How Long-Term Records of Fire Can Inform Fire Ecology and Management
We examine records of Holocene fires and erosional response recorded in alluvial fan sediments from Idaho and the Northern Rockies, and the southwestern US. This study uses a compilation of nine individual studies, >480 radiocarbon-dated fragments of charcoal, and thousands of individual deposits extending back to ~14 ka. Radiocarbon ages from single charcoal fragments provide evidence of individual fire events across several study sites; and when combined, these records show minima and maxima in fire activity throughout the Holocene for the western U.S. Our record is of fire-related sedimentation primarily from moderate- to high-severity fires; in the SW especially, recurrence intervals for fire related events (often 100s of years) are much longer than mean fire intervals 10s of years from tree rings. Characteristics of the alluvial deposits,e.g. deposit thickness, and debris flows vs. sheetfloods provide a measure of the geomorphic response to fire and an indication of fire severity and landscape response. Fire chronologies are from three general ecosystems: high elevation mixed conifer forests in the Northern Rockies, montane ponderosa and Douglas-fir forests in the Northern Rockies and Southwest, and low elevation sagebrush steppe and open piñon-juniper woodlands near the Snake River Plain. The combined studies yield the following results. 1) Climate variability drives ponderosa pine and Douglas-fir forests in both the Southwest and Northern Rocky Mountains to burn ‘at both ends of the spectrum’, where frequent low-severity fires are typical, but higher-severity fires burn during severe droughts following fuel buildup over wet decades. 2) Deposit types vary with ecosystem type; sheetfloods are more common in sparsely vegetated sites and over drier Holocene periods that produce open forests, whereas denser forests and/or infrequent severe fires often produce debris flows. 3) The late Holocene arrival of ponderosa, lodgepole and piñon pine at sites in the Northern Rockies temporally corresponds with an increase in fire activity, suggesting a link between vegetation and fire regime changes. 4) Fires in dry sage steppe ecosystems are generally fuel-limited, but burn during times of multi-decadal to centennial wet and variable climates; grazing and other post-Euroamerican land-use changes, as well as invasive species, particularly influence modern fire regimes at these sites. 5) At moist high-elevation lodgepole pine and mixed conifer sites in Yellowstone and central Idaho, episodic large fire-related debris flows suggest high severity burns, often during times of severe multidecadal drought. 6) While records reveal regionally coherent peaks (e.g. ~200, 500, 900, 1700 and 2600 cal yr BP), historic fire activity is not generally synchronous among sites. Regional differences in climate between the xeric northwestern, northern Rocky Mountain, and monsoonal southwestern sites may account for some of these asynchronies. 7) Recent, severe fires (~1985-2015) have burned in 8 of the 10 sites described; erosional response appears particularly anomalous in the Southwest, where the impacts of Euroamerican fire suppression and land use have been greatest. This begs the question of whether or not widespread and severe modern fires herald the arrival of a new, no-analog era of fire in the western US.