Tree-Ring Basics- From: http://tree.ltrr.arizona.edu/dendrochronology.html; The Laboratory of Tree-Ring Research, Tucson, Arizona
What is Dendrochronology?
Dendrochronology is the dating and study of annual rings in trees.
The word comes from these roots:
ology = the study of
chronos = time; more specifically, events and processes in the past
dendros = using trees; more specifically, the growth rings of trees
Dendrochronologist
a scientist who uses tree rings to answer questions about the natural world and the place of humans in its functioning
What do Tree Rings Tell Us?
The practical applications of the study of tree rings are numerous. Dendrochronology is an interdisciplinary science, and its theory and techniques can be applied to many applications. See our subdisciplines for examples. These research interests have in common the following objectives:
to put the present in proper historical context
to better understand current environmental processes and conditions
to improve understanding of possible future environmental issues
Why Not Just Count the Rings?
Ring-counting does not ensure the accurate dating of each individual ring. Numerous studies illustrate how ring-counting leads to incorrect conclusions drawn from inaccurate dating. Dendrochronologists demand the assignment of a single calendar year to a single ring. Various techniques are used to CROSSDATE wood samples to assure accurate dating.
Dating Method: Crossdating by Skeleton Plotting
The SKELETON PLOT is one method of crossdating tree rings.
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
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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
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Basics of Ring Formation
Understanding these concepts will help you succeed at this website's skeleton plotting and crossdating exercises. This page does not attempt to cover the details of wood formation that make tree rings possible, but rather provides an overview of common wood characteristics and anomalies that you will need to identify when you are crossdating.
Conifer Tree Ring
earlywood
appears light in color
cells have thin walls, large diameter
latewood
appears dark in color
cells have thick walls, small diameter
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(transverse or cross-sectional view)
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Angiosperm Tree Ring
earlywood
cells have large diameter vessels
latewood
cells: small diameter vessels
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(transverse or cross-sectional view)
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Ring Width Variation
This picture of a conifer wood sample shows The rings display much variation:
variation in total ring width:
a light and a dark band
variation in latewood width:
just the dark bands
variation in latewood density:
darkness of dark band
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Variation in these rings is due to variation in environmental conditions when they were formed.
Thus, studying this variation leads to improved understanding of past environmental conditions
and is the basis for many research applications of dendrochronology.
A key distinction of dendrochronology is that all trees rings being analyzed are dated to their correct year of formation. At first glance, it appears easy to date tree rings by just counting them, but reality is often more complicated than that. 
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