Dendrochronology

Trees and other woody plants grow by covering themselves with a new layer of tissue every year. When seen in a horizontal section, such wood layers appear as concentric tree rings, familiar to anyone who has looked at a tree stump. Because tree growth is influenced by the environment, tree rings are then natural archives of past environmental conditions. For instance, trees grow less when climate conditions are less favorable, producing narrower rings. The study of past changes recorded by wood growth is called dendrochronology.

Besides determining tree age, dendrochronological information has been used in four major fields of scientific research:

• reconstruction of climatic factors that control average wood growth from year to year (such as precipitation , temperature, air pressure, drought severity, sunshine)
• dating of abrupt events that leave permanent scars in the wood (fire, volcanic eruptions, earthquakes, insect defoliations, and hurricanes, for example)
• dating of archaeological wood (such as the pueblos of the American Southwest, churches, bridges, and paintings in Europe)
• the calibration of the radiocarbon time scale over the Holocene epoch, covering the last ten thousand years.

The application of tree-ring dating to archaeology is indeed closely linked to the development of dendrochronology as a modern science, a process that began in the early 1900s at the University of Arizona under the direction of Andrew Ellicott Douglass, an astronomer who first established and demonstrated the principles of tree-ring dating.

Most tree-ring samples consist of pencil-shaped cores drilled from the lower stem, allowing an estimate of wood growth without cutting the tree down. So-called increment borers used for coring allow for nondestructive sampling because they leave only a 5 millimeter-wide hole, and such small injury can be readily managed by a healthy tree. (As an analogy, extracting an increment core is likely to affect a mature tree's vigor as much as drawing a blood sample is likely to affect an adult animal's health.)

Dating and Cross-Dating

Tree-ring dating is the assignment of calendar years to each wood growth ring. This requires more than simply counting visible rings, because not every growth layer is always present or clearly noticeable, especially in very old trees. When only one or two trunk radii are available per tree, the chance of dating errors is greater than when examining entire cross-sections. To ensure dating accuracy, ring patterns from many different trees of the same species and location are matched with one another. This allows the creation of a master chronology for this location. This cross-dating exercise, which is similar in principle to matching fingerprints or deoxyribonucleic acid (DNA) sequences, is first done visually under a binocular microscope using 10 to 30 power magnification. Once a tree-ring sample has been properly surfaced , that magnification is high enough to distinguish individual wood cells. After measuring the thickness of each ring, cross-dating can be verified using specialized numerical procedures. While numerical cross-dating is based on alternating patterns of narrow and wide rings, visual cross-dating can incorporate other anatomical elements as well, such as the proportion and color of earlywood and latewood within individual rings.

Cross-dating has found other important applications in dendro-chronology. Once a (master) tree-ring chronology is established, a wood sample from the same species and area can be accurately dated by matching its ring-width patterns against the master. This procedure is commonly used in archaeological and historical investigations to date wood material, artifacts , and structures. In addition, as wood samples from older living trees are cross-matched against those from historic and prehistoric times, the length of the master chronology can also be extended farther back in time, a process that has allowed the development of tree-ring chronologies for the last ten thousand years, over the entire Holocene epoch.

The final tree-ring chronology is derived from the combination of all tree-ring samples into a single, average time series, which summarizes short- and long-term historical patterns for that species and site. Tree growth varies on multiple time scales, from interannual to interdecadal, and various numerical methods have been proposed to preserve (or discard) this information in the final tree-ring chronology. Such methods are grouped under the term standardization in the dendrochronological literature, and they are intended to minimize changes in growth rate that are not common to all trees. For climatological reconstruction, the final tree-ring chronology is statistically calibrated against instrumental records of climate, such as precipitation and temperature, to identify the main climatic signals present in the tree-ring record. The relationship between tree growth and climate is then extrapolated back into the past, and climatic changes are estimated from the tree-ring chronology itself. Because of the long life of many tree species, dendrochronological records tell of climate conditions occurring each year over hundreds, sometime thousands, of years, whereas instrumental weather records are commonly limited to the last decades, and seldom exceed one hundred years.

