Introduction

Forests are often considered to provide a key ecosystem service in the form of above ground carbon storage. How much carbon is stored in a hectare of forest? How does carbon accumulate and how is it released?

Putting numbers into context

In order to have an intuitive understanding of any number it must be put into context. Carbon dioxide is emitted to the atmosphere as a result of burning fossil fuels for heating, transport and industry. Land use change can also add carbon dioxide to the atmosphere. When looking at emissions reporting you should be aware that it can sometimes be reported in terms of carbon dioxide, and sometimes as carbon itself.The atomic weight of carbon is 12 atomic mass units, while the weight of carbon dioxide is 44, because it includes two oxygen atoms that each weigh 16. So, to switch from one to the other, use the formula: One ton of carbon equals 44/12 = 11/3 = 3.67 tons of carbon dioxide.

Currently Our World in data reports that carbon dioxide emissions per capita in the UK are around 5.2 tons. This is equivalent to 1.4 tons of carbon. So an average household will emit around 5 tons of carbon. Keep this number in mind as a reference.

A report from Natural England (Gregg et al. 2021) suggests that a total of 4,000 Mt CO2 (around 1,000,000,000 tons of carbon) is stored in UK forests. This is equivalent to around 10 years of total UK carbon emissions. This includes both above ground and below ground storage.

The carbon cycle

Different types of vegetation will have different carbon dynamics and store different amounts of carbon. In woodland most of the carbon captured through photosynthesis during the growing season is released in the winter months through ecosystem respiration. Gross primary productivity in woodlands may be around 22 tons of carbon per hectare per year with respiration amounting to a release of 20 tons. This leaves around 2 tons sequestered. Notice that this is just over half the amount produced by a household.

Over longer time frames carbon accumulates and is stored, both above ground and below ground. This simplified carbon cycle represents a deciduous forest in which leaves are shed in the autumn. Forests consisting of conifers can grow more quickly and store more carbon above ground.

Carbon storage in natural habitats

The habitats which store the greatest total mass of carbon all do so below ground. Forests differ in having a visible above ground carbon store, but even in many forests most of the carbon may be in the soil.

How much carbon is stored above ground in a stand of trees?

There are two approaches to this question.

  1. Carefully measure every element of the forest. Leaves, twigs, branches and tree trunks. This could provide a precise answer.
  2. Estimate the amount using simplifications. This will provide an imprecise answer, but is very useful when putting the question into a broad context.

To get a rough and ready estimate we can use the following method. We need a few quantities as input.

  1. An estimate of the average (mean) diameter of the trees.
  2. An estimate of the average (mean) height of the trees.
  3. An estimate of the number of trees per hectare. To get this we need an estimate of the distance between the trees.

Let’s say the mean diameter of the trees is 30 cm. The mean height is 10 m. Trees are spaced at 5 m intervals.

The area of the base of the tree is given by \(A = \pi r^2\)

So A = 0.15 ^2 * 3.14 = 0.07 \(m^2\)

The volume of the tree trunk if it were a cylinder is simply the area times the height.

V = 0.07 * 10 = 0.7 \(m^3\)

However, a tree narrows from the base to the tip. In forestry this is known as the form factor. Each species has a different form factor which also depends on growth conditions. However a reasonable estimate for many trees is around 0.6

V = 0.7 *0.6 = 0.42 \(m^3\)

As we are only interested in a very rough estimate that is correct to within the right order of magnitude we can simplify this to assume that each trees volume is around 1 \(m^3\). If the density of the wood was the same as water this would amount to 1 ton of material. Its actually rather less, but let’s continue to simplify. Around a half of the volume is carbon. So each tree would contain around 0.2 tons of carbon.

Now, how many trees are there in a hectare? If they are spaced at 5 m intervals then there would be 20 trees per 100m. A hectare is 100m by 100m. So the total number of trees is approximately 400

So a rough and ready estimate of the total amount of above ground carbon would be 80 tons

Reference

R Gregg, J. L. Elias, I Alonso, I.E. Crosher and P Muto and M.D. Morecroft (2021) Carbon storage and sequestration by habitat: a review of the evidence (second edition) Natural England Research Report NERR094. Natural England, York.