Dendrochronology

The growth rings of an unknown tree species, at Bristol Zoo, England

Introduction

Dendrochronology is the method of scientific dating based on the analysis of tree-ring growth patterns. The technique can date wood to exact calendar years, and tell much about the climate of specific years.

History

Beginnings of Dendrochronology

This technique was invented and developed during the 20th century originally by A. E. Douglass, the founder of the Laboratory of Tree-Ring Research at the University of Arizona.

Dendrochronology in Anthropology

Notable cases in Dendrochronology's history

Dendrochronology used to verify carbon dating techniques

Methuselah

Methuselah (estimated birth 2832 BCE) is a bristlecone pine in the White Mountains of California, which, at 4,838 years old, is the oldest living organism currently known and documented. It is named after Methuselah, a biblical figure reputed to have lived 969 years. Its exact location is currently undisclosed to the public as a protection against vandalism.

Prometheus

Prometheus (aka WPN-114) is the nickname given to the list of oldest non-clonal organism ever known, a Great Basin Bristlecone Pine (Pinus longaeva) tree about 5000 years old growing at treeline on a mountain in eastern Nevada, USA. The tree was cut down on August 6, 1964 by a graduate student and United States Forest Service personnel for research purposes. The cutting of the tree remains controversial. Different versions of the event and the decision-making process behind it exist, although a number of basic facts are agreed upon. The name refers to the mythological figure Prometheus, who stole fire from the gods and gave it to man.

The tree was a member of a population of bristlecone pine trees growing at treeline on the lateral moraine of a former glacier on Wheeler Peak, in what, since 1986, has been Great Basin National Park, in eastern Nevada. Wheeler Peak is the tallest mountain in the Snake Range, and the tallest mountain entirely within the state of Nevada. The bristlecone pine population on it is divided into (at least) two distinct sub-populations, one of which is accessible by a popular interpretive trail. Prometheus however, grew in an area reachable only by off-trail hiking. In either 1958 or 1961, a group of naturalists who admired the grove in which the tree grew gave names to a number of the largest or most distinctive trees, including Prometheus. The designation of WPN-114 was given by the original researcher, Donald R. Currey, and refers to the 114th tree sampled by him for his research in Nevada's White Pine county.

The ring count of a section of the tree by Currey was 4844. A few years later, this was increased to 4862 by Donald Graybill of the University of Arizona's Laboratory of Tree-Ring Research. However, the ring counts were done on a trunk cross section taken about 2.5 m (8 feet) above the original germination point of the tree, and so the innermost, lower rings were missed in the count. Adding in the years required to reach this height, plus a correction for the estimated number of missing rings (which are not uncommon in trees growing at treeline), it is probable that the tree was at least 5000 years old when cut. This makes it the oldest unitary (i.e non-clonal) organism ever known, exceeding the Methuselah tree of the White Mountains' Schulman Grove in California by about 200 years.

Whether Prometheus should be considered the oldest organism ever known depends on the definition of "oldest" and "organism" one uses. For example, certain sprouting or clonal organisms, such as creosote bush or aspen, could have older individuals if the entire clonal organism is considered. Under this criteria, the oldest living organism is a quaking aspen grove in Utah known as Pando, at 80,000 years old. In a clonal organism, however, the individual clonal stems are nowhere near as old, and no part of the organism at any given point in time is particularly old. Prometheus was thus the oldest non-clonal organism yet discovered, with its innermost wood over 5000 years of age. It is possible, however, that an older specimen occurs that has not yet been aged. Bristlecones are notoriously hard to age because of their extremely contorted growth, and cutting of old trees is no longer allowed.

In the 1950s dendrochronologists were making active efforts at finding the oldest living tree species, in order to use the analysis of the rings for various research purposes, such as the evaluation of former climates, the dating of archaeological ruins, and the basic question of finding the oldest living things. Bristlecone pines in the White Mountains of California and elsewhere were discovered by Edward Schulman to be older than any species yet dated. This spurred interest in finding very old bristlecones, possibly older than the Methuselah tree, aged by Schulman in 1957 at over 4700 years.

Donald R. Currey was a graduate student at the University of North Carolina at Chapel Hill studying the climate dynamics of the Little Ice Age using dendrochronology techniques. In 1963 he became aware of the bristlecone populations in the Snake Range and on Wheeler Peak in particular. Based on the size, growth rate and growth forms of some of the trees he became convinced that some very old specimens existed on the mountain, and cored some of them, finding trees exceeding 3000 years. Currey was not, however, able to obtain a continuous series of overlapping cores from WPN-114. Here, stories diverge. It is not clear whether Currey requested, or Forest Service personnel suggested, that he cut and section the tree in lieu of being able to core it. There is also some uncertainty as to why a core sample could not be obtained. One version has it that he broke or lodged his only long increment borer and could not obtain another before the end of the field season, another claims he broke two of them, while another implies that a core sample was too difficult to obtain and also would not provide as much definitive information as a full cross section of the tree would.

