How much carbon does one tree store?

Plants “breathe in” CO2 and “exhale” oxygen in the process storing carbon, but just how much does the average tree (and forest) store, and how do we measure this? Through photosynthesis, light energy from the sun and the CO2 that plants breathe in is converted to a range of sugars, cellulose and other organic compounds, all containing carbon. We can estimate the amount of carbon stored in a tree by calculating its biomass – the total weight of the stem, branches, leaves and roots.

Carbon stored in a single tree

Step 1

By measuring the diameter and height of a tree, we can then use a function to calculate the volume of its trunk. For small trees, the trunk shape is close to that of a cone. For larger trees, the shape of the trunk is a little more complex. Foresters will develop “volume equations” for specific species.

Step 2

Once we know the volume of wood in the trunk of a tree, we can estimate its biomass (the weight of dry plant matter) if we know the density of the wood. Density of wood may vary from around 160 kg/m3 for balsa wood, 400 kg/m3 for pine, and up to 1,000 kg/m3 for some hardwoods. An average figure is around 500 kg/m3.

For a trunk volume of 0.47 m3

& Wood density of 500 kg/m3

Trunk biomass = 236 kg dry plant matter.

Step 3

Once we know the amount of biomass in a tree stem we can estimate the total tree biomass using allometric equations, or average ratios between trunk biomass, the biomass of the branches and leaves, and that of the roots. A common ratio for crown biomass (leaves and branches) to trunk biomass is 0.25:1. Similarly a common average ratio of root biomass to total above ground biomass is 0.25:1. Based on these ratios, we can calculate the total biomass of a tree as follows:

Step 4

Now that we have the total weight of plant material for a tree, we can estimate the amount of carbon. Because plant material is made up of organic compounds that have an average carbon density of 0.5 kg carbon/kg dry plant material. The other 0.5 kg/kg of dry matter is largely oxygen and hydrogen, bound together in organic molecules. For a Trunk biomass of 368.75 kg: Carbon mass = 0.5 x 368.75 = 184.375 kg carbon.

Step 5

The international trading unit for carbon is measured in CO2 equivalents (CO2e). This may be calculated using the ratio of the molecular mass of carbon (12) to the molecular mass of CO2 (44). For a pure carbon mass of 184.375 kg: Equivalent mass of CO2e = 44/12 x 184.375 kg = 676.0417 kg (0.676 tonnes).

A simple rule of thumb is the ratio of CO2e to tree trunk volume. Based on the above calculations, this ratio is 1 m3 = 1.4 t CO2e.

Carbon stored in a forest

Now that we have estimated the mass of carbon stored in a tree, we need to extrapolate this to a forest. Many make the mistake of simply multiplying the volume measured from a single tree. However, trees in a forest compete with each other for resources (light, water nutrients), so that a tree growing in competition with others in forest will not grow as much as a tree growing on its own in a paddock. In addition, the number of trees per unit land area will decline over time as individual trees grow larger and the competition increases. Open grown trees have more access to resources than close spaced trees in a forest and will have move volume for the same age and height.

For the reasons above, we need to know how all the trees in a forest stand grow together, and how

this changes over time. For this, we establish the pattern of growth of a forest over time. This typically follows an “S” shaped or sigmoidal curve. The total biomass that can grow on a site is a dependent on the rainfall and quality of the soil. A typical growth curve for a forest on high quality soil and 600 mm annual rainfall is shown below.

The amount of stemwood volume is designated by the blue line, and the amount of carbon stored by the green line. It can be seen that after a forest is planted, it will continue to strongly absorb carbon from the atmosphere for around 30 to 60 years, after which growth (and carbon sequestration) plateaus.

Legacy Carbon

Forest bio-sequestration of carbon differs from many other forms of carbon abatement, as it is one of the few means of removing “legacy” carbon from the atmosphere where it is stored in living biomass, rather than simply avoiding new carbon emissions. If the forest is harvested for timber and replanted, then some of the carbon stored in the forest remains locked up in timber products such as telegraph poles, house framing and furniture, further reducing the amount of carbon in the atmosphere available for global warming.

Every cubic metre of wood in living trees, power poles, house frames and furniture keeps around 1.4 tonnes of CO2 out of the atmosphere.

Help with carbon management

TCT has conducted numerous carbon projects including: estimating and reviewing carbon stores for large forestry organizations; due diligence of carbon yield estimates for private investors, policy and legislation advice to carbon liable entities and; direct establishment and management of forest carbon sinks. Contact us to find out how we can assist you in meeting your carbon requirements.

Crown biomass = 0.2 x trunk biomass = 0.2 x 236 kg = 47.2 kg

Above ground biomass = Trunk Biomass + crown biomass = 236 kg + 47.2 kg = 283.2 kg

Root biomass = 0.25 x above ground biomass = 0.25 x 283.2 kg = 70.8 kg

Total biomass= Trunk biomass + Crown biomass + Root biomass = 236 kg + 47.2 kg + 70.8 kg = 354 kg