Tree Rings as Natural Records

Each year, a tree grows a new layer of wood just beneath its bark. In spring, when water is plentiful, the tree produces large, light-colored cells. Later in the season, growth slows and the cells become smaller and darker. Together, these two bands form one annual ring, so counting the rings reveals a tree's age.

The width of each ring also carries information. [A] In years with enough rain and mild temperatures, trees tend to form wide rings, while droughts or unusually cold summers produce narrow ones. [B] Because trees in the same region respond to the same weather, their ring patterns match. [C] By comparing samples from living trees with those from old buildings and preserved wood, scientists have built continuous records that stretch back thousands of years. [D]

These records have proved useful in many fields. Historians and archaeologists use them to determine, sometimes to the exact year, when wooden buildings and ships were constructed. Climate researchers, meanwhile, use them to reconstruct rainfall patterns from periods before weather instruments existed. The study of tree rings, known as dendrochronology, therefore allows researchers to read the environmental history stored inside ordinary pieces of wood.