Reading the Rings Dendrochronology and the Secret Diaries of Trees

Every tree on Earth is keeping a diary, and most of the entries are about the weather. The diary has one line per year, written in wood, and it has been kept without interruption for as long as the tree has lived. A scientist with a thin hollow drill can read it without cutting the tree down.

The tool is called an increment borer. It is a hollow steel screw that bites into the trunk and pulls out a pencil-thin core of wood, running from the bark to the center. Hold that core up to the light and you see stripes. Each pair of light and dark bands is one year of growth. In spring and early summer, a tree lays down wood that is pale and soft. Later in the season, growth slows and the wood turns dense and dark. Then winter stops the process, and the next spring starts a new layer on top. Count the bands and you know the tree's age. People have known that for centuries.

The deeper discovery is that the bands differ in width. A wet year with a long growing season produces a thick ring. A drought produces a thin one. A frost in the wrong week can leave a scar. The National Park Service notes that the same annual precipitation can produce differently sized rings, depending on the moisture a tree received during its particular growth season, and that small rings generally mark dry years. So each tree carries a record of the conditions it lived through, written in a barcode of fat and thin stripes.

Here is where the science becomes powerful. Trees growing in the same region, under the same sky, tend to share the same pattern of good and bad years. A pine on one hillside and a pine a few miles away will both show a thin ring in the same drought year. That means the pattern from one tree can be lined up against the pattern from another, and the match locks both into the calendar.