Reading the Rings of a Tree

[A] A tree in a temperate climate lays down one ring a year, and the width of that ring records how favourable the year was for growth. [B] Because the pattern of wide and narrow rings across a few decades is close to unique, a sample taken from a beam can be matched against a sample from a living tree, and the beam can then be dated to the year in which its outermost ring grew. [C] Chronologies assembled in this way now reach back several thousand years in some regions. [D]

The method is exact enough that its limits are easily forgotten. A tree checked by drought may lay down no ring at all in a bad year, and a tree that recovers within a season may lay down two, and both errors are caught only by comparing many samples from the same district. The larger difficulty lies not in the wood but in what the date is taken to mean. A ring gives the year in which the wood grew. A builder may have seasoned that timber for a decade, or pulled the beam out of an older roof and used it again. Dendrochronology dates the tree, and it is the archaeologist who dates the building, and those are not the same claim.