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.
Environmental inputs into the cambium are primarily regional climatic variations, changes in temperature, aridity, and soil chemistry, which together are encoded as variations in the width of a particular ring, in the wood density or structure, and/or in the chemical composition of the cell walls.At its most basic, during dry years the cambium's cells are smaller and thus the layer is thinner than during wet years.The ring widths were matched and cross-dated, and by the 1920s, chronologies were built back nearly 600 years. Haury, and dendrochronology conducted on Showlow eventuated the first single chronology for the southwest, extending over a period of over 1,200 years.The first ruin tied to a specific calendar date was Kawaikuh in the Jeddito area, built in the 15th century; charcoal from Kawaikuh was the first charcoal used in (the later) radiocarbon studies. The Laboratory of Tree-Ring Research was established by Douglass at the University of Arizona in 1937, and it is still conducting research today.A recent attempt to use tree ring analysis on olive trees (Cherubini and colleagues) revealed that too much variation of the cambium occurs in olives to make dendrochronology viable.
That study was one of ongoing attempts to determine a reliable chronology of the Mediterranean Bronze Age.
Fortunately for us, the looters left behind the tools they used to break into the mounds, wooden spades and stretchers (small handled platforms used to carry objects out of the tombs), which were analyzed using dendrochronology.
Tying tree ring fragments in the tools to established chronologies, Bill and Daly (2012) discovered that all three of the mounds were opened and the grave goods damaged during the 10th century, likely as part of Harald Bluetooth's campaign to convert Scandinavians to Christianity.
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.
It had long been known that three 9th century Viking period boat-grave mounds near Oslo, Norway (Gokstad, Oseberg and Tune) had been broken into at some point in antiquity.