A historic beam or archaeological object may preserve too few growth rings to be dated by conventional methods. According to the Cultural Heritage Agency of the Netherlands (RCE), this affects around 30 per cent of historic wood surviving in the Netherlands from the period 1300 to 1600. Some evidence of past building activity and woodland use is therefore left without a precise date.
Dendrochronology dates wood by matching its pattern of wider and narrower rings to an established sequence. With fewer rings, a reliable match is harder to find. A study published in PLOS ONE, led by Clara Rodríguez-Morata, examines another record held in those same rings: variations in oxygen isotopes. Researchers from Naturalis Biodiversity Center, RCE and Swansea University took part.
Isotopes are forms of the same element with different masses. Trees take them up through water. Heat and drought affect evaporation from leaves and the balance of lighter and heavier oxygen recorded in the wood. That balance changes from year to year, creating a pattern that can be compared with a dated reference sequence.
To build that reference, the researchers used previously dated oaks from the Twente-Westphalia region, spanning the eastern Netherlands and north-western Germany. Their sequence covers 1300 to 1600, with an isotope value for each year.
The comparison allowed individual samples with as few as 30 rings to be dated when a representative isotope sequence was available for the area where the wood originated. If the bark also survives, it may be possible to identify when the tree was felled.
Where no local reference sequence exists, combining patterns from three to five samples from the same period, covering a longer span of years, may improve comparisons with regional sequences. Further tests on wood from other sites and periods are needed to assess how widely the approach can be applied. The current sequence is for oak; extending the work to beech, alder and conifers is a future aim.
The method could make it possible to revisit finds that have remained undated. Bringing short-ring samples into the picture may give researchers more evidence for how woodlands were used and when building took place.
