Where do trees get their mass from?
As trees photosynthesise they use sunlight to combine carbon dioxide (CO2) from the atmosphere with water and nutrients from the ground to form carbohydrates which make up the tree’s biomass. CO2 is taken in at a certain rate and builds the mass of the tree over time. Biomass is a measure of the dry mass of woody and leaf matter in kg.
The carbon content of a tree is approximately 50% of its biomass (dry mass), though recent studies show this varies a lot between species and locations e.g tropical and boreal. The other 50% is made up of hydrogen, oxygen and nitrogen and other elements.
The tree pictured below shows a fallen, dying Sycamore in Priory Park, Reigate. The carbon that is stored in its biomass is either being released back into the atmosphere as CO2 or added to the carbon in the soil through decomposition. Respiration by soil biota will eventually lead to most of the carbon being released back into the atmosphere as CO2. The rest of the carbon will probably be transported away dissolved in water and eventually runoff into rivers, like the River Mole.
Dead Sycamore tree in Priory Park, Reigate… releasing captured carbon
The carbon content of a tree depends on how big it is, how old it is, the species of the tree (they grow at different rates and capture different amounts of carbon at different life cycle stages; usually, younger trees sequester carbon more quickly than older trees) and local conditions and the health and management of the tree also determine how much carbon is sequestered (taken in and stored by photosynthesis).
The tree pictured below is one of the biggest and oldest OAK trees in Priory Park… note the large GIRTH in comparison with the Labrador dog at the left foot of the trunk. The girth is 5 metres, which makes this oak around 300 years old. Note also that the tree has been pollarded to increase growth earlier in it’s life: this management will have increased carbon capture.
The size of a tree can be determined by the HEIGHT, GIRTH and root size. Roots also contain carbon, so we must include these to get an idea of how much carbon a tree has sequestered.
Very roughly speaking, a tree absorbs up to 20 kg CO2 per year = about 1 tonne of carbon by age 40. However, these figures vary a lot between species and locations. Add the enormous amount of carbon stored in forest soils to that of the trees, and forests are major carbon storage reservoirs.
How to measure carbon content in a tree
As a rough start, the key figure of carbon content is the SIZE of the tree. Bigger trees are usually older and have captured more carbon dioxide from the atmosphere during their life and should therefore contain more carbon!
The GIRTH (circumference) of a tree is a good basic measure of SIZE or how BIG the tree is. Girth is measured at chest height 1.3m above the ground.
Easy carbon content calculator method using just girth:
As the carbon content of a tree is approximately 50% of the biomass, we can estimate the carbon content by calculating the mass of the tree and dividing the dry weight by two. The CO2 captured by the tree in its life time is calculated by multiplying the carbon content by 3.67. Bingo! = CO2 sequestered by the tree.
What is the advantage of this “easy” method of calculating carbon sequestration in trees?
Surrey has 42,000 hectares of WOODLAND… it is one of the most forested counties in the UK. It has been calculated that the trees in Surrey remove 350,000 tonnes of carbon per year.
(note: possible error? the video above shows her measuring circumference, she then quotes the diameter)
A more complete but complex method is shown below.
The excel spreadsheet below can calculate a carbon content of a tree given the height and circumference entered in metres and cm respectively. The biomass calculated includes roots and above ground weight.
If you know the age of the tree the spreadsheet will also be able to calculate more accurately how much CO2 has been sequestered per year of its’ life. The age of a tree can also be estimated by knowing the average growth rate which varies between species…
How to measure the HEIGHT of a tree…
- Measure the HEIGHT of your tree
2. How to measure the circumference of your tree
To measure the girth (circumference) of a tree you need a tape measure wrapped round the trunk at 1.3m above the ground (commonly called breast height).
3. How to calculate the diameter of your tree
The diameter of the tree is needed to calculate biomass. Diameter = circumference/Pi
It is difficult to calculate the exact amount of CO2 sequestered by a tree per year due to the complex biological and climatic variables involved in tree growth.