NOVA Online | Tracking El Niño | How can you tell how old a tree is?

How can you tell how old a tree is?

To find out how old an individual tree is, all you need to do is count the number of rings. To discover how long ago a dead tree might have lived, you need to use a technique known as cross-dating. It works like this:

Diagram depicting aligning of rings from newly-cut, standing dead and fallen dead trees

How to build a tree-ring chronology.

Start with a living tree. A cross-section of its trunk will have a distinctive series of thick and thin rings. Next, find an older tree in the same area and compare tree-ring widths from the two cross sections. You should find a place where the ring patterns overlap (see illustration). Since the age of the living tree is known, once the pattern is matched, scientists can figure out the age of the older tree. By continually overlapping cross sections from older and older trees, scientists can find trees from a long time ago whose rings can provide information about past climates. Currently, the oldest tree-ring chronology dates back 10,000 years!

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Illustration: courtesy of Leonard Miller of the Ancient Bristlecone Pine Web site.

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