WHAT IT IS
Dendrochronology (obtaining the felling date of a tree by tree-ring dating) works by utilising the variation in width of the annual growth rings as influenced by climatic conditions common to a large area. It is these climate-induced variations in widths which allow calendar dates to be ascribed to undated timbers when compared with a firmly-dated sequence. If a tree-section is complete out to the bark edge then a precise date-of-felling can be determined. A narrow core is extracted from across the annual growth rings of the timber, with no detriment to its strength or stability.
WHY IT IS USED
House designs and plans were sometimes used over a long period. Architectural features are therefore only a very rough guide to the date of a house as it cannot be known whether a building was the first or last of that design, nor how quickly new design fashions spread within a region.
Someone intending to build a house would, however, cut down or purchase, and then collect on site, sufficient large timbers, usually of oak, for constructing the roof and other key elements. As seasoned timber was too hard to work and shape, unseasoned timber was used in houses built before c.1650. These early buildings would therefore usually have been erected very soon after the latest date of the felling of the main timbers. Obtaining the felling date for main roof timbers can thus give useful information about the date of erection of a building, and comparisons can then be made relating to the spread of that building or style of carpentry within a region, using houses which have survived without major alterations.
Astronomer A. E. Douglass, founding father of Dendrochronology, photographed at the Steward Observatory with Director K. Lundmark in the 1930s
A BACKGROUND TO DENDROCHRONOLOGY
The basis of dendrochronological dating is that trees of the same species, growing at the same time, in similar habitats, produce similar ring-width patterns. These patterns of varying ring-widths are unique to the period of growth. Each tree naturally has its own pattern superimposed on the basic ‘signal’, resulting from genetic variations in the response to external stimuli, the changing competitive regime between trees, damage, disease, management etc.
In much of Britain the major influence on the growth of a species like oak is, however, the weather conditions experienced from season to season. By taking several contemporaneous samples from a building or other timber structure, it is often possible to cross-match the ring-width patterns, and by averaging the values for the sequences, maximise the common signal between trees. The resulting ‘site chronology’ may then be compared with existing ‘master’ or ‘reference’ chronologies.
This process can be done by a trained dendrochronologist using plots of the ring-widths and comparing them visually, which also serves as a check on measuring procedures. It is essentially a statistical process, and therefore requires sufficiently long sequences for one to be confident in the results. There is no defined minimum length of a tree-ring series that can be confidently cross-matched, but as a working hypothesis most dendrochronologists use series longer than at least fifty years.
The dendrochronologist also uses objective statistical comparison techniques, these having the same constraints. The statistical comparison is based on programs by Baillie & Pilcher (1973, 1984) and uses the Student’s t-test. The t-test compares the actual difference between two means in relation to the variation in the data, and is an established statistical technique for looking at the significance of matching between two datasets that has been adopted by dendrochronologists. The values of ‘t’ which give an acceptable match have been the subject of some debate; originally values above 3.5 being regarded as acceptable (given at least 100 years of overlapping rings) but now 4.0 is often taken as the base value. It is possible for a random set of numbers to give an apparently acceptable statistical match against a single reference curve - although the visual analysis of plots of the two series usually shows the trained eye the reality of this match. When a series of ring-widths gives strong statistical matches in the same position against a number of independent chronologies the series becomes dated with an extremely high level of confidence.
BACKGROUND TO DENDROCHRONOLOGY
Dr M. C. Bridge FSA
Oxford Dendrochronology Laboratory Mill Farm
One can develop long reference chronologies by cross-matching the innermost rings of modern timbers with the outermost rings of older timbers successively back in time, adding data from numerous sites. Data now exist covering many thousands of years and it is, in theory, possible to match a sequence of unknown date to this reference material.
It follows from what has been stated above that the chances of matching a single sequence are not as great as for matching a tree-ring series derived from many individuals, since the process of aggregating individual series will remove variation unique to an individual tree, and reinforce the common signal resulting from widespread influences such as the weather. However, a single sequence can be successfully dated, particularly if it has a long ring sequence.
Growth characteristics vary over space and time, trees in south-eastern England generally growing comparatively quickly and with less year-to-year variation than in many other regions (Bridge, 1988). This means that even comparatively large timbers in this region often exhibit few annual rings and are less useful for dating by this technique.
When interpreting the information derived from the dating exercise it is important to take into account such factors as the presence or absence of sapwood on the sample(s), which indicates the outer margins of the tree. Where no sapwood is present it may not be possible to determine how much wood has been removed, and one can therefore only give a date after which the original tree must have been felled. Where the bark is still present on the timber, the year, and even the time of year of felling can be determined. In the case of incomplete sapwood, one can estimate the number of rings likely to have been on the timber by relating it to populations of living and historical timbers to give a statistically valid range of years within which the tree was felled. For this region the estimate used is that 95% of oaks will have a sapwood ring number in the range 11 - 41 (Miles 1997a).
Section of tree with conversion methods showing three types of sapwood retention resulting in A terminus post quem, B a felling date range, and C a precise felling date. Enlarged area D shows the outermost rings of the sapwood with growing seasons (Miles 1997a, 42)
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