How old is this tree? In what year did bugs attack it? When did it catch a disease—or catch on fire?

Some trees are hundreds or thousands of years old. No one could spend that long studying them! And yet, people can find out the answer to all of these questions. They can even learn all about the oldest of the forest’s oldies. How do they do it?

Easy: They look at tree rings. In order to learn about a tree’s age or history, scientists look inside its trunk. They count how many rings run from a tree’s center to its outside.

This study has a name: dendrochronology. Dendro means tree. A chronology is a way of measuring time. Each tree ring has two parts: a light and a dark part. The light parts form in the spring. The dark parts form in the late summer or fall. In a normal situation, a tree gets one new ring every year. If the growth was interrupted by bugs, a disease, or a fire, scientists will be able to tell. The rings will have scars in them.

In dry years, trees make small rings. In years when a tree has plenty of water, large rings appear. That’s a great help to scientists. They don’t just learn the history of the tree. They learn the history of a tree’s climate!