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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.
To eliminate individual variations in tree-ring growth, dendrochronologists take the smoothed average of the tree-ring widths of multiple tree samples to build up a ring history, a process termed replication.
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.
If the bark of the tree has been removed in a particular area, the rings will likely to be deformed as the plant overgrows the scar.
Each ring marks a complete cycle of seasons, or one year, in the tree's life.
European chronologies derived from wooden structures initially found it difficult to bridge the gap in the 14th century when there was a building hiatus, which coincided with the Black Death, Given a sample of wood, the variation of the tree-ring growths provides not only a match by year, it can also match location because the climate across a continent is not consistent.
Direct reading of tree ring chronologies is a learned science, for several reasons.
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.
A fully anchored and cross-matched oak and pine chronology in central Europe extends back 12,460 years, Timber core samples are sampled and used to measure the width of annual growth rings; by taking samples from different sites within a particular region, researchers can build a comprehensive historical sequence.
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).
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Critical to the science, trees from the same region tend to develop the same patterns of ring widths for a given period of historical study.