ELM Dendrochronology

Abstract

Elm is generally considered to be unsuitable for dendrochronology, usually having too few rings, or having abrupt growth-rate changes that do not result from the external weather conditions. Samples rarely match each other in the same structure. A further difficulty is that even where it is known that the sequence is complete (the bark is still present), it is often not possible to distinguish sapwood rings by their appearance, even under a microscope. This is a significant problem as elm has been an important structural component of many British vernacular buildings over many centuries, but whereas dendrochronological dating of oak has transformed our understanding of thousands of buildings, by 2015 only four instances of dating elm had been made, two of those involving a single timber. When elm has been encountered, it has generally been dismissed from further dendrochronological study as a result of these known issues, but no systematic study has been undertaken to see whether these prejudices are justified. In order to get some evidence-based information about how elm might behave dendrochronologically, Historic England initiated a study: Developing the dendrochronology of elm in historic buildings, Project 7350, funded through its Heritage Protection Commissions. This resulted in over 70 buildings being looked at, with several being sampled, and the results are discussed here. At some sites an elm site master sequence could be derived, but potential matches with local oak chronologies were generally not strong enough to be considered dated. In one instance good matches with local oak sites were found, but subsequent radiocarbon analysis found these matches to be erroneous. At another site, five trees gave an 89-year ring sequence, but no acceptable matches were found with oak chronologies. Radiocarbon dating and oxygen isotope dating both gave the same dating results at this site, however, showing that these two methods appear to give the best hope of dating elm in the future.