This is accomplished using wood specimens found preserved, for example, in historic buildings, or on the forest floor, or in peat bogs.
The technology involved is fascinating and impressive.The following article is abstracted from The Biblical Chronologist Volume 5, Number 1. The science of constructing chronologies from tree rings is called dendrochronology. Modern trees are known to produce one growth ring per year. (The idea that ancient trees grew more than one ring per year will be discussed below.) Therefore, by coring a living tree and counting rings from the present backwards, it is possible to determine the year in which each ring grew. The bristlecone pines in the White Mountains of California live to extremely old ages, some in excess of 4,000 years.The University of Arizona dendrochronology lab sports a (no longer living) specimen which contains over 6,000 rings.But for the specimen to be useful in extending the tree-ring chronology, the absolute calendar age of its rings must be determined.The annual growth rings vary in thickness each year depending on environmental factors such as rainfall.