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The dating of trees and ancient wood by measuring and counting the annual growth rings for, usually, archaeological purposes. But hey, some people might do it just for kicks.

Mostly, it is used for the dating of past events such as climate change. Larger rings mean more moist seasons, where a tree diverts energy to growth, smaller rings show harsher times.

In this way, dendrochronology can be used to trace rainfall, wind, snow, droughts, fires and volcanic activities throughout the life of the tree. The more varied the growth rings, the more a trees growth has been limited by changing environmental conditions. Similiary, the more constant the growth rings, the more a tree has led a fairly easy life, and climate conditions have been stable.

The use of tree rings to determine the age of trees was first developed by astronomer Andrew Ellicott Douglass in 1901. Initially looking for tree ring growth as an indicator of solar cycles, believing that solar flares affected climate, and therefore the amount of growth a tree might make in a year. What he ascertained, was that tree ring width does indeed vary with the amount of rainfall in a given year. Therefore, all trees within a certain area will grow at the same rate during wet or dry years. And, each tree will contain a record of rainfall that occurred during its life, and will include density, trace elements, and isotope composition.

Along with anthropologist Clark Wissler, Douglass worked to find a connecting tree ring sequence that would allow for dating pine trees in the American southwest. In 1929, after 12 years of research, they found a charred log near Show Low, Arizona, that enabled them to complete their sequence and assign calendar dates to archaeological sites for over 1000 years.

Nowadays, dendrochronology has been extended in the American southwest to 322 BC, by matching known patterns of dark and light rings to those recorded by Douglass and those who followed his research. There is also dendrochronology records for Europe, the Aegean, and an International Tree Ring Database which includes 21 countries.

First utilized by Andrew Ellicott Douglass at the University of Arizona in the early 20th century, dendrochronology is a method of dating archaeological digs and past environmental changes such as climatic shifts by studying the growth of rings in trees. Douglass' pioneering work began when he left the Lowell Observatory in 1901 and began teaching at the University of Arizona in 1906. The study of dendrochronology is closely related to the fields of botany, climatology and dendrology.

As a tree grows, it adds another layer of xylem cells to its trunk for each year it is alive. These cells are necessary for transport of water from the roots to the leaves of the tree. Given more rainfall, the tree needs more xylem cells to transport the water up its trunk, so the rings for wetter years are larger. The reverse is true during periods of drought.

There are various conceptual principles involved in this technique.1 These assumptions form the basis for all research conducted that involves this practice.

  • The Uniformitarian Principle: Stated simply, "the present is the key to the past." 2 The geological processes that affect the environment now are understood to have existed in some form in the past, affecting things in a similar manner. This principle is applied further by including that "the past is the key to the future". By understanding past environmental changes through studying the growth of tree rings, such changes can be better managed when they arise in the future.
  • The Principle of Limiting Factors: The rate of growth in plants can only proceed as quickly as allowed by that factor which is most limiting. This is most often applied to rainfall; if rain is especially scarce one year, making it inhibit growth more than other conditions, then the tree will only grow as much as rainfall permits it to.
  • The Principle of Aggregate Tree Growth: The overall tree growth for one year can be broken down into its composite parts, according to this principle, which are a collection of various environmental influences. These include climatic shifts, insect infestation, normal physical aging trends and human involvment. Therefore, in order to isolate one variable to study (such as climate), it is necessary to select a sample in which other factors are not a major influence.
  • The Principle of Ecological Amplitude: Species of trees have varying ability to grow in wide, narrow, or restricted types of habitats. For example, Pinus ponderosa is the most widely distributed of all pine species in North America, growing in a diverse range of habitats such as wet, dry, and various elevations. Therefore, ponderosa pine has a wide ecological amplitude. Conversely, Sequoiadendron giganteum only grow in restricted areas on the western slopes of the Sierra Nevada of California. Giant sequoia, therefore, have a narrow ecological amplitude. The application of this principle is in using it to find the idea trees for dendrochronological study, which are usually found at the outer margins of their natural range.
  • The Principle of Site Selection: This principle is an extension of The Principle of Ecological Amplitude. In order to fully isolate and study one variable, a site which has likely been affected by that variable is needed. For example, if studying the effects of excessive rainwater on past growth patterns, then a site would be selected which ideally exhibited current heavy rainfall patterns, perhaps in a tropical climate.
  • The Principle of Crossdating: A good method of verification of dating, a second sample is taken from a different tree from the same site. By checking the ring patterns on both trees, a more specific date can be determined and the possibility of error is reduced. For example, if desiring to date an ancient indigenous wooden structure, crossdating would be applied by sampling the trees from nearby to compare the ring patterns.
  • The Principle of Replication: This principle applies to collection of samples itself in that it is necessary, to reduce the risk of ignoring possible environmental variables, to sample more than one stem radius per tree, more than one tree per site, and several sites in a region. This assures that the researcher is not neglecting such factors as insect infestation, air pollution and runoff that can affect limited areas and invalidate results.

These principles of sampling are crucial in terms of forming an accurate chronology. When utilized correctly, the time period studied can be expanded much further:

"A chronology (arrangement of events in time) can be made by comparing different samples. Using a boring tool, a long slender core sample about .423 centimeters in diameter is extracted. Lets say the sample was taken from a standing 4,000 year-old (but long dead) bristlecone. Its outer growth rings were compared with the inner rings of a living tree. If a pattern of individual ring widths in the two samples prove to be identical at some point, we can carry dating further into the past. With this method of matching overlapping patterns found in different wood samples, bristlecone chronologies have been established almost 9,000 years into the past." 3

An instrument called a boring tool, or a core bore, is used to remove a long, thin slice of wood from living trees for analysis. If sampled correctly, the wood sample should be perfectly horizontal with respect to the ground so that measurement of the width of the rings is as accurate as possible. Lists of data are available at many different websites, however, a particularly excellent resource which has searchable data and links to other research institutions can be found at the World Data Center for Paleoclimatology's site: http://www.ngdc.noaa.gov/paleo/treering.html

1 Much thanks to Henri Grissino-Mayer who first compiled a list of these principles. His research website at The University of Arizona can be found at http://ltrr.arizona.edu
2 As first stated by James Hutton in 1785.
3 http://www.sonic.net/bristlecone/dendro.html

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