How do tree rings reveal the exact year a beam was cut and the weather of long ago?

A tree writes a diary in its trunk, one line per year. Each year the tree adds a new layer of wood just under the bark, and because growth speed changes through the seasons, that layer shows up as a visible ring. One ring generally marks one year of the tree's life. Count the rings on a cut trunk and you know its age.

The rings carry more than a count. A year with good moisture and a long growing season makes a wide ring, while a drought year may make a very narrow one. So every trunk records the weather it lived through. The inner part of each ring grows early in the season, quickly, and is less dense. The outer part, late wood, is denser.

Dendrochronology, or tree-ring dating, turns this into a clock. Trees from the same region tend to share the same pattern of wide and narrow rings, so researchers can match the pattern ring for ring between trees and build overlapping chronologies. A beam from an old building can be matched against those chronologies and its wood dated precisely, sometimes to the exact year the tree was cut.

Dating got its human face in the early twentieth century, when the astronomer A. E. Douglass founded the Laboratory of Tree-Ring Research at the University of Arizona. He hoped the rings would reveal the Sun's cycles of sunspot activity. His chain of reasoning was sunspots, then climate, then rings. Today rings help date houses, such as the Fairbanks House in Massachusetts, and they help scientists reconstruct climates from hundreds to thousands of years ago.