The diagram below shows the layers of the cambium, from the primary xylem (shown in dark green), outward to the cork cambium that forms new bark (shown in yellow). The layers in between produce different types of tissue for conducting water and minerals through the trunk. 

Each layer of new wood that is added to a tree forms a recognizable ring. The cambium produces large cells in the spring, when water is abundant and growing conditions are generally good. As it becomes drier later in the summer and the growing season comes to a close in the fall, the cambium produces smaller cells that have thicker walls. In the late autumn, growth stops. 

The new, large cells that are produced the following spring are easily distinguished from the previous year's growth as a distinctive ring. A ring composed of a light part (spring growth) and a dark part (late summer/fall growth) represents each year's growth.

Dendrochronology: the study of tree rings
Dendrochronology means the measuring of time using trees - dendro = trees, and chronology = system of measuring time. Dendrochronology was recognized as an important source of information on age and climate as early as the 1700s in Europe and in the early 1800s in the United States.

Reading the Rings
Scientists can use the tree rings described above for reconstructing past climate because trees are good environmental indicators. They produce very small rings during years of drought and large rings during years of good growing conditions. 

By counting the rings from the middle of the trunk, and studying the width of the rings, scientists can reconstruct an approximate calendar of wet and dry years. Scientists assume that the weather affected ancient tree ring growth the same way it does today. Scientists can also date the occurrence and frequency of fires by finding scars that appear in the growth rings. 

To read more about dendrochronology, see How Tree Rings Form and Bristlecone Pines.