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. Moden
scientific tree-ring dating, dendrochronology, stems from pioneering work in
5 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
10 the production of a new cell stops. The same process is repeated the following
year. In this waya year's growth (annual ring) is imprinted as newwood. 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.
15 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 atall 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
20 inequalities of climate from year to year. In years with abnormal drought, for
example, nanrow rings are produced and sometimes no ring at all. In this way a
fossil record is imprinted foras 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.
25 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.
