Diverse historical fire disturbance and successional dynamics in Douglas-fir forests of the western Oregon Cascades, USA

We created the first annually resolved records of historical fire occurrence coupled with precise estimates of tree establishment for the northern half of the west slope of the Oregon Cascades, a region that is home to some of the most productive forests on earth. Our reconstructions at 36 randomly located sites document exceptional diversity in historical fire disturbance and successional dynamics. Most stands where we collected data appear to have initiated following stand-replacing fire between 200 and 750 years ago, although many sites exhibited evidence of moderate-severity fire that created multi-aged stands. More than two-thirds of sites experienced multiple non-stand-replacing fires following stand initiation. A spatial generalized linear mixed model demonstrated that historical fire occurrence was negatively associated with average snow disappearance day and time since last fire and positively associated with drought. Significant variability in the number of fires, length of fire return intervals, and sample depth across sites made calculation of informative mean fire return intervals (MFRIs) difficult. Site-level annual probability of fire from our mixed model ranged from 0.039 to 0.003, equivalent to MFRIs of 26–389 years. We used fire and tree establishment records to infer the general location of several large historical fire events that likely burned as much or more area as the >50,000 ha fires that burned across our study region in 2020. We also identified periods of extensive burning and subsequent tree establishment that occurred across seven centuries within six large river drainages that made up our study region. Although tree establishment occurred for up to a century following stand-replacing fire at some sites, we show that these apparent long periods of establishment were relatively short pulses of regeneration separated by reburns. This study demonstrates that many highly productive Douglas-fir-dominated stands in western Oregon are significantly departed from historical fire disturbance regimes. Management that emphasizes rapid re-establishment of closed canopy forest conditions following fire and development of old-growth forest conditions in the absence of fire may fail to provide for the unique and highly valued ecosystem services associated with these forests.