Tree-ring chronologies have been developed from a number of species in all continents where trees exist. In the western United States, most tree-ring records are derived from conifers, because they are very common, reach old ages, and, as softwoods, they are easier to sample than hardwoods. However, not all trees are equally suitable for dendrochronological studies. In temperate, high-latitude and high-elevation climates, wood growth is usually constrained to the warm season, and tree rings are easily recognizable. Cross-dating is easier when year-to-year variability of tree growth is higher, because this causes a greater number and degree of pattern differences in tree-ring series. When ring widths are less variable, common, climatically influenced patterns are more difficult to discern. Site conditions are therefore very important in dendrochronological studies because they affect tree-ring variability, which is an expression of the sensitivity of tree growth to climate. Other factors being the same, trees growing in difficult environ-ments—on steep, rocky slopes, at the latitudinal or altitudinal edge of their natural range—attain greater age, grow more slowly, and show higher year-to-year changes than trees of the same species found in more mesic sites, on flat terrain, and/or deeper soils.

To date, tree-ring studies of tropical trees have been limited by the fact that wood growth layers are not visually identifiable, especially in species found at low elevations. Anatomical features and the lack of pronounced seasons allow wood growth in tropical lowlands to occur throughout or erratically during the year, making the identification of synchronous growth patterns among trees a difficult task. Even outside the tropics it is not always possible to reliably cross-date tree-ring patterns among individuals of the same species and site. A notable example is the world's tallest tree, the California coast redwood (Sequoia sempervirens ), whose rings are not uniform around the stem. This causes different radii from the same tree to include a widely different number of rings, which prevents the development of a reliable tree-ring chronology. Such ring discontinuities are species specific and apparently unrelated to climate.

see also Forestry; Palynology; Record-Holding Plants; Trees; Wood Anatomy.

Franco Biondi

Bibliography

Baillie, M. G. L. Tree-ring Dating and Archaeology. Chicago: University of Chicago Press, 1982.

Fritts, Harold C. Tree Rings and Climate. New York: Academic Press, 1976.

Nash, Stephen E. Time, Trees, and Prehistory: Tree-ring Dating and the Development of North American Archaeology, 1914-1950. Salt Lake City: University of Utah Press,1999.

Schweingruber, Fritz H. Tree Rings: Basics and Applications of Dendrochronology. Boston: D. Reidel Publishing Co., 1988.

Stokes, Marvin A., and Terah L. Smiley. An Introduction to Tree-ring Dating. Tucson, AZ: University of Arizona Press, 1996.

Dendrochronology

Dendrochronology is the science of dating events and variations in environment in former periods by comparative study of growth rings in trees and aged wood. In scientific terminology, tree growth rings are used as proxy indicators for past environmental variations. The term dendrochronology is derived from the Greek terms dendron for tree, chronos, meaning time, and logos meaning the science of.

Dendrochronology is governed by a set of principles or scientific rules. These principles have their roots as far back as 1785 (the Principle of Uniformitarianism) and have continued to evolve as recently as 1987 with the Principle of Aggregate Tree Growth. Some are specific to dendrochronology, such as the Principle of Aggregate Tree Growth, while others, like the Principle of Replication, are basic to many disciplines. All tree-ring research must adhere to these principles, or else the research that results could be flawed.

Dendrochronology is an important technique in a number of disciplines, including archeology, paleontology, paleobotany, geomorphology, climatology, and ecology. Forensic applications concern the dating of wooden objects and matching objects with crime scenes using the wood’s morphological features.

Plant anatomical features have long been used by archeologists and paleontologists to date and to characterize archeological sites. Since the 1930s they have become increasingly more common in forensic applications. The cell wall is particularly important for two reasons: it is not easily digested by most organisms and, therefore, persists when other plant features are destroyed, and the size, shape, and pattern of cell walls is often specific according to species.

