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Ron Towner from the Laboratory of Tree-Ring Research at the University of Arizona explains the principles behind dendrochronology and why this dating method is valuable to archaeologists. Ron demonstrates how to accurately count tree-rings, and discusses the importance of patterns and master chronologies. Trees are often used to make analogies about the past. Family trees, the tree of life, getting back to your roots. But beyond the powerful imagery that trees give us to represent our history, what can trees actually tell us about the past? Dendrochronology is the scientific method of tree-ring dating.
Visible rings result from the change in growth speed through the seasons of the year; thus, critical for the title method, one ring generally marks the passage of one year in the life of the tree. Removal of the bark of the tree in a particular area may cause deformation of the rings as the plant overgrows the scar. The rings are more visible in trees which have grown in temperate zoneswhere the seasons differ more markedly. The inner portion of a growth ring forms early in the growing season, when growth is comparatively rapid hence the wood is less dense and is known as "early wood" or "spring wood", or "late-spring wood"  ; the outer portion is the "late wood" sometimes termed "summer wood", often being produced in the summer, though sometimes in the autumn and is denser.
Many trees in temperate zones produce one growth-ring each year, with the newest adjacent to the bark. Hence, for the entire period of a tree's life, a year-by-year record or ring pattern builds up that reflects the age of the tree and the climatic conditions in which the tree grew. Adequate moisture and a long growing season result in a wide ring, while a drought year may result in a very narrow one.
Direct reading of tree ring chronologies is a complex science, for several reasons. First, contrary to the single-ring-per-year paradigm, alternating poor and favorable conditions, such as mid-summer droughts, can result in several rings forming in a given year. In addition, particular tree-species may present "missing rings", and this influences the selection of trees for study of long time-spans.
For instance, missing rings are rare in oak and elm trees.
A Few Notes on Trees
Critical to the science, trees from the same region tend to develop the same patterns of ring widths for a given period of chronological study. Researchers can compare and match these patterns ring-for-ring with patterns from trees which have grown at the same time in the same geographical zone and therefore under similar climatic conditions.
When one can match these tree-ring patterns across successive trees in the same locale, in overlapping fashion, chronologies can be built up-both for entire geographical regions and for sub-regions.
Moreover, wood from ancient structures with known chronologies can be matched to the tree-ring data a technique called cross-datingand the age of the wood can thereby be determined precisely.
Dendrochronologists originally carried out cross-dating by visual inspection; more recently, they have harnessed computers to do the task, applying statistical techniques to assess the matching. To eliminate individual variations in tree-ring growth, dendrochronologists take the smoothed average of the tree-ring widths of multiple tree-samples to build up a ring historya process termed replication.
A tree-ring history whose beginning- and end-dates are not known is called a floating chronology. It can be anchored by cross-matching a section against another chronology tree-ring history whose dates are known.
A fully anchored and cross-matched chronology for oak and pine in central Europe extends back 12, years,  and an oak chronology goes back 7, years in Ireland and 6, years in England. The dendrochronological equation defines the law of growth of tree rings. The equation was proposed by Russian biophysicist Alexandr N. Tetearing in his work "Theory of populations"  in the form:.
With the neglection of natural sinusoidal oscillations in tree mass, the formula of the changes in the annual ring width is:.
The formula is useful for correct approximation of samples data before data normalization procedure. Dendrochronology makes available specimens of once-living material accurately dated to a specific year.
Timber core samples are sampled and used to measure the width of annual growth rings; by taking samples from different sites within a particular region, researchers can build a comprehensive historical sequence.
The techniques of dendrochronology are more consistent in areas where trees grew in marginal conditions such as aridity or semi-aridity where the ring growth is more sensitive to the environment, rather than in humid areas where tree-ring growth is more uniform complacent. In addition, some genera of trees are more suitable than others for this type of analysis.
For instance, the bristlecone pine is exceptionally long-lived and slow growing, and has been used extensively for chronologies; still-living and dead specimens of this species provide tree-ring patterns going back thousands of years, in some regions more than 10, years. For the period back to 12, B.
Dendrochronology practice faces many obstacles, including the existence of species of ants that inhabit trees and extend their galleries into the wood, thus destroying ring structure. European chronologies derived from wooden structures initially found it difficult to bridge the gap in the fourteenth century when there was a building hiatus, which coincided with the Black Death however there do exist unbroken chronologies dating back to prehistoric times, for example the Danish chronology dating back to BC.
