In climates that have seasons where the tree grows or stands dormant, a tree’s trunk has growth rings, also called tree rings.
Growth rings form under the bark of the tree during every growing season. So, the newest rings are the outside ones near the bark and the oldest ring is the one formed from the first year growth of the tree and that is in the very centre of the trunk. If the growing season is good, e.g. when there is a lot of rain, the tree ring will be thick. If the year is particularly dry, then there will be less growth and the tree ring will be thin.
By counting the number of rings from the bark to the middle, you can tell how old the tree is. By looking at the thickness of the rings you can also get an idea of how much rain fell in each year. In the cross-section of the trunk shown, you can even tell from the scar that the tree was damaged by a forest fire in that year.
In cool temperate climates, where trees do not grow in the low temperatures of winter, a new ring of wood grows under the bark of the tree every summer.
In tropical climates where trees do not grow in the dry season, a new ring of wood grows every wet season.
We have to be careful on the equator. If the climate has two wet seasons (the long and short rains) and two dry seasons, then two rings may form each year. So, to get the age of a tree you have to count the rings and divide by 2 to get the tree’s age.
When trees grow in a swamp or by a permanent river and have a supply of water for 12 months of the year, then, in the tropics, it will keep growing all the time and, in this case, the trunk will not show growth rings.
Dendrochronology
Dendrochronology is the science of dating wooden object by studying their tree rings. By studying first the tree rings of a particularly old tree, we can get a reference pattern for climate over hundreds of years. If we then find a fragment of wood in an old building or boat, we can compare the tree rings with the pattern in the reference tree and use this to tell us in which year the building or boat was made.
Very clear accounts of how Tree-ring dating is used in archaeology can be found in these links:
http://www.pbs.org/time-team/experience-archaeology/dendrochronology/
https://www.nationalgeographic.com/culture/archaeology/how-tree-rings-date-archaeological-site/
Activity
Find a tree that has recently been sawn down.
What can you find out about the story of the tree from its growth rings?
- Which is the newest ring?
- Which is the oldest ring?
- Estimate the age of the tree by counting the number of growth rings.
- What can you say about the weather conditions in each year from now to when the tree was a small sapling? (Remember, the wider the tree rings the better the growing season).
- Measure the diameter of the tree.
- Try to estimate the ages of some trees that have been planted in recent years by measuring their diameter. Ask the owner to check whether your estimate coincides with when they were planted.