Annual growth rings occur because the xylem cells become gradually smaller in radius as the growth season proceeds into the dormant season. There is an abrupt change in size from small, late season cells to

the large, early season cells of the following spring. The approximate age of a temperate forest tree can be determined by calculating the annual growth rings in the lower part of the trunk. The variation in ring width reflects environmental conditions. Wide rings signify favorable growing conditions, absence of disease and pests, and favorable climatic conditions. Ring patterns of several samples from a given geographic area subject to similar environmental conditions are cross-dated, giving standardized chronologies (curves) for different species in different areas, to which specimens of unknown origin can be compared. Tree rings record responses to a wider range of climatic variables, over a larger part of the Earth, than any other type of annually dated proxy record.

Tree ring analysis is a common technique for dating masterworks by European painters, many of which were painted directly on wood. If the samples are in good condition, analysts can pinpoint the exact year when the tree from which the wood for the painting was taken was cut down. For example, a Peter Paul Reubens painting originally dated 1616 was shown to be at least 10 years younger, and a painted wall panel recovered from a house in Switzerland in the 1970s was determined to have been painted on spruce harvested in 1497.

Likewise, dendrochronology techniques are useful in determining the provenance of wooden art objects and musical instruments. In one case, two violins forming part of an inheritance were purported to have been made by Antonio Stradivari. The sounding boards of the instruments were x-rayed and compared to standard curves for spruce from the Alpine region of northern Italy, where Stradivarius is known to have worked. The oldest rings from the samples dated to 1902 and 1894 respectively for the two violins. Furthermore, these oldest rings were not the outermost rings of the wood from which the violins were constructed. Allowing for a period of seasoning before the wood could be used to make the instruments, analyses showed that the violins could not have been made before 1910. Given that Stradivari did his best work at the turn of the 17th century, the instruments were deemed to be fakes.

It can be a challenge to estimate the time since death for a body when only bones remain. Plant roots, like their above-ground counterparts, exhibit annual growth rings that can be useful in pinning down the postmortem interval, or at least the time since the body came to be at the location where it was found.

In one criminal case, the discovery of human remains lying across a black spruce (Picea mariana ) leader (branch) that subsequently grew up around the remains provided an opportunity to use the growth ring pattern to estimate the postmortem interval. These remains were discovered in an advanced state of decomposition, and it was clear that relevant insect evidence was not forthcoming. The asymmetrical growth of the leader resulted in a correspondingly asymmetrical pattern of its growth rings. As the date of cutting the leader was known, it was possible to evaluate the asymmetrical growth pattern to provide an estimation of the postmortem interval. Fine polishing of the cross section and computerized quantification of ring widths enabled an estimation of the displacement of the leader, and hence the time the decedent’s body was so positioned. By charting the ring-width differential for the leader, the actual date of disappearance was confirmed.

See also Crime scene investigation; Forensic science.

Nicolas Dittert

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.

Dendrochronology

Dendrochronology is the science of dating events and variations in environment in former periods by comparative study of growth rings in trees and aged wood. In scientific terminology, tree growth rings are used as proxy indicators for past environmental variations. The term dendrochronology is derived from the Greek terms dendron for tree, chronos, meaning time, and logos meaning the science of.

Dendrochronology is governed by a set of principles or scientific rules. These principles have their roots as far back as 1785 (the Principle of Uniformitarianism) and have continued to evolve as recently as 1987 with the Principle of Aggregate Tree Growth. Some are specific to dendrochronology, such as the Principle of Aggregate Tree Growth, while others, like the Principle of Replication, are basic to many disciplines. All tree-ring research must adhere to these principles, or else the research that results could be flawed.

Dendrochronology is an important technique in a number of disciplines, including archeology, paleontology, paleobotany, geomorphology, climatology, and ecology. Forensic applications concern the dating of wooden objects and matching objects with crime scenes using the wood's morphological features.

