Dendrochronology for Non-Specialists
Art historians and students not fully informed about this dating tool are often confused by statistical ranges for dates and ask how such a seemingly straightforward technique can produce varied results? Perhaps when the methodology and variables involved are understood this will be less perplexing. Let us start with how dendrochronology works.
The natural place to begin any discussion of tree ring dating is botany. Various species of trees like oak, chestnut, pine, elm, and fir, for example, produce annual growth rings whose width depends on moisture and climatic conditions. There is also a difference in the nature of the wood from the exterior bark, the outermost growing layers where the sap flows (the sapwood) down to the heartwood and core of the tree.
Oak: Heartwood/Sapwood Boundry (top Image)
Photo: W.G. Simpson
The essential concept behind dendrochronology is based on the fact that trees of the same species grown at the same time and place will produce practically identical ring patterns. Hence, when detailed measurements of the variations in the width and character of the annual rings have been accumulated and classified for large numbers of trees from the same species and region, a chronology of ring widths derived from actual dimensions can be established. Since tree rings reflect seasonal changes, only trees from temperate and arid regions have annual rings. Because oak is so long-lived and its outer wood (sapwood) easily distinguishable, it provides ideal samples for study. This is especially important for medieval architecture, since oak, above all other species was the premier building material for large-scale carpentry.
A Master Chronology
A so-called master chronology must begin ultimately with living trees of a particular species, such as oak or pine. The trees are felled or bored through to obtain a core sample and the rings inside the bark are counted to determine the age of the tree. Each ring represents a year's growth. Thus, each ring width measured in millimeters can be assigned to a particular calendar year. By taking great numbers of samples, the ring widths are averaged to establish a mean sequence of size and years, and this is called a master chronology.
| Oak: Heartwood/Sapwood Boundry (bottom Image) Photo: W.G. Simpson
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 How a Master Chronology is formed Drawing by Lynn Courtenay
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The key to establishing a master chronology spanning millennia involves bridging, which occurs when a recently dated sample begins before the first ring of the master chronology so that the cross-dated sequence then forms a bridge to an earlier time period. Thus, by working gradually backward to join ring sequences of an unknown date (called floating chronologies) to a master chronology based on trees of a known felling date, the time span of the original chronology continually expands. Once cross-matched rings are identified, even prehistoric sites can be fixed in real time, and today sites and shipwrecks thousands of years old can be dated by this method.
One of the most important, but not the longest, master chronologies used for dating medieval buildings in continental Europe is Hollsteinšs Master Oak Chronology (1980) for northern central Europe extending to 700 B.C. In the last twenty years, however, new regional master chronologies have refined earlier pioneering works. As this science has evolved, dendrochronology has become a primary tool for dating early buildings. As an important by-product of this endeavor, we know much more about medieval woodlands, climate changes, the regional characteristics of timber during certain time periods, and the choices carpenters and patrons made concerning the quality and quantity of wood employed in a given building. Thus the symbiotic relationship between historic ecology and dendrochronology has proved highly beneficial to both.
Procedures
As in any statistically-based method, several securely-dated samples (ring-width data) augment the ease with which a reliable match with a master chronology is achieved. Climatic differences dictate that master chronologies be established for individual regions; eight to twelve core samples are normally taken to date each phase of construction of a building so that representation of different parts of the structure are included in the dating process. Also important, each sample must have enough rings present to establish an accurate match. Taking the core samples at the correct angle and place requires skill and knowledge of timber construction. Core samples with less than 50 rings are considered unsuitable. Most critical, however, in obtaining a reliable felling date is the presence or absence of sapwood and how much.
The Heartwood-Sapwood Boundry for Oak
If the bark or the last annual ring are preserved, the precise year and season in which the tree was felled can be determined; however, in most cases (especially in elite buildings) carpenters cut away much of the sapwood in squaring the timber for construction. The question then arises: how can an approximate felling date be determined from partial evidence? The solution to this problem again relates to a botanical understanding of oak. Since the transition between the porous sapwood (where fluid transfer occurs) and the dense heartwood of oak is climate related and is reportedly fairly constant within a given region, estimations of the amount of sapwood can be derived from statistics based on complete sapwood.
Dendrochronologists use two methods of determining the earliest plausible felling date for timber with incomplete sapwood. In England, where dendrochronology has been advanced for several decades, the tendency is to use regional, microclimates to determine the number of rings of sapwood normally present. For example, chronologies for Warwickshire in the West Midlands, indicate that sapwood rings for oak vary from 15 to 40 in number and a general average of ca. 30 rings (30 years) has been used when more refined data is absent.
Elsewhere, the calculation of sapwood has been related to actual measurement of width, as in John Fletcher's early work, where an average of about an inch of sapwood (assuming a consistent average ring width of 1mm) equals about 25 years growth. The sapwood-width method has been recently applied to dating oak in France, Switzerland, and Belgium. For example, for eastern Belgium, Patrick Hoffsummer calculates that the average amount of sapwood from the last hardwood ring to be 2 cm wide, which in turn averages to 16 rings, or 16 years with an additional variable +/- 5 for the earliest possible felling date.
Thus, if the ring marking the boundary between heartwood and sapwood is present, then a felling date range can be given, e.g. from a boundary ring dated to 1500, the range for a Warwickshire building would be 1515 to 1540. If only heartwood is present, with a last ring of 1500, then the earliest possible felling date would be 1515, usually printed as 1515+, but felling could have taken place much later.
As a caveat, students of medieval architecture need to realize that dendrochronology at best tells us exactly when a particular tree was felled, but it does not indicate when it may have been actually used or re-used. Thus one also needs to look carefully at the integrity and consistency of the carpentry per se. There are some cases where timber has been stockpiled in advance of construction, although the normal procedure with oak was to use it green (soon after felling), since it was easier to work with the tools available before the timber dries, shrinks, and hardens. Nonetheless, in cathedral building extended over a considerable period of time, an hiatus in construction might easily occur after a certain amount of timber has already been felled.
There is also plenty of evidence in both ecclesiastical and domestic buildings for the re-use of timber, which remained a valuable commodity throughout the Middle Ages and hence was salvaged. The application of dendrochronology has, however, made re-use more apparent when tell tale signs like inappropriate joints and empty mortises are not conspicuous. In sum, if the timber in question is suitable for this dating method, dendrochronology can be extremely reliable and hence an invaluable aid to archaeology and the study of early buildings.
Example of re-used timber with an empty lap mortice,
from the Prior's House, Ely Cathedral Precinct.
Photo: Lynn Courtenay
Suggested Further Readings
Baillie, M.G.L., Tree Ring Dating and Archaeology, Chicago, 1982.
Baillie, M. G. L.. - A slice through time : dendrochronology and precision Dating. London : Batsford, 1995.
Fletcher, J. ed. Dendrochronology in Europe, Principles, Interpretations and applications to Archaeology and History (Greenwich: British Archaeological Reports, international ser., 51, 1978)
Hillam, J., Morgan, R.A., & Tyers, Ian, "Sapwood estimates and the dating of short ring sequences," Application of Tree-Ring Studies (Greenwich: BAR, int. ser. 3 (1987), 165-85.
Hoffsummer, Patrick. Les charpentes de toitures en Wallonie, Typologie et dendrochronologi (XIe - XIXe siècle), Études et Documents, Monuments et Sites; Namur, 1995. [contains extensive bibliography!]
Hollstein, E. Mitteleuropaische Eichenchronologie, Mainz am Rhein, 1979;