Dendrochronology

Introduction

Dendrochronology is the study of growth rings within the internal structure of trees that allows age dating of the tree and interpretation of events affecting tree growth and their timing. Growth rings, or simply tree rings, can be seen in a cross section of the trunk of a cut tree. The rings manifest themselves as sequential bands of graded color that ring the entire trunk of the cut tree. Because each ring tends to terminate in a color shade that is different from the color shade of the ensuing band, they are readily distinguishable.

The rings themselves are the result of annual new growth of the tree trunk in the vascular cambium, the outer layer of woody material in the tree trunk that lies beneath the tree’s outer bark. The changing shade of color within each ring is the result of the diminishing rate of growth of new woody material as the seasons change. The inner part of each ring is the spring-growth part and the outer part of each ring is the summer-growth. Trees in areas of adequate moisture develop one ring per year; however, it is possible for two or more rings to form under conditions of sudden water restriction during the spring-summer growing season. These multiple rings may be distinguished from normal annual growth rings by their relatively narrow width and lack of substantial color gradation. Rings vary in the intensity of shading and some rings are nearly uniform, for example some pine trees, so the wood looks more or less homogenous.

Historical Background and Scientific Foundations

Trees growing in the same area tend to develop the same pattern of tree rings as long as they experience the same conditions. This allows for correlation of dates and events affecting tree-ring growth across areas such as river basins, mountain ranges, and plains. This technique, which is usually done using computer-based statistical analysis of tree-ring width and other parameters, can be used to understand the recent natural history of a forested area.

At the outset of dendrochonological studies, simple measurements of tree ring widths and patterns of ring widths were used in side-to-side comparisons. Modern analytical techniques involve measuring and then comparing between many trees to attain a statistically valid average of ring widths for each annual cycle of tree growth. Anchored dendrochronologies are those set of average tree-ring widths that have a known start and end date. Floating dendrochronologies do not have known start or end dates, but may be anchored by additional study and comparison to selected parts of known, anchored dendrochronologies. There is an anchored dendrochronology for approximately the last 10,000 years.

The process of obtaining dendrochronological data depends upon the possibility for sampling. If the tree has already been felled, a sample may be obtained by cutting a section of the trunk. If the tree is to be left standing or if the sample is of some wood that must remain in place (for example, archaeological structures), a core is extracted by drilling a hole in the wood with a special drill device that extracts a continuous, cylindrical sample of wood from the hole drilled. The surfaces of both the slice and the wood core must be polished and prepared for analysis, usually by photographing the surfaces and then subjecting the images to digital analysis by scanning.

Impacts and Issues

Modern dendrochronology has moved beyond the realm of age dating of trees. For example, dendrochronology has been used as a mechanism for checking the accuracy and precision of radiometric dating, which is based on laboratory analysis of the ratio of carbon isotopes present in once-living matter. By careful study, dendrochronological records of trees, particularly long-lived species like the bristle-cone pine, have been correlated with climatic records in ice cores and the records of sedimentary layers (varves) in large lakes and oceans. Modern dendrochonology is an integral part of the database used to help understand the recent natural history of climate change on Earth as well as being part of more local studies of the more local changes in specific ecological areas. Further, modern dendrochronology is closely tied to many archaeological investigations that involve age-dating as well as analysis of climate change and human impacts on civilization. In this regard, some related fields of modern dendrochronology are dendroclimatology and dendroarchaeology.

See Also Climate change; Forests; Ice cores

BIBLIOGRAPHY

Books

Baillie, M. G. L. A Slice Through Time: Dendrochronology and Precision Dating. New York, Cambridge University Press, 1995.

Web Sites

Oxford University. “Oxford Dendrochronology Laboratory.” December 2007. (accessed 26 March 2008).

WORDS TO KNOW

DENDROARCHAEOLOGY: The analysis of wooden material from archaeological sites using the techniques of dendrochronology.

DENDROCLIMATOLOGY: The study of past climates using the techniques of dendrochronology.

RADIOMETRIC DATING: Use of naturally occurring radioactive elements and their decay products to determine the absolute age of the rocks containing those elements.

VARVE: An annual layer or series of layers of sediment deposited in a body of still water.

David T. King Jr.