dendrochronologie

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About dendrochronology

Dendrochronology is a branch of science which is based on the investigation and dating of the annual rings of wood. With the measurement and analyses of the annual rings of woods to 1/100 mm accuracy we can discover the time when any given tree was cut out - in optimal cases – within half a year of certainty. Accordingly in the case of the analysis of historical wooden structures, wooden artifacts and archeological wooden remains, Dendrochronology is the most certain method of dating.

The method is based on the fact that the trees from the same species which live close to each other, year by year grow similar kinds of annual rings – first of all, because of the similar climatic conditions. In this way if we measure two wood samples from trees which lived at the same time, which are at least 30 to 50 years old, the yearly change of the width of the ring of wood (growth or decrease) is similar. The difference can be illustrated at its best on a diagram by a curve, which is why the classical form of contrasting a series of wood rings is done by setting the measured curves against each other. In accordance with the historical theory the yearly wooden rings of a tree which is at least 30 years old is unique in such a way that any individual tree cannot exist again throughout history;  in this way it reflects only that period of time, when that particular tree lived. By combining more unique series of annual ring data characteristic for a certain territory and wooden species, a so called chronology can be created. The chronology – back from our days into the past – contains the annual rings of many historical wooden elements (for example vernacular wooden houses, church roof structures, medieval painted panels, wooden remains after archaeological excavations), where the series of data can be combined based on the theory of lapping. Based on the common period of time of 70 to 90 years of the series of annual rings in a 150 year old oak cut out in 2015 and a 100 year old oak cut out in the middle of the 20th century, a comparison can determine the correlation between the two series of data, so with the help of the tree cut out in 2015 the time can be defined when the older tree was cut out. With the continuation of these series toward the past theoretically an infinite annual ring curve can be combined. The dating of annual rings of wooden samples from an unknown period and relating it to a calendar year we can specify, a dated chronology can be created. Dendrochronological dating is the most accurate definition of our time. If the wood sample contains the most exterior annual ring under the bark, which is also the latest one it is called the finishing ring (waney edge, with German technical term: Waldkante), and this can define not only the year when the tree was cut out but also the period of that year when the cutting out of the tree happened, in autumn-winter or spring-summer time. All this is disclosed by the final ring under the bark of the tree: if the last of the annual rings contains only early or spring tract, the tree was cut out after its vegetation period began, at the end of spring, or the beginning of summer; if the latest annual ring contains later tract, the cutting out of the tree must have happened after the end of the vegetation period, in autumn-winter time. If the sample does not contain the marked ring under the bark, in the case of fir trees we can establish that the tree was cut out one year later.This is called “post quem” dating. In the case of oak though, the heartwood (or inside part) of the tree can be clearly differentiated from the outer living part which carries the nutrition and is called “sapwood”.

This fact becomes important in a wood sample; because the number of annual sapwood rings is approximately the same throughout a tree’s life and trees from the same territory have close similarities in this aspect (the Transylvanian average according to our research up to the present time is 15+2). So in the case of oak samples, which do not have finished annual rings under the bark but contain sapwood annual rings, the time of the cutting out of the tree can be better deduced. In this case we assign a four year period for the most approximate cutting out period (for example around 1624-1628).

 

Sometimes the dating of the last year of the wood or wood remains is not necessarily identical with that building or structure from which the sample was taken originally. However in many cases it is questionable whether the wood from which the sample was taken, is at its original place in that building period or whether it was used as a secondary piece from a building demolition.

 
 
 
 
 
© 2016 Erdélyi dendrokronológiai laboratórium