A SYNTHESIS OF ALLUVIAL RECORDS OF WILDFIRE IN THE WESTERN USA: A COMPLEX INTERPLAY OF CLIMATE AND VEGETATION

We examine records of Holocene fire and erosional response in alluvial fan sediments of the interior western US, including 10 study areas and >480 ¹⁴C dates. Single charcoal fragments date individual fire events, and show minima and maxima in fire-related sedimentation from moderate- to high-severity fires. Alluvial deposit characteristics, e.g. thickness, and debris flows vs. sheetfloods provide a measure of postfire geomorphic response and an indication of fire severity. Chronologies are from high elevation mixed conifer forests in the Northern Rockies, ponderosa and Douglas-fir forests in the Northern Rockies and SW, and low elevation sagebrush steppe and piñon-juniper woodlands near the Snake River Plain. They yield the following results: 1) Climate variability drives ponderosa pine and Douglas-fir forests in both the SW and Northern Rockies 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 environment; sheetfloods are more common in sparsely vegetated sites and in drier Holocene periods with open forests, whereas dense forests and infrequent severe fires often produce debris flows. 3) The late Holocene arrivals of ponderosa, lodgepole and piñon pine at Northern Rockies sites correspond with increased fire severity, linking vegetation and fire regime changes. 4) Fires in dry sage steppe are generally fuel-limited, but burn during prolonged wet and variable climates; grazing, post-Euroamerican land-use, and invasive species, particularly influence modern fires. 5) At moist high-elevation lodgepole and mixed conifer sites in Yellowstone and central Idaho, episodic large debris flows indicate high severity burns, often during severe multidecadal droughts. 6) Regionally coherent peaks exist ca. 200, 500, 900, 1700 and 2600 cal yr BP, but fire activity is not generally synchronous among sites. Differences in climate among sites likely account for some asynchroneity. 7) Recent severe fires have burned in 8 of 10 sites described; erosional response appears particularly anomalous in the SW, where impacts of fire suppression and land use are greatest. Widespread and severe modern fires may herald the arrival of a no-analog era of fire in the western US.