How much CO₂ does a tree absorb?

How much carbon dioxide is absorbed by a tree? As you might expect the answer is “it depends”. It depends on the species, the age of the tree and its growth. It also depends on the season. A tree will remove carbon dioxide from the air as part of photosynthesis during the growing season and as a tree drops its leaves and these leaves are broken down on the forest floor there will be a release of carbon dioxide (satellite images and accompanying computer simulations of carbon dioxide concentrations show this effect well; see for example the NASA Orbiting Carbon Observatory 2 website [1] or simulation video [2]). The net amount of carbon dioxide locked into the tree will be the carbon that is stored as new growth of the trunk and branches (and roots).

It follows that a small tree, like a dwarf fruit tree, will lock in less carbon than a huge beech or oak. High growth trees like poplars will lock in more over a shorter period.

For the reason of slow growth, we also must be careful what we claim is the carbon stored: it will happen slowly so just because we may have planted a tree in the autumn we can’t claim that has offset a tonne of our emissions already!

So how much carbon does a tree sequester?

What is wood made of? Wood is made up of varying quantities of cellulose, hemicellulose and lignin, depending on the species and in particular whether the tree is deciduous or coniferous. By dry mass, a typical tree may be around 50% carbon. The rate of increase in mass of a tree appears to depend on the tree size [3], so the bigger the tree, the greater the mass added and therefore the greater the carbon sequestered. A growth model for 41 European species from [3] is reproduced in Figure 1 with a high mass and high growth species indicated. It is insightful to analyse this single example to show what it means in terms of growth over time. Figure 2 uses this model and we can see that this example does sequester nearly 4 tonnes of carbon but it takes almost 100 years after the tree trunk passes the 10cm diameter mark to get there (the methodology in [3] only considered trees starting from 10cm diameter, so this model may not be appropriate for saplings). Importantly we can see that the carbon sequestered after 30 years is only around half a tonne and that after 50 years is around one tonne. We would need to leave the tree alone for it to sequester increasingly large levels of carbon as it grows. It also becomes a more valuable and powerful carbon store as it increases in size. Finally it is important to know what will happen to the tree when it dies or is cut down: all the good work locking in carbon could literally go up in smoke if it is burnt!

An accessible paper on carbon sequestration of UK trees is [4] and it shows that a wild cherry can sequester 1.5 tonnes of carbon over 45 years, an oak can sequester 2.9 tonnes of carbon over 100 years and a fast-growing poplar can sequester 0.7 tonnes of carbon over 25 years. These are equivalent to 5.6, 10.5 and 2.4 tonnes of carbon dioxide for the three tree species.

Don’t let the slow growth put you off. If we really want to make net zero by 2050 (or some people are aiming for 2030) then it means we must start planting trees now!

[1] NASA Orbiting Carbon Observatory 2 website, https://www.nasa.gov/mission_pages/oco2/index.html, retrieved February 2021.

[2] NASA: A year in the Life of Earth’s CO2, https://youtu.be/x1SgmFa0r04, 17 November 2014, retrieved February 2021.

[3] Stephenson, N., Das, A., Condit, R. et al. Rate of tree carbon accumulation increases continuously with tree size. Nature 507, 90–93 (2014). https://doi.org/10.1038/nature12914

[4] Cannell, M. G. R., “Growing trees to sequester carbon in the UK: answers to some common questions”, Forestry, Vol. 72, No. 3, 1999, pp.237—247