Wolffia using a increment borer to age-date an old sierra juniper (Juniperus occidentalis var.australis) on a steep 9,000 foot ridge of Pine Mountain in the San Gabriel Range of southern California.Often the borer does not reach the center of the trunk, so the total number of years must be extrapolated from the radius of the trunk.The radius (r) can be determined from the circumference of the trunk (C=2πr), or from special tape measures that give the diameter directly.Not all trees can be measured or used without additional analytical techniques: not all trees have cambiums that are created annually.In tropical regions, for example, annual growth rings are not systematically formed, or growth rings are not tied to years, or there are no rings at all.The 1906 ring pattern in wood Sample A (which was cut from Stump A) correlates with a 1906 ring pattern in Sample B which was cut from an older, undated Stump B. By matching up similar spaced rings in Samples B, C and D, the ages of ancient timbers can be determined.As long as the wood samples being compared have some ring patterns that coincide, time may be extended back through an unbroken succession of growth rings.
This remarkable tree was approximately 1400 years old, and grew on this rugged mountain ridge during the time of Mohammed.
See the cal BP discussion for additional information about radiocarbon calibration.
Tree-ring dating works because a tree grows larger--not just height but gains girth--in measurable rings each year in its lifetime.
The rings are the cambium layer, a ring of cells that lie between the wood and bark and from which new bark and wood cells originate; each year a new cambium is created leaving the previous one in place.
How large the cambium's cells grow in each year--measured as the width of each ring--is dependent on seasonal changes such as temperature and moisture availability.