Given a sample of wood, the variation of the tree-ring growths not only provides a match by year, but can also match location because climate varies from place to place.
This makes it possible to determine the source of ships as well as smaller artifacts made from wood, but which were transported long distances, such as panels for paintings and ship timbers. Dates from dendrochronology can be used as a calibration and check of radiocarbon dating .
Dendroclimatology is the science of determining past climates from trees primarily from the properties of the annual tree rings. Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous.
Dendrochronology has become important to art historians in the dating of panel paintings. However, unlike analysis of samples from buildings, which are typically sent to a laboratory, wooden supports for paintings usually have to be measured in a museum conservation department, which places limitations on the techniques that can be used.
In addition to dating, dendrochronology can also provide information as to the source of the panel. Many Early Netherlandish paintings have turned out to be painted on panels of "Baltic oak" shipped from the Vistula region via ports of the Hanseatic League. Oak panels were used in a number of northern countries such as England, France and Germany.
Wooden supports other than oak were rarely used by Netherlandish painters. Since panels of seasoned wood were used, an uncertain number of years has to be allowed for seasoning when estimating dates. Consequently, dating studies usually result in a " terminus post quem " earliest possible date, and a tentative date for the arrival of a seasoned raw panel using assumptions as to these factors.
However, dendrochronology revealed that the wood dated from the second half of the sixteenth century. It is now regarded as an original sixteenth-century painting by an unknown artist. On the other hand, dendrochronology was applied to four paintings depicting the same subject, that of Christ expelling the money-lenders from the Temple. The results showed that the age of the wood was too late for any of them to have been painted by Hieronymus Bosch. While dendrochronology has become an important tool for dating oak panels, it is not effective in dating the poplar panels often used by Italian painters because of the erratic growth rings in poplar.
The sixteenth century saw a gradual replacement of wooden panels by canvas as the support for paintings, which means the technique is less often applicable to later paintings. The dating of buildings with wooden structures and components is also done by dendrochronology; dendroarchaeology is the term for the application of dendrochronology in archaeology.
While archaeologists can date wood and when it was felled, it may be difficult to definitively determine the age of a building or structure in which the wood was used; the wood could have been reused from an older structure, may have been felled and left for many years before use, or could have been used to replace a damaged piece of wood.
The dating of building via dendrochronology thus requires knowledge of the history of building technology. Herbchronology is the analysis of annual growth rings or simply annual rings in the secondary root xylem of perennial herbaceous plants. Similar seasonal patterns also occur in ice cores and in varves layers of sediment deposition in a lake, river, or sea bed.
Dendrochronology, or 'tree ring dating' as it is often known, can provide an invaluable insight into the history of a building by revealing the year in which the . Dendrochronology, or tree-ring dating, is the study of growth rings in deciduous trees to identify absolute dates of wooden objects. Tree rings are created by the tree as it grows in girth, and the width of a given tree ring is dependent on climate, so a stand of trees will all have a near-identical pattern of tree rings.
The deposition pattern in the core will vary for a frozen-over lake versus an ice-free lake, and with the fineness of the sediment. Sclerochronology is the study of algae deposits. Some columnar cacti also exhibit similar seasonal patterns in the isotopes of carbon and oxygen in their spines acanthochronology. These are used for dating in a manner similar to dendrochronology, and such techniques are used in combination with dendrochronology, to plug gaps and to extend the range of the seasonal data available to archaeologists and paleoclimatologists.
Archaeology Dating Lecture Part 1
A similar technique is used to estimate the age of fish stocks through the analysis of growth rings in the otolith bones. From Wikipedia, the free encyclopedia. Method of dating based on the analysis of patterns of tree rings. Further information: Wood. Main article: dendroclimatology.
Main article: Dendroarchaeology. This section does not cite any sources. Please help improve this section by adding citations to reliable sources.
Unsourced material may be challenged and removed. Trees portal. Archived from the original on Douglass, A. Climatic Cycles and Tree Growth.
Washington, D. From p. For this study the name "dendro-chronology" has been suggested, or "tree-time. Studhalter, R. James H.
The potential then, even with these two simple sets of data that we may extrapolate from the tree ring data, is enormous.
It is an accurate and reliable dating method with a large number of uses in environmental studiesarchaeology and everything in between. The method has gone from strength to strength and is now a vital method across multiple disciplines. From the s, several seminal studies began at the University of Arizona 67 studying the bristlecone pine of California and hohenheim oak in Germany. Thanks to the work of these studies, we now have an 8, year chronology for the bristlecone pine and in the region of 12, year chronology for the oak.
