Paleo Slide Set: Tree Rings: Ancient Chronicles of Environmental Change
Extending a chronology based on living trees further back in time through crossdating.

Dendrochronology is the analysis of tree rings, including the dating of annual rings 
and study of patterns of ring characteristics, such as widths, density, and isotopic 
composition. In mid- to upper latitudes, or areas where there is seasonality in 
temperature and/or precipitation, many species of trees form annual growth rings. 
Because the same set of environmental factors influence tree growth throughout a region, 
the patterns of ring characteristics, such as ring widths, are often common from tree 
to tree. These patterns can be matched between trees in a process called crossdating, 
which is used to assign exact calendar year dates to each individual ring. Dated and 
measured rings from a number of trees in a region are combined to form a tree-ring chronology. 
The chronology provides two main types of information:

The chronology can be used as a tool for dating events that caused tree death or a marked 
change in the appearance of a ring or set of rings. The death date can be used to date the 
tree cutting involved in the construction of wooden dwellings. Scars can record the timing 
of events such as fire, flood, avalanche, or other geomorphological events, while sequences 
of suppressed or larger rings record events such as insect infestation, effects of pollution, 
or changes in forest dynamics.

The chronology is an average of coherent variations in growth from a number of trees. 
It enhances the common pattern of variation or "signal" -- usually related to climate -- 
while the non-common variance, or "noise" is dampened. Chronologies from trees that are 
sensitive to climate can be used to reconstruct past variations in seasonal temperature, 
precipitation, drought, streamflow, and other climate-related variables.



Photo Credits:

Laboratory of Tree-Ring Research, The University of Arizona