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Tree-ring Analysis of Arbutus menziesii : Suitability for Dendrochronology

In order to establish an estimate of the ages of large Pacific madrone (Arbutus menziesii) trees growing atop the bluffs on Magnolia Boulevard in Seattle, Washington, I obtained stump-discs from madrones cut by the Seattle Parks Department in June 1994. Each disc was sectioned into smaller pieces to allow processing with standard woodworking techniques. After initial processing, each slab was sanded with progressively finer grades of sandpaper until ring boundaries were distinguished. Slabs were then examined for possible marker years that allow cross-dating among slabs. Ring counts determined the minimum age of the samples to be 113, 91, 78, 76 and 67 years. Exact ages could not be determined. High year to year variability in ring-widths was observed, but numerous bole scars associated with irregular growth rings throughout the samples prevented accurate cross-dating. Although the madrone stump-discs used in this study could not be cross-dated, the high year to year variability in ring-widths and some commonality among fast and slow growth years suggest Pacific madrone growth is sensitive to environmental conditions. Carefully sampled madrone may be suitable for dendrochronological analysis. The seasonal nature of temperate climates often results in the production of a single annual ring of diameter growth in trees. Variations in the size of annual rings provide information on differences in an individual’s growth on a year to year basis. In a local or regional area, shared patterns of tree growth ring variation indicate historical influences such as climate change and disease on tree growth (i.e., dendrochronology) (Fritts 1976 and Swetnam, et al. 1985). Matching of relatively fast and slow growth ring patterns among trees is called cross-dating (Swetnam, et al. 1985). Cross-dating allows comparisons of growth from different trees on the same site because the specific year that each ring was formed can be determined. 2