Dendrochronology

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.