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Trees don’t lie about their years

19 October 2011
Published in Culture
by Christina Kolyva

Autumn always makes me think of trees. There is nothing I like better on a Sunday morning in autumn than to take a long walk in the woods, advice idly stepping on crisp fallen leaves, with the sunlight filtering through golden branches, birds singing and squirrels working away frantically to top up their winter supplies. There is something about the longevity of forest trees, the assurance and solemnity emanating from their unwavering presence that always manages to captivate me. And even though trees cannot speak to tell stories about endless stringent winters and sweet-smelling summers, they do have their own voice, if one knows where to look.

Trees grow a new layer of wood every year. Due to changes in growth speed through the seasons, annual growth usually comprises two layers: an inner layer, called earlywood, which is lightly coloured because the growth is rapid early in the growing season and the new wood is less dense and thus lightly coloured; and an outer layer, called latewood, which is darker and denser. The process is actually not dissimilar to the way varve or ice core patterns are formed. When a tree is felled this biannual growth can be seen on the cross-section of the trunk in the form of rings. From this view it is also possible to distinguish the sapwood from the heartwood, that is to say the young wood closer to the bark where sap flows, and the old wood forming the core of the trunk respectively.

The thickness of the tree-rings depends particularly on climate and factors such as rainfall, temperature and sunlight. Favourable conditions for growth result in a wide ring and vice versa. Trees growing in the same region will have similar tree-ring patterns, because of their mutual response to climate changes.

Dendrochronology or tree-ring dating is a method of determining the age of wooden objects, based on their tree-ring pattern. The idea in a nutshell: you have an object of unknown age but with visible tree-ring pattern and you compare it, like a fingerprint, to patterns developed on akin tree species grown during a known period under similar climatic conditions, until you get a match, ring for ring. This process is called cross-dating and, depending on the state of the undated wood, dendrochronology in theory can provide annual or even sub-annual resolution. The technique works only with clear annual growth rings; tropical species where annual ring boundaries are not visible or orchard trees whose growth is dependent on the ardour of the gardener rather than Mother Nature herself are not suitable for cross-dating.

The ‘fingerprint database’ against which you compare the tree-ring pattern that needs cross-dating is called reference or master chronology. For a specific tree species and region, it is produced by overlapping the ring patterns of successively older timber, starting with living trees of known chronology and progressively superimposing older timbers from buildings or even archaeological sites to gradually extend the chronology into the past. The pattern to be matched is the relative increase and decrease in ring width year after year.

Dendrochronology has found numerous art-historical applications, ranging from panel paintings to antique furniture. It is necessary for the wooden object to contain an adequate number of clearly visible rings, usually no less than sixty, and there must be a master chronology for the particular tree species and geographical area or you might end up… barking up the wrong tree! The application I myself find most intriguing is the dating of stringed instruments of the violin family – violins, violas, cellos and double basses – an application that instrument appraisers, sellers, players, authenticators and purchasers alike are gradually starting to acknowledge.

The back, sides and neck of stringed instruments are typically made of maple or sycamore, which are not suitable for dendrochronological investigation, but the front of the belly is usually made of Norway spruce, which is ideal for the technique. It is a lucky coincidence that wood with optimal acoustic properties, such as spruce of good quality without defects, knots and whatnot, is also dendrochronology-friendly. On the other hand, heavily restored instruments, patches of transplanted or later wood, painted year rings, dents, scratches, varnish that obstructs the wood grain, send dendrochronologists climbing up a tree…!

To obtain the desired symmetry on the front of the instrument, the craftsman starts with a wedge of wood, split from a cylindrical slice of tree trunk and with the oldest growth at the thin end of the wedge. This is split down the middle and the two half-wedges are opened like a butterfly and joined usually along the bark side of the wood. Therefore symmetrical ring patterns should be displayed on the two pieces with the youngest growth rings towards the centre. More than two pieces might be used in larger instruments, such as double basses.

Dendrochronological findings are used as an adjunct to information about the instrument based on styling or labelling criteria and historical records, and although dendrochronology can obviously not identify the maker, in some cases it can prevent erroneous or fraudulent attribution. Proof that the tree was still growing when the attributed maker was already deceased, for example, is somewhat suspicious. Dendrochronology provides the terminus post quem date of manufacture – the date after which the instrument could have been built based on the youngest identifiable tree-ring. Depending on the completeness of the sampled timber (if a significant amount of sapwood is preserved) it is possible to determine the date the tree was felled with some accuracy. Uncertainty in this estimation is however introduced because of the removal of an unknown number of outermost rings during the planning process prior to joining the front pieces. Even if the felling date is determined accurately, wood stored for many years before use or reused wood from older structures can introduce errors in the cross-dating, which are further amplified in cases of wood imported from another climatic region, if the wrong reference chronology is used.

How would you like to take a peep at the workshop of a master violinmaker? Have you seen The Red Violin? I’ve always felt a bit disappointed they didn’t show the early stages of the construction of the violin. Apparently tree-ring dating can provide information not only about chronology, but also about the technique and working habits of the luthier, which could indirectly help support or refute an attribution. Examples of this are: the way the pieces are joined at the centre of the front piece (bark to bark, bark to core or a combination), whether the front pieces are from the same wedge or not, use of common trees and exchange of pieces between makers, the provenance of the wood and the amount of time it was kept in storage for seasoning.

Although I love a fireplace with a crackling fire during winter, I have to admit that next time I have a fire to stoke, every log I throw will probably feel like burning a piece of history…

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