Relative Scale of Skeleton Plots

Trees living within a homogeneous stand or forest:
  • Usually exhibit the same relative pattern of growth variation through time, BUT
  • Often have absolute growth rates that differ substantially due to living in different microsites:
    • Difficult to compare the ring-growth variation of two or more specimens from actual wood samples
    • For example, see figure:

      three cores with different growth rates

      1. All three samples above have exactly the same pattern of variation
      2. Top sample has a growth rate less than that of the middle sample
      3. Middle sample has a growth rate less than that of the bottom sample

Problem: Because of different growth rates, it's impossible to align the common pattern of variation using the actual wood samples

Solution: Skeleton plots can standardize scales:
  • Represent growth variation of samples onto separate strips of graph paper
  • Equalize the absolute scales of ring growth
  • Easily compare growth variation of two or more specimens
Note the three skeleton plots of the figure above:
  • Have the same relative scale
  • Are easy to compare to one another
  • Clearly show the same pattern of variation of each sample:
When you try skeleton plotting for yourself, you can move the virtual core left and right:

  • Put the cursor anywhere on the core above
  • Click and hold the mouse button
  • Move the mouse left or right
  • Don't worry that the core goes off the edges – it always comes back!

Note: The virtual core comes with every 10th ring prenumbered – which won't be true in real life, so number your wood carefully!

When you try skeleton plotting for yourself, you can change the scales of the graphics:
magnification options
  • There will be magnification circle buttons, as on the left
  • You may enlarge or reduce the virtual core to see rings better
    • Choose between 1x, 2x, or 3x
    • Hint for magnifying a ring:
      1. There will be a vertical blue line marking the middle of your screen, as on the right
      2. Put the ring that you wish to see better at the blue line
      3. Then magnify – at 3x magnification, narrow rings are apparent
  • You may also enlarge or reduce the graphs
    • Choose between Large, Medium, and Small
zoom magnification

Paul R. Sheppard
Laboratory of Tree-Ring Research, The University of Arizona
Tucson, Arizona 85721 USA
office: (520) 621-6474, fax: (520) 621-8229
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