Wildfire is at the center of many forest conservation and management efforts across North America. Wildfire is cited as both a threat and an essential tool to promote resilient forest ecosystems in the face of a changing climate. We evaluated how much characteristic and uncharacteristic fire Pacific Northwest forests experienced between 1984 – 2014 from the perspective of ecosystems’ historic fire regimes. Historic and contemporary fire regimes were defined by both fire extent and severity. We characterized historic fire regime reference conditions for all forests across Oregon and Washington, USA using modified LANDFIRE biophysical setting models. Extent and severity of contemporary large wildfires (1984 – 2014) were mapped using relative differenced Normalized Burn Ratio (RdNBR) data from the Monitoring Trends in Burn Severity (MTBS) program. To correct known problems with MTBS fire severity mapping, we applied consistent RdNBR fire severity thresholds based upon pre-post fire plot data and LANDFIRE fire severity definitions.
Across our study region we found large deficits in observed versus expected wildfire extent. Between 1984 – 2014 Pacific Northwest forests experienced 1.5m ha burned in large wildfires (>405 ha) compared to an expected 30 year burned area of 17.1 – 23.8m ha. The largest overall departure of contemporary fire regimes from historic reference conditions was a deficit of 9.2m ha of low-severity fire in dry forests historically maintained by frequent, low-severity fire (Fire Regime Group I). Similarly, we found a deficit of 3.9m ha of low and moderate-severity fire in forests historically characterized by a mixed-severity fire regime (Fire Regime Group III). Historical high-severity fire regimes (Fire Regime Groups IV & V) had a slight deficit of high-severity (0.2m ha) and excess of low-severity fire (<0.1m ha). Surprisingly, we did not find an overall excess of high severity fire across our study region (high-severity fire deficit 0.8m ha). However, high-severity fire constituted a greater relative proportion of total burned area than expected for historic fire regimes (34% observed versus 9.5 - 13.8% expected). These results support the focus of land management agencies and public lands stakeholders on restoring low-severity fire regimes within dry forests across the Pacific Northwest. Also, these results highlight that despite increases in annual burned area over recent decades, the Pacific Northwest continues to experience far less wildfire compared to historic fire regimes. Important next steps include evaluating the spatial patterns produced by contemporary versus historic fire regimes.