Exceptional variability in historical fire regimes across a western Cascades landscape, Oregon, USA

Detailed information about the historical range of variability in wildfire activity informs adaptation to future climate and disturbance regimes. Here, we describe one of the first annually resolved reconstructions of historical (1500–1900 ce) fire occurrence in coast Douglas-fir dominated forests of the west slope of the Cascade Range in western Oregon. Mean fire return intervals (MFRIs) across 16 sites within our study area ranged from 6 to 165 years. Variability in MFRIs was strongly associated with average maximum summer vapor pressure deficit. Fire occurred infrequently in Douglas-fir forest stands seral to mountain hemlock or silver fir, but fire frequency was much shorter than predicted by theory in other forest types. MFRIs within Douglas-fir stands seral to western hemlock or grand fir ranged from 19 to 45 years, and MFRIs in stands seral to Douglas-fir ranged from 6 to 11 years. There was little synchrony in fire occurrence or tree establishment across 16 sites separated by 4 km. The lack of synchrony in fire suggests that large, wind-driven fire events that are often considered to be characteristic of coast Douglas-fir forests were not an important driver of succession in our study area during the last ~400–500 years. Climate was more arid than normal during fire years in most forest types, but historical fire in stands seral to Douglas-fir was strongly associated with antecedent moisture and less strongly associated with drought. We interpret the extraordinary tempo of fire we observed in stands seral to Douglas-fir and the unique climate pattern associated with fire in these stands to be indicative of Indigenous fire stewardship. This study provides evidence of far more frequent historical fire in coast Douglas-fir forests than assumed by managers or scientists—including some of the most frequent fire return intervals documented in the Pacific Northwest. We recommend additional research across the western Cascades to create a comprehensive account of historical fire in highly productive forests with significant cultural, economic, and ecological importance.