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It can be anchored by cross-matching a section against another chronology (tree-ring history) whose dates are known.
New growth in trees occurs in a layer of cells near the bark.
A new layer of wood is added in each growing season, thickening the stem, existing branches and roots, to form a growth ring.
Growth rings, also referred to as tree rings or annual rings, can be seen in a horizontal cross section cut through the trunk of a tree.
Moreover, wood from ancient structures with known chronologies can be matched to the tree ring data (a technique called cross-dating), and the age of the wood can thereby be determined precisely.
Cross-dating was originally done by visual inspection; more recently, computers have been harnessed to do the task, applying statistical techniques to assess the matching.
First, contrary to the single ring per year paradigm, alternating poor and favorable conditions, such as mid-summer droughts, can result in several rings forming in a given year.
In addition, particular tree species may present "missing rings", and this influences the selection of trees for study of long time spans.
Growth rings are the result of new growth in the vascular cambium, a layer of cells near the bark that is classified as a lateral meristem; this growth in diameter is known as secondary growth.
Visible rings result from the change in growth speed through the seasons of the year; thus, critical for the title method, one ring generally marks the passage of one year in the life of the tree.
Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed in order to analyze atmospheric conditions during different periods in history.
Dendrochronology is useful for determining the timing of events and rates of change in the environment (most prominently climate) and also in works of art and architecture, such as old panel paintings on wood, buildings, etc.
The techniques of dendrochronology are more consistent in areas where trees grew in marginal conditions such as aridity or semi-aridity where the ring growth is more sensitive to the environment, rather than in humid areas where tree-ring growth is more uniform (complacent).