We are all familiar with the use of barcodes in our daily lives, especially in supermarkets. Tree rings, too, act as a kind of barcode, containing information on the growth conditions of trees. A tree forms a unique sequence of wider and narrower rings, and the widths of these rings are influenced by weather conditions the tree endures each year. Therefore, when researchers possess a long tree ring series with a known date of origin, they can use the ring series as a reference point, comparing parts of it to other similar tree ring patterns and, in doing so, calculate a tree’s age and longevity.
This method of dating wood with dendrochronology (the scientific study of tree rings) has many uses—even, surprisingly, in the field of art history. In earlier centuries, it was a common practice to paint on wooden panels; thin oak planks were glued to form a support for a painting. Tree ring patterns, visible on the edge of such panels, can now tell us whether the dates of the trees used fit with the historical period to which a painting is attributed. This method can be used to confirm the origin of unidentified paintings, or to identify whether a painting is genuine, or a later copy or falsification.
These dendrochronological methods are at work at the Department of Geography of University of Tartu, Estonia. In addition to dating wood panels, researchers can determine the region of origin of the timber used for works of art. In the sixteenth and seventeenth centuries, during a wood shortage in western Europe, much timber was imported from the forest-rich regions of eastern Europe. Gdansk/Danzig and Riga were important timber export harbours, and logs were rafted there from hinterlands as far as Belarus and Ukraine. Researchers in Tartu study tree rings of Flemish paintings maintained in the Estonian Art Museum in Tallinn and determine growth years of the panel trees. The similarities of the ring series to reference series from various countries can also help identify the probable region of origin of the wood. Knowing the region of origin, in turn, gives us an estimate of the number of light-coloured rings that would appear in oaks of this region. Light-coloured rings under the bark, called sapwood, are water-conducting living wood layers in oak trees. Because the perishable light-coloured sapwood is trimmed away in the course of panel-making, estimating the approximate number of these rings helps to specify the time of painting.
A recent co-operative effort involves the investigation of four paintings depicting the same subject, that of Christ expelling money-lenders from the temple. One of these paintings is stored in Tallinn, one in Glasgow, one in Copenhagen, and one in a private collection. These paintings have been attributed to the school of Bosch and Bruegel, and dendrochronological research has helped scientists and art historians test this hypothesis. Researchers Aoife Daly from Dublin and Alar Läänelaid from Tartu measured tree rings from the end-grains of the panels and established the approximate painting time of three paintings of the four, exchanging data and comparing results with each other. It appeared that the Tallinn and Copenhagen copies were coeval, painted after 1562, while the private painting was made about 30 years earlier. The results showed that none of these paintings could have been created by Bosch (1450-1516) himself, nor could the private painting have been created by Bruegel the Elder (ca. 1525-1569). This conclusion is significant not only for art historical and material studies: the age and provenance of a painting also dictates its value.
Dendrochronological investigations become possible with efficient international co-operation between tree-ring researchers, art historians and conservators. The author of this paper has dated painting panels in Tallinn and in Helsinki. Baltic oak chronologies compiled by Jennifer Hillam and Ian Tyers from Great Britain are of great help in this process. Anne Crone from Sheffield asks Baltic researchers to check the dates of pine series of Scotland. In Tartu, we then compare our tree-ring series with those of Tomasz Wazny and Andrzei Zielski from Poland. The Gotland pine chronology created by Thomas Bartholin plays an indispensable role in dating pine wood in the Baltics. Māris Zunde from Latvia, Rūtilė Pukienė and Adomas Vitas from Lithuania exchange data with Estonian researchers in order to date wooden items and archaeological findings.
Just as, hundreds of years ago, timber and art was shipped from one part of Europe to another, so today does the international co-operation of scientists support the study of the trade and culture relations of prior ages.
University of Tartu