A casual look at a cross section of a tree trunk will reveal a pattern of distinctive rings. As a tree grows, the numbers of rings increase outward from the center. Since each ring usually represents a year of growth, the age of the tree can be roughly determined from counting each ring. However, climatic perturbation such as drought or unusually wet cycles can complicate the simple annual pattern. Beyond merely indicating the number of circular growth marks, tree rings can also reveal information about climate.

When a tree grows annually, it is actually increasing the size and number of xylem–or the woody portion of a tree exclusive of the bark. This growth tends to occur in a seasonal pattern that can be observed in the growth rings. For example, trees in temperate areas tend to grow mostly in the summer when leaf buds form. The winter portion of the cycle begins the dormant period when the leaves fall from the tree. Since the size of the ring is dependant upon moisture and temperature, the cycle is depicted within the rings. When soil moisture is abundant, growing trees form large, thin-walled cells. When less water is available, small, thick-walled cells are added. When viewing a stump, the contrast between smaller and larger cells can be seen.