Dendrochronology is a science which involves the study of growth ring patterns in trees, and using these growth patterns to establish a chronology of dates. In some areas, fully "anchored" chronologies lasting tens of thousands of years have been established by dendrochronologists through painstaking inspections of vast numbers of trees and lumber artifacts. Using dendrochronology, scientists can learn a number of interesting things about the natural environment and human history.
This science relies on the fact that living trees create distinctive layers known as growth rings for each year of life, and that each growth ring will vary in composition and appearance depending on the conditions at the time of the growth. During drought years, for example, a growth ring might be extremely slender, reflecting the fact that the tree didn't grow very much. In temperate years, a growth ring might be fat, reflecting ideal growing conditions for the tree.
By looking at the pattern of rings in a cross-section of tree trunk, a scientist can learn about the weather conditions that the tree witnessed. By analyzing isotopes and minerals from each growth ring, it is even possible to learn about volcanic eruptions and other major events in the area. Tree-ring dating can sometimes explain subtle cultural shifts spurred by climate change, making it of interest to anthropologists and archaeologists as well as scientists who are interested in the environment and climate change.
While studying a single tree can certainly be interesting, the real science in dendrochronology lies in comparing trees across generations, using a distinctive growth ring as a marker to create a timeline. If, for example, 1873 created a characteristic growth ring, a scientist could find that ring in timber from a tree which grew from 1776 to 1897 and in a tree which grew from 1849 to 1913, thereby creating a timeline from 1776 to 1913. By looking for another distinctive growth ring in, say, 1812, the scientist could go further back with an older piece of wood or tree trunk, and so forth.
A dendrochronologist can use cut timber in dwellings and other structures for dendrochronology studies, along with whole tree trunks, fossils, and living trees. By studying the pattern of the tree rings, scientists can learn about climate change and the ways in which subtle changes have an impact on the natural environment. Dendrochronology is an especially fruitful period of study in desert conditions, where the environment preserves wooden artifacts; this may explain why the science originated in Arizona.