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

Abstract

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.

Pre-contact Indigenous fire stewardship: a research framework and application to a Pacific Northwest temperate rainforest

“…Our fire history reconstruction also shows that fires were relatively small compared to most contemporary fire perimeters with few fires recorded at more than one of the systematically located sites separated by ∼4 km (Johnston et al, 2023). Perhaps most remarkable, despite the fact that we've documented fires at lower elevations (<1,000 m) in nearly every calendar year from 1693 through 1895 (with multi-year gaps noted after the 1850s), there is little evidence that these fires resulted in large or severe events.…”

“…Fire-return intervals lengthened with distance from areas of oak and pine, but do not begin to match the region-wide expectations of long fire-return intervals except at the highest elevation sites most distant from archaeological sites (Johnston et al, 2023). The least frequent fire sites (longest fire return intervals) were in closed canopy forest stands at the highest elevations with the lowest density of archaeological sites.…”

“…There are few cross-dated tree ring reconstructions of fire dynamics for PNW rainforests, although recent efforts in the central western Cascades have sought to fill this gap (Tepley et al, 2014;Merschel, 2021;Johnston et al, 2023). We created annually resolved fire histories at 16 sites systematically located approximately 4 km apart to form a 12 km by 12 km grid that provides an objective sample of variability in historical fire across the range of environmental conditions present across our study area.…”

“…These fire histories extend back in time from between 1350 CE to 1700 CE depending on the site. More than 99% of reconstructed fires occurred prior to 1890 CE, after which management by the U.S. Forest Service emphasized "protection" of forests against fire (Graves, 1910;Johnston et al, 2023).…”

“…We then propose a transdisciplinary research framework for overcoming previously intractable research questions about Indigenous fire stewardship in highly productive montane forest settings. We further describe our application of this framework to the Middle Fork study area on national forest land in the western Oregon Cascades where recent dendroecological research (Johnston et al, 2023) reveals far more frequent fire than predicted by theory. Although our framework has been tailored to the PNW and, further, to the empirical context of our study area, we suggest that the overarching approach is broadly generalizable and the methods for operationalizing these hypotheses may have applicability elsewhere around the globe.…”