Dendrochronology

Dendrochronology

What is Dendrochronology? Dendrochronology is the dating and study of annual growth rings in trees. The word comes from these roots: dendros = trees; more specifically, the growth rings of trees chronos = time; more specifically, events and processes in the past ology = the study of In other words, the “study of tree rings to understand past events and processes”. What Do Tree Rings Tell Us? The practical uses of the study of tree rings are numerous. Dendrochronology is an interdisciplinary science, and its theory and techniques can be applied to many uses. These research interests have in common the following goals. • to put the present in proper historical context • to better understand current environmental processes and conditions • to improve understanding of possible future environmental issues

Dendrochronology History Archaeological tree-ring dating began in 1917 when Andrew Ellicott Douglass, the founder of dendrochronology, first examined prehistoric wood samples. Twelve years later, the “bridging of the gap" at Show Low, Arizona joined dated living-tree and "floating" archaeological chronologies and began routine archaeological tree-ring dating. In 1937, the University of Arizona founded the Laboratory of Tree-Ring Research (LTRR) to continue Douglass' dendrochronological research. After World War II, the Laboratory collected all other Southwestern archaeological tree-ring collections - Museum of Northern Arizona, Gila Pueblo, Laboratory of Anthropology, Navajo land Claim and I. F. Flora - and the Robert E. Bell Collection of samples from the eastern United States. At the same time, LTRR's own Douglass Collection continued to grow through regular additions. As a result, the Laboratory has become the repository for all Southwestern archaeological tree-ring material and many samples from elsewhere. LTRR houses more than 360,000 archaeological samples from the Southwest, the Great Basin, the Great Plains, the Midwest, Alaska, Mexico, and the Near East. These research collections provide the ultimate certification of the dates and constitute an unmatched reservoir of materials for further archaeological and dendrochronological research. The gathering of archaeological tree-ring collections at LTRR provided the opportunity for an large-scale study of all Southwestern tree-ring material. Between 1963 and 1975, the "Dendrochronology of Southwestern United States" project organized and reanalyzed the existing collections, an exercise that quadrupled the numbers of dated samples and sites. As a result of these and subsequent analyses, the continuous regional ring chronology has been extended back to 322 B.C., and more than 60,000 dates have been produced from more than 5,000 sites.

The Laboratory of Tree-Ring Research History In 1937, the scientific study of tree-rings in America was established by the creation of the Laboratory of Tree-Ring Research (LTRR) here at the University of Arizona in Tucson. By that time, founder Andrew Ellicott Douglass had been working with wood for over 30 years, and so the tradition of dendrochronology here in Tucson is older than the Lab itself. The LTRR has long since given up the distinction of being the nation's only dendrochronology lab, but we have helped to found many dendrochronology labs around the world. Our unique heritage is still a source of pride, and it encourages us today to remain dedicated to our ideals: excellence in research, excellence in teaching, and excellence in outreach. Andrew Ellicott Douglass

Dendrochronolgy Vocabulary • Cookie A tree cookie is a sliced portion of a tree bole that can show each and every annual ring on a viewable plane. A tree cookie can be one of the best teaching aids to kids and adults on things happening in a tree and environmental effects on trees. • Dendrochronlogy is the dating and study of annual growth rings in trees. • Dendrochronologist is a scientist who uses tree rings to answer questions about the natural world and the place of humans in its functioning. • Increment Borer an instrument used to remove a narrow core sample from the tree • Living-tree chronology is a chronology series beginning in the present or recent past and getting older in time using only cross-dating of non-archaeological tree samples. • “Floating” chronology is a chronology series based on archaeological tree ring samples which is not connected to the present with living-tree chronologies. • “Bridging of the gap” is when the floating chronologies were connected to the living-tree chronologies.

