Fire dating from tree rings in western Cascades Douglas-fir forests: an error analysis

Cross dating, the matching of tree-ring patterns to determine absolute dates or tree-ring series, is a valuable technique for dating wildfires. However, most recent fire history studies conducted in Pacific Northwest Douglas-fir forests have not employed cross dating. The error associated with non-cross-dated, field-counted, fire history data was assessed at four sites in Douglas-fir forests of the western Cascades, Oregon. Fire scar and tree origin years were dated in the field by counting tree rings on minimally prepared stump surfaces. Wood samples from these same stumps were then prepared in the laboratory, where tree rings were recounted and cross dated. Fire histories from field-counted, laboratory-counted. and cross-dated efforts were compared.
Cross dating required 22 times the effort of the field-counted fire history reconstruction, and 87% of fire-scarred samples could be cross dated. The field-counted data generally underestimated ages of fire scar and tree origin years, and fires reconstructed from field-counted data were estimated as having occurred from 1 to 16 years more recently than they actually did. Field counted scar years were within 10 years of their true values for about 75%, and within 20 years for about 87% of observed cases.Errors in fire frequency estimates were small unless an incorrect number of fires was reconstructed. Also, the error associated with careful laboratory counts on well-prepared surfaces was minimal (mean error of 1.5 years) even when cross dating was not conducted. We recommend that future fire history studies in the Pacific Northwest employ cross dating.