Dendrochronology

Description

Dendrochronology (from Greek dendron = tree, chronos = time and logos = word or speech) is a method to date wood of unknown age. A variety of objects of cultural history can thus be dated, such as subfossil, archaeological, and historical wood used in construction or for tools, including furniture, musical instruments and objects of art.

Due to its very precise dating results with the possibility to date tree rings to the exact year they were formed, the method has established itself in recent decades as the most accurate method for determining the age of wood. Applications range from prehistory and early history, monument preservation, and art historical research to the geosciences. Under ideal conditions, dendrochronology provides unique chronological insights until the last ice age.

Application

Generally, wood samples with a clear tree ring pattern can be da-ted, regardless of whether the wood is moist, dry or charred. The preservation status is also irrelevant as long as the annual ring pattern can be clearly be seen. The best chances for a successful age determination are with regularly grown trunk wood that is largely free of knots and injuries.

At the beginning of each analysis there is the determination of the wood species. This is essential, since the availability of corresponding annual ring calendars for the species at hand and for the region is decisive for the dating success. For oak, for example, the best documented chronologies are usually available, as it was the most preferred timber for constructions and other use until modern times. Gradually, chronologies could also be developed for various conifers (fir, pine, spruce) for the last 1000 years and partially for prehistoric periods. But there are also regional chronologies for beech, ash, alder, poplar and elms (especially if they occur simultaneously with oak). Some scatterporous deciduous wood species such as poplar, willow or lime are not suitable for the construction of reference series, mainly because of their rare use.

After determining the wood species a suitable sample is prepared and then the ring widths are measured with an accuracy of 1/100 mm with a special measuring system. In the case of parti-cularly valuable objects (e.g. paintings or musical instruments), the rings can also be measured nondestructively using computer tomography, photographs, impressions or directly visible surfaces. The resulting annual ring pattern is then compared with the corresponding reference chronologies both statistically and optically. In the case of matching patterns a date of the feeling of the tree can beobtained – under particularly favourable conditions even up to a seasonal resolution.

Basics

Dendrochronology is based on the measurement of annual ring patterns measured in samples (i.e. the sequence of narrow and wider rings) of trees. Many trees in temperate zones produce one growthring each year so that for the entire period of a tree‘s life, a ring pattern builds up that reflects the age of the tree and the climatic conditions in which the tree grew. Trees that have grown over the same time period in the same region tend to develop the same patterns of ring widths for a given period of chronological study, because they record the same information on weather and climate and other factors that influence the growth rate. When one can match these treering patterns across successive trees and also in different tree species in the same locale, in overlapping fashion, chronologies can be built up—both for entire geographical regions and for climatic subregions. Thus long tree ring calendars have been built up ranging up to 12500 years before present. In addition to the possibility to date wood the tree ring patterns are also excellent archives for studies in palaeoclimatology and ecology with high chronological resolution that are increasingly used to reconstruct past environmental conditions.

These known chronologies are the basis for dating wood from ancient structures. If their tree ring pattern can be matched to the existing treering data (a technique called crossdating), the age of the wood can be determined precisely. Accordingly, much time and work is invested to build up such reference data that comprises collecting wood samples, their photographic docu-mentation and inventarisation, their preparation for measurement and determination of the ring widths, and lastly the eva-luation of the patterns and their compilation in data bases. The actual dating of wodd samples may appear short and simple at first sight but it build upon the quality of the reference samples, similar to a tree that decisively depends on its roots.

Limitations

In addition to the limitations that the availability of chronologies inevitably sets for the species and region under study, the sample composition also plays an important role. Thin branches and wood from roots is very problematic and usually not datable. Similarly, partially decayed wood, e.g. by weathering, burial conditions, fungal attack or by vermin is not usable. However, in the end the number of identifiable tree rings is the decisive factor. Their number should be at least 50 and for dating complex features at least four to five specimens should be available, because not every one may yield a dating result. A larger number of specimens from the same feature raises the probability that even samples with a low number of rings can be included in the dating process. Furthermore, a series of samples may reveal the development of constructions better tha a single sample. Under such circumstances dendrochronology not only delivers exceedingly precise dating results but is also also indispensable for the calibration of radiocarbon dates, especially for increasing the resolution of the calibration curve in problematic sections. In combination with the analysis of the isotope ratios of the light elements the tree rings can also be used to determine the provenance of wooden artefacts (dendro provenancing).