The crossdating and skeleton plotting techniques take time and effort to learn. Before attempting these exercises, it may be helpful to review the basic information about dendrochronology and wood formation at the LTRR's website (http://www.ltrr.arizona.edu). A useful reference about the techniques of dendrochronology is "An Introduction to Tree Ring Dating" by Stokes and Smiley. It may be difficult for a novice to learn these techniques simply by trying these exercises. Asking for help from a local dendrochronologist, forester, or school teacher who has some experience teaching these techniques is a good way to get started.
Contents of an Exercise
Each of these exercises contains at least some of these elements:
- Site Description and Map: general background information about the origin of the material; purpose of the original study which collected the samples, site map
- Data Sheet: use this spreadsheet for organizing information about individual samples and your progress in crossdating them
- Wood Samples: these hand-drawn "cores" and "slabs" are simplified versions of real wood samples; skeleton plot each of them, then combine the individual plots into a composite skeleton plot-an undated or "floating" chronology which you will date against the master chronology
- Master Chronology: the established chronology against which your composite skeleton plot should date; cut out the strips and paste them end to end to create one long strip
Steps to Completing an Exercise
- Orient your cores or slabs with the pith (tree center) to the left and bark to the right.
- The left-most ring (partial or whole) on a wood sample is the year "0" since you do not yet know the actual calendar year. Use the wood-marking conventions described in the Crossdating Cheat Sheet to mark the cores and slabs.
- Create skeleton plots for each wood sample.
- Crossdate the skeleton plots amongst themselves. Lay them out in from of you in chronological order.
- Create one long composite chronology skeleton plot that averages the bar heights of each of your individual skeleton plots for each year (for year 32 for example, average the bar heights of all of the individual trees which were alive in year 32, and use that value as the composite bar height for year 32).
- Cut out the master chronology and paste together the strips so that you have one long chronology.
- Crossdate your composite chronology against the master chronology. When you have found the exact match, assign beginning, end, and decadal dates along the length of the composite chronology.
- Align your newly-dated composite chronology with the individual chronologies, and assign the beginning, end and decadal dates to each individual chronology. Record these on the data sheet.
- Think about the implications of your results. When did these trees grow? Is there any distinct pattern to the earliest years or latest years? Where pith is present, the earliest year may be close to the germination date; where bark is present, the last year is the death date.
- Check your answers.