Toward a better understanding of climate and human impacts on late Holocene fire regimes in the Pacific Northwest, USA

Abstract

In order to fully appreciate the role that fire, both natural and anthropogenic, had in shaping pre-Euro-American settlement landscapes in the Pacific Northwest (PNW), it is necessary to develop a more robust method of evaluating paleofire reconstructions. Here we demonstrate an approach that includes the identification of charcoal morphotypes (i.e. visually distinct charcoal particles), and incorporates both paleoecological and archaeological data sets, to more specifically determine both the nature of past fire regimes (i.e. fuel type and fire severity) and the likely ignition source of those fires. We demonstrate the usefulness of this approach by reconstructing the late Holocene fire and vegetation histories of Lake Oswego (Clackamas County), Oregon, and Fish Lake (Okanogan County), Washington, using macroscopic charcoal and pollen analysis of sediment cores. The histories were compared with climatic records from the PNW as well as archaeological, ethnographic, and historical records from the Lower Columbia River Valley and Southern Columbia Plateau cultural regions. Our results indicate that while centennial-to-millennial-scale climate change had limited influence on the fire regimes at the study sites during the past ∼3800 years, the use of fire by Native Americans for a variety of reasons, particularly after ca. 1200 calendar years before present (AD 750), had a far greater impact. Charcoal morphotype ratios also indicate that fires in the two watersheds were fundamentally different in their severity and impact, and led to major shifts in the forests and woodlands surrounding Lake Oswego, but helped maintain the ponderosa pine-dominated forest at Fish Lake. The elimination of fire from the two study sites during the past 100–300 years is likely the combined result of Euro-American contact and the arrival of disease in the PNW, as well as 20th-century fire suppression and grazing effects on fuel continuity, which has implications for future forest management and restoration efforts in the PNW.

Fire, leaf harvest, and abiotic factors drive demography of an ecologically and culturally important understory plant

“…, Walsh et al. , ). This past fire suppression and exclusion can impact current fire severity and size, particularly in drier ecosystems with shorter fire return intervals (Hantson et al.…”

Shifting Perspectives in Assessing Socio-Environmental Vulnerability

“…Frameworks in North America have broadened to consider the influence of humans in the evolution of natural ecosystems. For example, the use of fire by indigenous peoples, which has occurred for thousands of years in North America, is increasingly regarded as part of the natural disturbance regime under which many ecosystems in the Western United States have evolved [73,74]. Accordingly, particular ecological communities, including groves of large California black oaks (Quercus kelloggii) and Oregon white oaks (Quercus garryana), camas grounds (Camassia spp.…”

The Charcoal Quantification Tool (CharTool): A Suite of Open-source Tools for Quantifying Charcoal Fragments and Sediment Properties in Archaeological and Paleoecological Analysis

“…Finally, CharTool incorporates an optional charcoal fragment morphotype classification component for each charcoal fragment that is selected by the user. Several studies have suggested that combustion conditions and fuels consumed during a fire determine the morphology and structure of charred fragments (Courtney Mustaphi and Pisaric 2014; Enache and Cumming 2006;Jensen et al 2007;Walsh et al 2018). Thus, charcoal fragment morphology is increasing used as a metric for estimating fuel sources and intensities of landscape fires.…”