Cross dating in dendrochronology douglass

Dendrochronologists originally carried out cross-dating by visual inspection; more recently, they have harnessed computers 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.During the first half of the twentieth century, the astronomer A. Douglass founded the Laboratory of Tree-Ring Research at the University of Arizona.Douglass sought to better understand cycles of sunspot activity and reasoned that changes in solar activity would affect climate patterns on earth, which would subsequently be recorded by tree-ring growth patterns (i.e., sunspots → climate → tree rings).In addition, particular tree-species may present "missing rings", and this influences the selection of trees for study of long time-spans.For instance, missing rings are rare in oak and elm trees.A tree-ring history whose beginning- and end-dates are not known is called a floating chronology.It can be anchored by cross-matching a section against another chronology (tree-ring history) whose dates are known.

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.Diagram of secondary growth in a tree showing idealised vertical and horizontal sections, a new layer of wood is added in each growing season, thickening the stem, existing branches and roots, to form a growth ring Horizontal cross sections cut through the trunk of a tree can reveal growth rings, also referred to as tree rings or annual rings.Growth rings result from new growth in the vascular cambium, a layer of cells near the bark that botanists classify as a lateral meristem; this growth in diameter is known as secondary growth.Dendrochronology is useful for determining the precise age of samples, especially those that are too recent for radiocarbon dating, which always produces a range rather than an exact date, to be very accurate.However, for a precise date of the death of the tree a full sample to the edge is needed, which most trimmed timber will not provide.

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