Fire and vegetation history in the eastern Cascade Mountains, Washington

Dendrochronological techniques were used to reconstruct a 433-yr fire history, and to characterize the historical fire regime (frequency, size, season, severity) of the Teanaway River drainage in the eastern Cascade Mountains of Washington, USA. General Land Office section corner data were used to reconstruct aspects Of the late-19th-century structure and composition of the forests in the study area. Systematic fire-sear surveys (similar to30 000 ha; 92 sites; 257 fire-scarred cross-sections; 1569 individual fire scars) revealed that fire frequency was quite variable spatially; Weibull median probability intervals ranged from seven to 43 years. Fire extent also varied widely; most fires were relatively small (<1000 ha), although several large fires (>4000 ha) were detected. Mean and median fire sizes were 1795 ha and 988 ha, respectively. Large fires occurred every 27 years, on average (every 11 years, on average, between 1708 and 1889), and coincided with periods of annual and seasonal drought (Palmer Drought Severity Index and winter Southern Oscillation Index). Intervals from 1 to 37 years occurred between fires of >4000 ha. Over 80% of fires occurred late in the growing season, or after the onset of cambial dormancy. Sampling locations in dry forest types (dominated by ponderosa pine) yielded fire-scarred cross-sections with numerous fire scars leading us to infer that most historical fires were of low intensity, leaving the overstory structure intact. This inference is corroborated by the composition and structure of the historical forest, which was characterized by a preponderance of very large (>100-cm diameter) ponderosa pines. Mesic forest types (dominated by grand fir and Douglas-fir) likely exhibited a wider range of fire severities. Moderate and occasional high-severity understory fires or crown fires did occur within the study area, as indicated by the scarcity or lack of fire-scar evidence and the presence of relatively even-aged forests at several mesic forest sites. Historical section corner data indicate that small amounts of these forest types occurred in the study area. Fire frequency and size declined dramatically circa 1900, coincident with the advent of commercial timber harvesting, although most fires, despite their reduced number and size, continued to burn in the late summer or fall.