Cross-dating is comparing tree-ring information between two different trees to match-up sections of there three-ring chronologies. • Cross-section a cross section is a cut through something (such as a tree) at an angle perpendicular to its axis in order to view its interior structure. • Annual ring is the amount a tree grows in one year as seen in its tree rings. • Earlywood is growth of the tree ring in spring (or rainy season); looks light colored. • Latewood is growth of the tree ring during fall/winter (or dry season); looks dark colored. • Pith is the wood at the center of the tree. • Repository is a location where things are stored for future examination and review. For example, a library is also called a book repository. • Interdisciplinary is when specialists/scientists from different areas of study (i.e., disciplines) work together toward understanding or studying a common problem.

Anatomy of aTree Ring

Anatomy of a Tree RingA tree during the winter months stands dormant, but during the spring (or during the wet season, in some areas), it begins to grow. It actually experiences a growth spurt -- with new cells growing between the previous year's tree ring and the tree's bark.The cells that grow during this initial period are large. As the season progresses, however, the cells that form are smaller and smaller. These smaller cells appear darker.The difference in appearance between the smaller latewood cells and the following season's earlywood cells creates a striking delineation. The ring represents one year in the life of the tree.

Spring/Summer Fall/Winter = 1 year

- crossdating (dendrochronology's fundamental technique) matching ring-growth characteristics across many samples from a homogeneous area (area of similar environmental conditions) - permits identification of EXACT year of formation for each ring - skeleton plotting' is one method of crossdating - skeleton plotting (one method of crossdating) the process of marking a tree's ring width variation on graph paper strips (the 'skeleton plot') - similar patterns of variation in individual plots (representing individual trees) are matched among trees

Skeleton PlotsHere we have tree-ring samples from two trees that grew not too far from each other. Notice how the patterns for both are the same except that, for one, the rings are compressed, which shows that it had a slower growth rate. There could be any number of reasons for the difference -- perhaps the tree that produced the compressed sample was shaded by other trees, for example. There is a way to present tree-ring information that allows easy comparison of trees with different growth rates. Called skeleton plotting, it offers other advantages as well.Every tree-ring sample contains a record for every year of growth. That's an awful lot of information, especially if what you want to do is crossdating. Some of this information is more relevant than other information. Narrow rings occur less often than normal-width rings, for example, so that information is more useful. What the skeleton plot does is to extract the most pertinent information and record it on a graph. Each vertical line in the above graph represents one year. The yellow bars that appear on three of the graph's lines represent narrow rings. The narrower the annual ring, the longer the line. The plot records other information as well. The "b" indicates a year with an unusually wide tree ring, for example, while a dashed line (not seen in this graph) would mark a year in which that tree failed to produce a visible ring. With skeleton plotting, tree-ring scientists are able to accurately crossdate tree-ring samples.

Skeleton PlotsHere we have tree-ring samples from two trees that grew not too far from each other. Notice how the patterns for both are the same except that, for one, the rings are compressed, which shows that it had a slower growth rate. There could be any number of reasons for the difference -- perhaps the tree that produced the compressed sample was shaded by other trees, for example. There is a way to present tree-ring information that allows easy comparison of trees with different growth rates. Called skeleton plotting, it offers other advantages as well.Every tree-ring sample contains a record for every year of growth. That's an awful lot of information, especially if what you want to do is crossdating. Some of this information is more relevant than other information. Narrow rings occur less often than normal-width rings, for example, so that information is more useful. What the skeleton plot does is to extract the most pertinent information and record it on a graph.Each vertical line in the above graph represents one year. The yellow bars that appear on three of the graph's lines represent narrow rings. The narrower the annual ring, the longer the line. The plot records other information as well. The "b" indicates a year with an unusually wide tree ring, for example, while a dashed line (not seen in this graph) would mark a year in which that tree failed to produce a visible ring. With skeleton plotting, tree-ring scientists are able to accurately crossdate tree-ring samples.

http://www.ncdc.noaa.gov/paleo/slides/slideset/18/18_364_bslide.htmlhttp://www.ncdc.noaa.gov/paleo/slides/slideset/18/18_364_bslide.html

Year 0 10 20 Start here 3 different trees (all same species), cut at the same time, but all have different growth.

Other Issues to Consider

Dendrochronology