A tree's growth rate changes in a predictable pattern throughout the year in response to seasonal climate changes, resulting in visible growth rings.
Each ring marks a complete cycle of seasons, or one year, in the tree's life.
It is also used in radiocarbon dating to calibrate radiocarbon ages.
New growth in trees occurs in a layer of cells near the bark.
The key issue in sample pretreatment is that there is no method, or methods, that can be universally applied to all types of material from archaeological or geological contexts.
Pretreatments are designed to remove the contaminating substances that have affected the sample during its post-depositional history.
Samples for dating need to be converted into a form suitable for measuring the content; this can mean conversion to gaseous, liquid, or solid form, depending on the measurement technique to be used.
In theory, both organic and inorganic components can be dated.
Samples used for radiocarbon dating must be handled carefully to avoid contamination.
Not all material can be dated by this method; only samples containing organic matter can be tested: the date found will be the date of death of the plants or animals from which the sample originally came.
In 1859, the German-American Jacob Kuechler (1823–1893) used crossdating to examine oaks (Quercus stellata) in order to study the record of climate in western Texas.
During the first half of the 20th century, the astronomer A. Douglass founded the Laboratory of Tree-Ring Research at the University of Arizona.