Plant anatomical features have long been used by archeologists and paleontologists to date and to characterize archeological sites. Since the 1930s they have become increasingly more common in forensic applications. The cell wall is particularly important for two reasons: it is not easily digested by most organisms and, therefore, persists when other plant features are destroyed, and the size, shape, and pattern of cell walls is often specific according to species.

Annual growth rings occur because the xylem cells become gradually smaller in radius as the growth season proceeds into the dormant season. There is an abrupt change in size from small, late season cells to the large, early season cells of the following spring. The approximate age of a temperate forest tree can be determined by calculating the annual growth rings in the lower part of the trunk. The variation in ring width reflects environmental conditions. Wide rings signify favorable growing conditions, absence of disease and pests, and favorable climatic conditions. Ring patterns of several samples from a given geographic area subject to similar environmental conditions are cross-dated, giving standardized chronologies (curves) for different species in different areas, to which specimens of unknown origin can be compared. Tree rings record responses to a wider range of climatic variables, over a larger part of the Earth, than any other type of annually dated proxy record.

Tree ring analysis is a common technique for dating masterworks by European painters, many of which were painted directly on wood. If the samples are in good condition, analysts can pinpoint the exact year when the tree from which the wood for the painting was taken was cut down. For example, a Peter Paul Reubens painting originally dated 1616 was shown to be at least 10 years younger, and a painted wall panel recovered from a house in Switzerland in the 1970s was determined to have been painted on spruce harvested in 1497.

Likewise, dendrochronology techniques are useful in determining the provenance of wooden art objects and musical instruments. In one case, two violins forming part of an inheritance were purported to have been made by Antonio Stradivari. The sounding boards of the instruments were x-rayed and compared to standard curves for spruce from the Alpine region of northern Italy, where Stradivarius is known to have worked. The oldest rings from the samples dated to 1902 and 1894 respectively for the two violins. Furthermore, these oldest rings were not the outermost rings of the wood from which the violins were constructed. Allowing for a period of seasoning before the wood could be used to make the instruments, analyses showed that the violins could not have been made before 1910. Given that Stradivari did his best work at the turn of the 17th century, the instruments were deemed to be fakes.

It can be a challenge to estimate the time since death for a body when only bones remain. Plant roots, like their above-ground counterparts, exhibit annual growth rings that can be useful in pinning down the postmortem interval, or at least the time since the body came to be at the location where it was found.

In one criminal case, the discovery of human remains lying across a black spruce (Picea mariana ) leader (branch) that subsequently grew up around the remains provided an opportunity to use the growth ring pattern to estimate the postmortem interval. These remains were discovered in an advanced state of decomposition , and it was clear that relevant insect evidence was not forthcoming. The asymmetrical growth of the leader resulted in a correspondingly asymmetrical pattern of its growth rings. As the date of cutting the leader was known, it was possible to evaluate the asymmetrical growth pattern to provide an estimation of the postmortem interval. Fine polishing of the cross section and computerized quantification of ring widths enabled an estimation of the displacement of the leader, and hence the time the decedent's body was so positioned. By charting the ring-width differential for the leader, the actual date of disappearance was confirmed.

see also Crime scene investigation; Decomposition; Identification; Paint analysis.

dendrochronology An absolute dating technique using the growth rings of trees. It depends on the fact that trees in the same locality show a characteristic pattern of growth rings resulting from climatic conditions. Thus it is possible to assign a definite date for each growth ring in living trees, and to use the ring patterns to date fossil trees or specimens of wood (e.g. used for buildings or objects on archaeological sites) with lifespans that overlap those of living trees. The bristlecone pine (Pinus aristata), which lives for up to 5000 years, has been used to date specimens over 8000 years old. Fossil specimens accurately dated by dendrochronology have been used to make corrections to the carbon dating technique. Dendrochronology is also helpful in studying past climatic conditions. Analysis of trace elements in sections of rings can also provide information on past atmospheric pollution.