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:
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!
How to build a tree-ring chronology.