An introduction to dendrochronology
How Tree-Rings can be dated
In most tree species in the British Isles a single tree-ring is produced each year and the tree-ring’s width varies in response to environmental factors, particularly rainfall and temperature. Generally speaking the milder the weather the larger the tree-ring. A warm British winter or wet summer will generally have a positive effect on growth (dependent on site factors) and give rise to a wider ring in that year. A harsh winter or dry/hot summer generally has a negative effect on a tree's growth causing a narrow tree-ring in that year.
Weather characteristics and the likely effect on the development of the tree-ring produced in that year by a living tree:-
|Weather which encourages narrow tree-ring growth||Weather which encourages wide tree-ring growth|
|Harsh Winter||Mild Winter|
|Hot Summer||Mild Summer|
In the short 83 year sequence exampled in the graph below which plots each successive year's ring width agaist time, the result is a distinctive trace of narrow and wide rings. Tree-ring dating matches the specific annual variation in a tree-ring sequence (in other words, the pattern of narrow to wide rings), to the corresponding years of an established chronology to enable dating.
A 83-year long dated sequences of tree-rings widths. (HS = Heartwood/sapwood boundary)
Dating the past
The unique sequence of tree-rings resulting from the pattern of weather conditions match up with the yearly records for weather we have for England over approximately the last 300 years. However, the pattern of weather is recorded in the tree-rings from before this time and when trees were growing at the same time they will broadly show the same unique sequence of tree-rings from the climatic conditions they experienced at that time.
The first 150 tree-ring sequence of a 300 year old tree growing today can be matched to half of a 300 year old tree felled 150 years ago. The tree-ring sequence of these two trees (one of known date and the second tree of possibly unknown date) overlap by 150 years. The two 300 year tree-ring sequences may be matched over the 150 year sequence of overlap and thus combined to create a 450 year long tree-ring sequence of known date. Overlapping timbers from modern forests, buildings and archaeology has allowed the annual tree-ring record to be extended backwards over the last 10,000 years. The overlapping of tree-ring sequences and combining of timbers from different sources allows the creation of a "reference or master chronology" generally of known date which can be used with compare to timbers of unknown date and allow them to be dated.
While as illustrated below the theory of dendrochronology is relatively simple, specialist experience, techniques and statistics are all required to provide a scientific date. Numerous reference chronologies have been developed, and through statistical analysis it is generally possible to date timbers from over most of the UK. However, different regions of the country experienced different climate, and dating may be problematic in some areas where the reference chronology coverage is weak and still in the process of development.
Representation of the principle behind tree-ring dating.