One line a year
Why Does a Tree Have Rings?
Each ring is a year of growing. Reading the widths tells you what the weather was doing before anyone was writing it down.
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Step 1 of 5
A trunk grows outwards, not upwards.
New wood is added in a thin layer all round the outside, under the bark. A branch two metres up stays two metres up for the tree's whole life.
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Step 2 of 5
Spring wood and summer wood look different.
In spring the tree grows fast and makes wide, pale, open cells. Later in the year growth slows and the cells are small and dark. That dark band is the line you see.
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Step 3 of 5
So one pale band plus one dark line is one year.
Count the dark lines from the middle outwards and you have the tree's age. It works because the growing stops each winter and starts again each spring.
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Step 4 of 5
The width of each ring records the year it had.
A warm wet year makes a wide ring. A drought or a cold summer makes a narrow one. Every tree in a district has the same pattern of good and bad years.
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Step 5 of 5
Which lets those patterns be matched up and joined together.
The pattern in a living tree can be matched to the outer rings of an old beam, and that beam to an older one. Chains like this reach back thousands of years.
Rings only work where there is a winter. Trees in the wet tropics grow all year and often make no clear rings at all.
The short version
A tree adds one ring a year — dark line for the slow late growth — and the width of each ring records how good that year's weather was.
Try it yourself
Find a cut log or a stump and count the dark lines. Look for a run of thin ones together: that was a hard few years.