Distinctive patterns of growth rings in increment cores from old-growth Douglas-fir (Pseudotsuga menziesii) stands identify A.D. 1800 as a more precise date for the eruption of tephra layer T by Mount St. Helens, Washington. Layer T was previously inferred to date to about A.D. 1802. Growth patterns also establish A.D. 1480 as the date of eruption of the earlier layer Wn, previously estimated as dating to about A.D. 1500. The timing of radial tree growth places a small limitation on the seasonal resolution of these new tree-ring dates.
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