Dendrochronology is a technique that involves calculating the age of a tree, or a piece of wood, by counting growth rings. As well as growing upwards, trees grow outwards each year by adding new woody tissue just underneath the bark, in an area known as the vascular cambium.

In the spring, trees produce large, thin-walled cells under the bark, but as growth slows towards the end of the summer, they add denser, thicker-walled ‘latewood’ cells that appear darker in colour, leaving visible rings in the trunk that mark each year of growth.

How does dendrochronology work? © Dan Bright
How does dendrochronology work? © Daniel Bright

Dendrochronology has a wide range of applications, including dating archaeological remains and calibrating radiocarbon dating. As the width of each ring depends on how much new tissue was added during the growth season, which is influenced by the climate and environmental conditions that year, tree rings can reveal information about past climatic changes. However, like all techniques, dendrochronology has limitations.

First, it requires a full cross-section of a tree, right up to the bark, which is not always present in trimmed timber samples. Additionally, damage to the tree (such as removing the bark) can deform tree rings as the plant grows over the scar.

Some trees can be more reliably dated than others. Oak trees, for example, reliably produce a single ring each year, whereas alder, pine and birch have more erratic growth cycles and may skip a ring in some years, or double up in others.

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Asked by: Helen Stevenson, via email

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Dr Claire Asher is a science journalist and has a PhD in Genetics, Ecology, and Evolution (GEE) at the University of Leeds. She also works part time as Manager of the UK Robotics and Autonomous Systems (RAS) Network, based at Imperial College London. Asher is also the author of Brave Green World: How Science Can Save Our Planet.