Dendrochronology is the technique of dating wood through the measurement and analysis of the growth-patterns of the parent tree. It is the most precise dating technique available to building historians and archaeologists because, given a complete sample with bark edge, the calendar year in which the tree was felled can be identified. The statistical correlations on which the dating method is based also enables the source of the wood to be provenanced. Other growth-ring parameters (Maximum density (Polge 1970); Blue Intensity (Wilson et al. 2011) and stable isotopes (McCaroll and Loader 2004)) can also be used but all the dating work reported below is based on ring-width measurement.
In Scotland there is a commercial dendrochronological laboratory at AOC Archaeology Group where Anne Crone specialises in the analysis of standing buildings and archaeological materials. In the School of Geography and Geosciences at the University of St Andrews Rob Wilson is working on the development of a native pine network based on living trees and sub-fossil data. Coralie Mills is a research fellow at St Andrews and provides freelance dendrochronological services for both the cultural and natural heritage sectors.
As in the rest of the UK dendrochronological studies in Scotland have focused primarily on oak (Quercus sp.), because this is the species most commonly used in construction from the Neolithic until the post-medieval period. Chronological and geographical coverage in Scotland is patchy and the existence of robust local chronologies influences the likelihood of successful dating. The bulk of the oak data for Scotland has come from the sampling of standing buildings, with only small amounts coming from archaeological and other sources. Consequently, there is more data for the medieval and post-medieval periods than for earlier periods. From the medieval period one issue dominates cultural dendrochronology in Scotland and that is the distinction between native-grown and imported timber, and the implications this has for woodland history and the timber trade. Until the late 15th century, long-lived native oak was widely available and material with long ring sequences from this period has been found on many urban medieval sites (Crone 2000a) and in buildings such as Glasgow Cathedral, Caerlaverock Castle (Baillie 1997) and Darnaway Castle (Stell & Baillie 1993). Many of the native oak chronologies begin as early as the mid-9th century AD (Crone 2006). Native oak has been identified in only a few later buildings, all late 16th/ early 17th century in date, and this timber tended to be young, poorly-grown oak; consequently the dating of native oak in these later periods remains problematic.
From the late 15th century imported oak was used almost exclusively in building, presumably because native-grown oak was in short supply. There is now an extensive network of oak chronologies throughout Europe and this has facilitated the identification and dating of imported oak in Scotland. Oak boards from the eastern Baltic and beams from Scandinavia have been identified by dendrochronology in many buildings dating from the late 15th century to the late 17th century (ie Crone 2008; Crone & Gallagher 2008). Consequently, a sizeable corpus of Scottish 'import' data now exists which makes it relatively straightforward to date and provenance imported oak.
Sampling underway in the roof of the Mansion House, Drum Castle, Aberdeenshire. Native oak timber, felled over a period of years from AD 1603 to at least AD 1612 was used in the construction of this roof. This is one of a very few buildings of this date which was built with native oak, which probably came from the nearby Forest of Drum. The timber was small in cross-section and knotty, with wavy, irregular grain and compressed growth, probably signalling poor management of the Forest in the previous century, © AOC Archaeology Group.
Considerable progress has been made in the analysis of Scots pine (Pinus sylvestris) in recent years. As with oak it is necessary to be able to distinguish between native-grown and imported pine. By the 17th century European sources of oak were also dwindling and pine imported from Scandinavia and the eastern Baltic replaced oak as the main building timber in Scotland. Although there are still only a relatively small number of dated pine assemblages from Scotland the growing network of master pine chronologies throughout Europe means that dating imported pine in post-medieval and early modern buildings is becoming more routine (Crone & Sproat 2011).
This is not the case with native-grown pine. A programme is underway to develop a continuous reference chronology for the last millennium, primarily for climatic reconstruction, but it will also have applications for the dating of cultural material (Wilson et al. 2011). Commercial exploitation of Scotland's pinewoods began in the 17th century the timber was intended for both shipbuilding and construction. However, at present the tree-ring data comes mainly from the isolated remnant pine woodlands in the Highlands and may not be representative of the timber used in construction. Nonetheless, two vernacular buildings in Upper Deeside have now been successfully dated against local reference chronologies (Mills & Crone 2012).
There is very little chronological coverage earlier than the mid-9th century AD, when many of the medieval chronologies begin. Attempts have been made to date numerous prehistoric and early historic sites which have produced oak timbers but these have been unsuccessful (ie Crone 1998a; 1998b; 2002), largely because the sites are geographically remote from all the reference chronologies available for the these periods but also because they have not produced timbers in sufficient numbers to construct robust site chronologies with strong climatic signals. All the sites that have been successfully dated lie in south-west Scotland, and their dating has relied on their proximity to northern Ireland or northern England, where there are regional reference chronologies constructed from trees which probably grew in similar environmental conditions to those in SW Scotland.
Currently, only two prehistoric sites have been dendro-dated; oak timbers from two crannogs, Cults Loch and Dorman's Island, Whitefield Loch, both in Wigtonshire, have been successfully dated indicating building activity in the 5th centuries and 2nd centuries BC (Cavers et al. 2011). Buiston crannog, Ayrshire produced one of the largest assemblages of oak timbers retrieved from an archaeological site in Scotland and consequently, a robust site chronology was constructed, covering the years AD 250 - 615, with indications of building activity on the crannog in the late 6th century and into the 7th century AD (Crone 2000b). Structural timbers from the Northumbrian monastic settlement at Whithorn have also been successfully dated, producing a site chronology covering the years AD 278 - 752 (Crone 1997).
The only other species which has been used in dendrochronological studies in Scotland is alder (Alnus glutinosa), mainly because it has been used extensively
in the construction of crannogs. There are no master reference chronologies for alder so it cannot be used directly to obtain calendrical dates, its greatest value lying in its potential to provide site-specific chronological relationships. However, it was used successfully at Buiston crannog where the alder chronology could be indirectly linked to the dated oak chronology through stratigraphic connection, thus providing calendrical dates for some of the structures. Work on alder from other crannogs has highlighted problems relating to sequence length, tree structure and growing conditions.
Much of the dendro-dating of medieval and early modern Scottish material is now relatively routine and improvements in provenancing is dependent to a large extent on the continuing work of European colleagues. Work is still needed on strengthening the later sections of the native oak chronologies, particularly the 16th and 17th centuries, while the development of the native pine network is critical if we are to date much of the vernacular building stock. Efforts should focus on the development of a continuous tree-ring chronology for the 1st millennium BC, based on crannog timbers, which would help to resolve many chronological issues for this period.