This enormous and comprehensive data set is fundamental to both European and North American studies of the palaeoclimate and prehistory 8. There is one major drawback to dendrochronology and that is that we can only date the rings in the tree. This says nothing about either when the particular tree was felled, nor about the date it was used 8.
In past times, good quality timber may have been reused 10 and for the archaeologist, it is important to check other records against the new data. Some trees are also better than others for study 5. Tree species vary greatly. In this article we make the assumption that growth is annual with a distinct growing season. Most tree species are reliable; oak is the most reliable tree type for tree rings - with not a single known case of a missing annual growth ring.
Birch and willow are not used at all because of the erratic nature of their growth cycle. Since the changes to the climate since the industrial revolution, some of the more recent dendrochronology records have become erratic 9 and in higher elevations, tree ring data has declined - we are seeing more variability than ever before In times before we had modern treatment of wood, people often drained trees of sap after felling and prior to use of the timber.
The removal of the sap, and sometimes the heartwood, can seriously affect the wood's reliability as an artefact for dating A good dendrochronology study depends heavily on a lack of a repeated pattern.
We expect, due to the changing nature of the climate, that each year will have a distinct pattern in the record 9. No pattern is likely to be repeated perfectly but it is certainly possible.
Tree ring dating (dendrochronology) 3 February Tree ring dating (dendrochronology) has been used in an attempt to extend the calibration of carbon dating earlier than historical records allow, but this depends on temporal placement of fragments of wood (from long-dead trees) using carbon dating. Dendrochronology is the dating and study of annual rings in trees. The practical applications of the study of tree rings are numerous. Dendrochronology is an interdisciplinary science, and its theory and techniques can be applied to many applications. See our subdisciplines for examples. These research interests have in common the following. Call or email us for more information about dating your building. Oxford Tree-Ring Laboratory Michael Worthington and Jane Seiter, PhD 25 E. Montgomery St. Baltimore, MD [email protected]
All permutations must be examined and, if necessary, check the record against known external information. Part of the dendrochronological record is also to measure the amount of carbon in the tree sample, because of this lengthy record we will know the exact date that a tree ring was created inside the living organism.
This ongoing record then, is vital to dating organic material through radiocarbon dating. The amount of radiocarbon isotope in the artefact is compared against tree ring data for calibration, and it is always calibrated against organic material of known age 8. The comprehensive nature of the tree ring record is the perfect database against which to calibrate when we are trying to date organic materials.
Radiocarbon Tree-Ring Calibration
Most records will be unique and this should, in theory, give an absolute date for the artefact; if they have an identical level of the isotope, we can safely conclude that they are of the same age Finding a precise year is rarely so clear-cut so a range of dates is selected, hence that radiocarbon dates always come with an error factor.
It is certainly the oldest datable footpath in the world if we define footpath as something artificial and deliberately created for the purpose of getting around a geographical area, rather than a path that has evolved from trampling.
Until the s, it was notoriously difficult to date waterlogged archaeological sites, which was frustrating for researchers because organic material such as wood rarely finds itself in areas where it might easily survive. Until this time, there was next to no chronology for the prehistoric period in England 15, p Dendrochronology helped this enormously and when part of the Sweet Track was found in waterlogged soil on the Somerset Levels, it gave researchers into the Iron Age and earlier periods hope that over the following decades was certainly realised.
The Somerset Levels were waterlogged most of the year in prehistoric times, not drained until the post-medieval period, and the track ran for nearly 2km from high ground to what was then an island on the levels The tree ring data taken from some of the surviving extensive timbers that survived because they were waterlogged managed to effectively date the track itself and settlements nearby to around BC at the time of completion 15, p This was a date that researchers suspected, albeit far more broadly than before confirmation, but from that point dendrochronology became a fundamental tool in dating archaeological remains.
In the fight against climate change, it is to the past that we look in order to work out what our future might look like. The study of tree ring data is vital for understanding what our regional and global palaeoclimate looked like at any time, especially in light of the lack of other sources where we might get such information. The method has undergone immense improvement in the last 20 years. Where most climatologists look at how humans are affecting the climate, dendrochronology for climate science is focussed on the changes on vegetation that results from the natural processes of climate chang 16 p The method of change may have been different, but the results are the same and it can tell us much about increasing levels of carbon in the past.