NOVA Online | Methuselah Tree

Skeleton Plots

Here 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.

Tree-ring skeleton plot

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.

Anatomy of a Tree Ring
Growth and Sensitivity
Skeleton Plots

To find out more about tree-ring dating, visit The Laboratory of Tree-Ring Research at the University of Arizona. The site includes interactive features that allow you to create your own skeleton plots.

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