2. Principles of Dendrochronology

This section outlines the principles and approaches underpinning dendrochronology across the various sub-disciplines and reflects the many connections between them. It also briefly introduces some of the newer methodological approaches which are detailed in later sections.  

The annual growth rings from trees can provide a rich range of environmental information from the effects of changing site ecology over space and time to the influence of climate on growth (Cook 1987). Underpinning tree-ring science (or dendrochronology) is the ability to assign a precise calendar date to each ring formed by the tree. Through the careful and scientific matching of wide and narrow rings (and other measured properties) from different trees of (usually) the same species growing within a similar climate region, it is possible to cross-match or synchronise the ring sequences against reference series.  This means it is possible to date each ring exactly to the calendar year they were formed. The fact that tree-ring parameter time series can be synchronised over large regions reflects the common influence of climate expressed in the growth of trees. This is fundamentally the greatest strength of dendrochronology – that each annual ring can be exactly dated to the year (Douglass 1941; Baillie 2012). 

The origin of the word Dendrochronology is translated as the study of dating tree-rings (dendron = tree, chronos = time, logo = the study of). The term now also includes the interpretation of the physical and chemical information contained within them. To understand tree-growth and the range of applications of dendrochronology in the environmental and social sciences, one must understand that there are multiple factors that influence the variability of tree ring parameters from year-to-year. This variability can be described by assuming that growth within a tree is a function of an aggregate of multiple factors and can be expressed by the following conceptual equation (Cook 1987): 

TRt = At + Ct + D1t + D2t   

TR is the measured tree-ring parameter for each year (t) and this value is related to; A = the so-called biological age trend which reflects ontogenetic changes of the tree form as it matures (i.e. for ring-width data, this is often expressed as a negative exponential trend in values due to the increasing girth dimensions of a tree through its life); C = the climatic influence on tree-growth; while D1 and D2 reflect unique tree-specific (D1 – e.g. growth release due to felling of a neighbouring tree due to increase light) or stand-wide (D2 – e.g. impact of disease, regional disturbance etc) ecological signals. As the growth of trees is an amalgam of a range of environmental signals, dendrochronology is therefore a broad discipline that encompasses many applications in the social and environmental sciences including archaeological dating, ecology, geomorphology and palaeoclimate reconstruction. In fact, within the International Tree-Ring Databank, there are literally 1000s of tree-ring data archived representing the full range of dendrochronological studies over the last 50 years.

Within the United Kingdom, dendrochronological methods have been applied mostly to archaeological dating, stand level woodland/landscape dynamics and climate reconstruction over the past 40-50 years (Baillie et al 1985; Briffa et al 1985; Crone and Mills 2012; Edwards and Mason 2006; Gagen et al 2019; Loader et al 2019; Mills and Crone 2012; Rydval et al 2017; Summer et al 2008; Wilson et al 2013; 2017). 

Variables that can be measured from tree-rings 

Multiple parameters can be measured from the rings of trees. Tree ring-width is the standard staple for most dendrochronological research and previous publications provide ample information on this TR parameter (Fritts 1971; Schweingruber 2012). However, in the last couple of decades other parameters such as wood density, quantitative wood anatomy and stable isotopes have come to the fore to enhance all aspects of dendrochronological research – including archaeological dating.

Section 6 Dendroclimatology details the use of ring-density and related parameters for studying past climate while section 8 Stable Isotope Dendrochronology refers specifically to stable isotopes and their utility for archaeological dating and dendroclimatology. In the coming decades, these new novel parameters will increasingly be used for dating of archaeological material to enhance what can be done using ring-width alone.  


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