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Each year a tree’s growth ring has two parts; one is wide and light colored, and the other is narrow and dark. This grows during the wet spring and early summer when the tree has a lot of sap, and the cambium cells giving rise to the trunk growth are large and thin walled.As the summer winds down and the transition to the cooler autumn occurs, the tree’s growth rate slows. More about the photograph) Lesson plans and essays for teachers and students explore North Carolina's past before European contact.

The relatively abundant rainfall and milder winters tend to blur trees’ annual growth layers; most look so much alike that creating a bridge by matching rings is difficult.

Also, the East’s humid, temperate climate decays wood beams quickly, so a sequence is limited in how far back in time it reaches.

By counting the dark ring segments, scientists can tell a tree’s age if the cross section of the trunk is complete. Based at the University of Arizona in Tucson, Douglass wanted to know how sun spot activity affected climate, and his research soon led him to pioneering tree-ring analysis.

Because the width of tree rings varies with growing conditions, scientists also learn about local climate during the tree’s lifetime by comparing the rings’ different widths. For instance, higher rainfall and a longer growing season produces a wider ring than a year with low rainfall and prolonged cold. Douglass was among the first to notice that trees in a geographic area develop the same growth-ring patterns because they experience the same climatic conditions.

Experience told him this narrow ring would be in all the region’s trees, but at different positions on the stump because of their different ages.