Cherry | 630

Chestnut | 560

Elm | 570

Holly | 750

Larch | 525

Lime | 560

Oak | 740

Scots Pine | 510

Sycamore | 500

Willow | 500

Yew | 670

So, 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 per year. 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!

How does it work?

The formula works by making some assumptions about the shape of the tree:

1. 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.

2. 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 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?

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.

The bottom line

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.

Explore More

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! To see what laser scanning looks like in practice, watch this video of laser scanning in Gabon: