Radiocarbon dating and tree-ring
dating, in combination, have provided a very powerful tool to establish a
time spectrum for more recent dates in the past. The initial idea for
dating by tree rings can be traced back to 1811. Modern scientific
tree-ring dating, dendrochronology, stems from pioneering work in early
1900’s.
Usually, but not always,
trees produce one ring each year. This ring is formed by the cambium, which
lies between the old wood and the bark. In spring, wood cells with large
lumens are manufactured, but in summer and autumn, the cells become smaller
and more thick-walled until with the onset of winter the production of a
new cell stops. The same process is repeated the following year. In this
way a year’s growth (annual ring) is imprinted as new wood. The demarcation
line between summer and autumn wood of the previous year, with its
characteristic small cells, and the spring wood of the year following, with
its large cells, enables annual rings to be counted relatively easily.
Growth rings, however, are
not always the same thickness. They vary for several reasons. Environmental
factors rigidly control the degree of growth of an annual ring or determine
whether, in fact, an annual ring appears at all in any particular year.
Thus in a specific locale or, more accurately, a specific climatic
province, tree-ring counts will reflect climatic conditions and variations
due to inequalities of climate from year to year. In years with abnormal
drought, for example, narrow rings are produced and sometimes no ring at
all. In this way a fossil record is imprinted for as long as the wood
remains intact. From this pattern a historical template can be constructed
to correlate one set of growth rings in one tree with a set of growth rings
in another tree or piece of timber.
Another important factor is
that tree-ring growth varies with age of the tree. As the tree matures, the
rings become narrower, and this results in the central rings being wider
than those on the outer part of the tree.
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