In addition, there are conflicting views over the uniqueness of Prometheus in the Wheeler Peak grove. It is reported that Currey and/or the Forest Service personnel who authorized the cutting believed the tree was just one of many large, very old trees in the grove, while others, at least one of whom was involved in the decision-making and tree cutting, believe that the tree was clearly unique — obviously older than other trees in the area. At least one person involved says that Currey knew this to be true at the time, although there is no known admission from Currey himself that he knew this, and others have disputed that the tree was obviously older than others.

Another uncertainty is that it is not clear how the cutting of such an old tree was necessary given the topic Currey was studying. Since the Little Ice Age started no more than 600 years ago, many trees could presumably have provided the information he was after for that time period. However, in Currey's original report (Currey, 1965) he refers to the Little Ice Age as encompassing the period from 2000 BC to the present, thus defining the Age over a much longer time period than is currently accepted. Whether this was the common sentiment at the time is not known. In the article, Currey indicates that he sectioned the tree as much from the question of whether the oldest bristlecones were necessarily confined to California's White Mountains (as some dendrochronologists had been claiming) as from its usefulness in regards to studies of the Little Ice Age.

Whatever the rationale, the tree was cut and sectioned in August 1964, and several pieces of the sections hauled out to be processed and analyzed, first by Currey, then by others in later years. Sections, or pieces of sections have ended up in various places, some of which are publicly accessible, including: the Great Basin National Park visitor center (Baker, NV), the Ely Convention Center (Ely, NV), the University of Arizona Laboratory of Tree-Ring Research (Tucson, AZ), and the US Forest Service's Institute of Forest Genetics (Placerville, CA), among others.

It has been argued that the cutting of the tree was an important factor in the move to protect bristlecones in general, and the Wheeler Peak groves in particular. There had been a movement to protect the mountain and contiguous areas in a national park before the tree was cut, and 22 years after the incident the area did gain national park status.

The exact location of the now-oldest tree, Methuselah, is kept secret by the administering agency, the U.S. Forest Service. Because of the importance of the species in dendrochronogical research, all bristlecone pines, standing or down, are now protected.

Trees

Many trees in temperate zones grow one growth ring each year, the newest ring being under the bark. For the entire period of a tree's life, a year-by-year record or ring pattern is formed that reflects the climatic conditions in which the tree grew. Adequate moisture and a long growing season result in a wide ring. A drought year may result in a very narrow one. Trees from the same region will tend to develop the same patterns of ring widths for a given period. These patterns can be compared and matched ring for ring with trees growing in the same geographical zone and under similar climatic conditions. Following these tree-ring patterns from living trees back through time, chronologies can be built up. Thus wood from ancient structures can be matched to known chronologies (a technique called cross-dating) and the age of the wood determined precisely. Cross-dating was originally done by visual inspection. Nowadays, computers are used to do the statistical 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. This process is 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 either the beginning or the end section against the end sections of another chronology (tree ring history) whose dates are known. Fully anchored chronologies which extend back more than 10,000 years exist for river oak trees from South Germany (from the Main and Rhine rivers). A fully anchored chronology which extends back 8500 years exists for the bristlecone pine in the southwest US (White Mountains of California).

In areas where the climate is reasonably predictable, trees develop annual rings of different properties depending on weather, rain, temperature, etc. in different years. These variations may be used to infer past climate variations — see dendroclimatology.

Timber core samples are used to measure the width of annual growth rings. By taking samples from different sites and different rock strata within a particular region, researchers can build a comprehensive historical sequence that becomes a part of the scientific record; for example, ancient timbers found in buildings can be dated to give an indication of when the source tree was alive and growing, setting an upper limit on the age of the wood. Some trees are more suitable than others for this type of analysis. Likewise, in areas where trees grew in marginal conditions such as aridity or semi-aridity, the techniques of dendrochronology are more consistent than in humid areas. These tools have been important in archaeological dating of timbers of the cliff dwellings of Native Americans in the arid Southwest.

Scientific value A benefit of dendrochronology is that it makes available specimens of once-living material accurately dated to a specific year to be used as a calibration and check of radiocarbon dating. The bristlecone pine, being exceptionally long-lived and slow growing, has been used for this purpose, with still-living and dead specimens providing tree ring patterns going back thousands of years. For dating purposes, in some regions sequences of more than 10,000 years are available.

The dendrochronologist faces many obstacles, however, including some species of ant which inhabit trees and extend their galleries into the wood, thus destroying ring structure.

Similar seasonal patterns also occur in ice cores and in varves (layers of sediment deposition in a lake or river). 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. 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 archeologists.

While archaeologists can use the technique to date the piece of wood and when it was felled, it may be difficult to definitively determine the age of a building or structure that the wood is in. 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.

Method

Dendrochronologically dateable trees

The Palm Tree Problem

Interpreting rings

Inaccuracies

Skipping rings

Adding rings