Are tree-ring chronologies reliable?
[EDITOR'S NOTE: This is the first of a two-part series on “Dating in Archaeology. Like other radiometric methods, radiocarbon dating faces technical problems. Some early papers on tree rings: J.C. Kapteyn. Tree-ring dating: Factors pertaining to accuracy. Objectives and methods in New England tree-ring studies. Seasonal changes in cellular growth near the bark of a tree leave rings buried in its wood. The size of those records is tied to the growth of the.
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.
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.
Radiocarbon Tree-Ring Calibration
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 history, a 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.
A typical form of the function of the wood ring in accordance with the dendrochronological equation with an increase in the width of wood ring at initial stage.
Dendrochronological equation defines the law of growth of tree rings. This is a result of the annual cycle of seasons.
The idea that ancient trees grew more than one ring per year will be discussed below. Therefore, by coring a living tree and counting rings from the present backwards, it is possible to determine the year in which each ring grew. There are some very old living trees on earth.
The bristlecone pines in the White Mountains of California live to extremely old ages, some in excess of 4, years. The University of Arizona dendrochronology lab sports a no longer living specimen which contains over 6, rings. Generally, it is not possible to construct a complete sequence of tree rings back through the historical periods using only living trees.
Chronologies derived from living trees must be extended. This is accomplished using wood specimens found preserved, for example, in historic buildings, or on the forest floor, or in peat bogs. The rings in a non-living specimen can be counted to determine the number of years the specimen spans. But for the specimen to be useful in extending the tree-ring chronology, the absolute calendar age of its rings must be determined.
The annual growth rings vary in thickness each year depending on environmental factors such as rainfall. By matching ring-width patterns in a specimen of known age starting with living specimens to ring-width patterns in an older specimen, the proper placement of the older specimen is determined. Tree-ring chronologies have been extended to 10, years before present in this way.
Some critics of dendrochronology suggest that the process of pattern-matching is highly error-prone. Are the long tree-ring chronologies inaccurate due to the inability of dendrochronologists to accurately match tree-ring patterns? We could discuss the details of pattern-matching technique or the probability of error, but there is another, more quantitative way, to determine if the long tree-ring chronologies are accurate or not.
One can use the amount of radiocarbon in the individual tree rings. Because radiocarbon is everywhere the same in the atmosphere at any given time, tree rings which grew in the same year should have the same amount of radiocarbon.
Furthermore, radiocarbon in the atmosphere fluctuates from year to year in a somewhat erratic fashion. This allows different dendrochronologies to be compared over multiple years to see if they show the same pattern of radiocarbon fluctuations.
An Independent Check Early in the history of the science of dendrochronology, a tree-ring chronology using bristlecone pines from the White Mountains of California was developed. Separate dendrochronologies were then developed, also in America, using other types of trees, such as Douglas fir. These separate chronologies did not extend as far back in time because these types of trees are shorter-lived.
However, they did agree with the bristlecone chronology as far back as it could be checked by the shorter chronologies. That is, rings of the same putative dendrochronological age were found to contain the same amount of radiocarbon, and to give the same pattern of fluctuations over time. These measurements demonstrated the basic validity of the science of dendrochronology. If the method had a large component of random error due to inaccurate pattern matching, how could such detailed agreement between the radiocarbon in the rings of two independent dendrochronologies be possible?
The internal agreement of these American dendrochronologies confirmed that dendrochronologists are able to accurately match ring patterns. But another independent check came along which was even better than the Douglas fir chronology.
Apologetics Press - Dating in Archaeology: Radiocarbon & Tree-Ring Dating
European Tree-ring Chronology While American scientists were building bristlecone pine and Douglas fir chronologies, European scientists were actively building a very long tree-ring chronology using oak trees.
The more recent part of the chronology was constructed from oak logs used in various historic buildings. The more ancient part of the chronology was constructed from oak logs preserved in peat beds, for example. The European oak chronology provided an excellent check of the American dendrochronologies. The two were obviously independent. Ring-width patterns are determined by local environmental factors, such as temperature and rainfall.
The patterns in America could not bias the work on patterns in Europe, because the specimens came from two different local climates, separated by an ocean. The scientists worked independently of one another. Also, oak trees and bristlecone pine or Douglas fir trees are very different.
Bristlecones, for example, are evergreens which grow very slowly, at high altitude, in a cold, arid environment, and live for thousands of years. None of these things are true of the oaks used in the European chronology. They are deciduous, grow relatively rapidly, at low altitudes, in relatively warm, moist environments, and live for only hundreds of years.