Dendrochronology is the dating of objects or historical events by the study of the annual growth rings of trees. Tree trunks grow in diameter by producing a thin layer of woody cells around the circumference. These cells arise from an actively dividing tissue, called the cambium, just beneath the bark. In temperate regions, such growth occurs only seasonally, creating annual layers, or rings.
At the beginning of the spring-summer growing season, conditions for growth are generally optimum; the woody cells produced are larger and more thin-walled than those produced later when conditions are less favorable. The change from one season's layer of large woody cells to that of the next makes one annual ring. By counting the annual rings inward from the outside, the dates of droughts, fires, and other occurrences that affect tree growth can be reliably established. A chronology of over 7,000 years has been established with this technique.