Dendrochronology

An absolute chronometric dating technique for measuring time intervals and dating events and environmental changes by reading and dating the pattern (number and condition) of annual rings formed in the trunks of trees. The results are compared to an established tree-ring sequence for a particular region with consideration to annual fluctuations in rainfall which result in variations in the size of the rings laid down by trees on the outside of their trunks. These variations, given favorable conditions, form a consistent pattern; and sections or cores taken from beams in ruins have been matched to provide a long chronology over large areas. The method is based on the principle that trees add a growth ring for each year of their lives, and that variations in climatic conditions will affect the width of these rings on suitable trees. In a very dry year growth will be restricted, and the ring narrow, while a wet and humid year will produce luxuriant growth and a thick ring. By comparing a complete series of rings from a tree of known date (for example, one still alive) with a series from an earlier, dead tree overlapping in age, ring patterns from the central layers of the recent tree and the outer of the old may show a correlation which allows the dating, in calendar years, of the older tree. The central rings of this older tree may then be compared with the outer rings or a yet older tree, and so on until the dates reach back into prehistory. Problems that arise are when climatic variation and suitable trees (sensitive trees react to climatic changes, complacent trees do not) are not be present to produce any significant and recognizable pattern of variation in the rings. Another problem is that there may be gaps in the sequences of available timber, so that the chronology 'floats', or is not tied in to a calendrical date or living trees: it can only be used for relative dating. Also, the tree-ring key can only go back a certain distance into the past, since the availability of sufficient amounts of timber to construct a sequence obviously decreases. Only in a few areas of the world are there species of trees so long-lived that long chronologies can be built up. This method is especially important in the southwestern United States, Alaska, and Scandinavia, dating back to several thousand years BC in some areas. Dendrochronology is of immense importance for archaeology, especially for its contribution to the refining of radiocarbon dating. Since timber can be dated by radiocarbon, dates may be obtained from dendrochronologically dated trees. It has been shown that the radiocarbon dates diverge increasingly from calendrical dates provided by tree-rings the further back into prehistory they go, the radiocarbon dates being younger than the tree-ring dates. This has allowed the questioning of one of the underlying assumptions of radiocarbon dating, the constancy of the concentration of C14 in the atmosphere. Fluctuations in this concentration have now been shown back as far as dendrochronological sequences go (to c 7000 BC), and thus dating technique is serving the further research on another. In 1929, A.E. Douglass first showed how this method could be used to date archaeological material. The long-living Bristlecone Pine (Pinus aristata) of California has yielded a sequence extending back to c 9000 bp. In Ireland, oak preserved in bogs has produced a floating chronology from c 2850-5950 bp.

The construction of chronologies from tree-ring sequences. Dendrochronology can be used as a dating method for timbers in buildings and other archaeological sites. It has also been used with great success for the calibration of radiocarbon dates. Tree-ring sequences are also used as indicators of environmental change. Principles. It is common knowledge that the age of a tree may be determined by counting the number of tree rings. Depending on the growing conditions in the particular years of their growth, tree rings differ in width and structure; for instance, during good growing years a thicker ring is added than in poorer years. Trees of a similar age in the same area will all be similarly affected and so will have similar sequences of ring widths; therefore sequences can be correlated between trees. Correlation can also be carried out with overlapping parts of tree-ring sequences from older trees, dead trees and timbers preserved on archaeological sites, In this way, an overlapping sequence of tree-ring widths can be built up and ideally such a sequence would extend back unbroken from the present day. However, timbers linking parts of the sequence may be missing and in these cases ‘floating chronologies’ are built up. Floating chronologies have to be dated approximately by radiocarbon, but as more timbers are found, it is hoped to join them to the sequence which is anchored in the present day, thus providing an absolute chronology. Once a master chronology has been constructed, other timbers from buildings and sites can be dated by matching them into the sequence. Range. The long-living Bristlecone Pine (Pinus aristata) of California has yielded a sequence extending back to c9000 bp. In Ireland, oak preserved in bogs has produced a floating chronology from c2850-5950 bp. Similar chronologies are being built up throughout Europe. Accuracy. Correlation between sequences is never perfect and has to be accomplished statistically. In addition, rings may be missed out in some years (between 3.4 per cent and 0.86 per cent of rings may be missing in one tree); other years may produce multiple rings (this is much rarer). Accuracy therefore varies, but the method appears to be much more accurate than any of the other dating methods. Problems. To use dendrochronology as a dating method, a sequence has to be built up for the area in which sites exist. This chronology will then apply to only one species of tree in one particular area. The size of the area will vary, but Bristlecone Pine sequences can be correlated between trees up to 1600 km apart. Radiocarbon calibration. Once the Bristlecone Pine absolute chronology had been established,small blocks of wood (clO rings) were sampled at intervals down the sequence. These were dated by radiocarbon and the difference between the two sets of dates used to construct calibration tables. Dendrochoronology and climate. Since tree ring widths are dependent on environment, it is possible to use the sequence of width changes to reconstruct environmental (particularly climatic) history for an area.