Annual Rings

How much rain has fallen on the earth in the past? Man has not always kept weather records. Because scientists need a way to learn about past rainfall, they study rings. A tree's trunk grows bigger each year. Beneath its bark, a tree adds a layer of new wood each year it lives. If you look at a tree stump, you can see the layers. They are called annual rings. On some trees all of the rings are of the same width. But the ponderosa pines that grow in the American southwest have rings of different width. The soil in the southwest is dry. The pines depend on rainfall for water. In a year of good rainfall, they form wide rings. In a dry year, they form narrow ones. Scientists do not have to cut down a pine to see its rings. With a special tool, they can remove a narrow piece of wood from the trunk without harming the tree. Then they look at the width of each ring to see how much rainfall in the year it formed. Some pines are hundreds of years old and so have hundreds of rings. These rings form an annual record of past rainfall in the southwest.