Two Post-Glacial Sagebrush Steppe Fire Records at the Wildland-Urban Interface, Eastern Cascades, Washington
Recent increases in large fires in the rapidly developing wildland-urban interface (WUI) areas of central Washington, where development intermixes with wildland fuels, contribute to federal firefighting costs exceeding of $1 billion annually. In addition, cheatgrass (Bromus tectorum) invasion and anthropogenic-caused warming shorten fire return intervals while lengthening fire seasons. These climatic, ecological, economic, and social factors combine with fuel accumulation resulting from historic fire suppression to threaten lives and property in the WUI. To plan for safe growth in WUI areas, long-term fire histories are needed to expand understanding of past fire regimes in an understudied ecosystem, sagebrush steppe. In this thesis, two ca. 15,000-year-old fire histories were developed from study sites in Okanogan County, Washington. Lake sediment cores were analyzed using macroscopic charcoal analysis and CharAnalysis software. Relationships between past fire, vegetation, human activities, and climatic drivers, such as the Early Holocene Warm Period, the Medieval Climate Anomaly, the Little Ice Age, and El-Niño Southern Oscillation, were considered. Results suggest that although climate influences eastern Cascades sagebrush steppe environments over millennial timescales, fuel availability represents a limiting factor over shorter intervals, with wet years allowing sagebrush and other fine fuels to proliferate, and fires occurring in subsequent dry intervals. The performance of CharAnalysis in sagebrush environments suggests that constant charcoal influx from frequent, low-severity fire events renders individual fire difficult to resolve from macroscopic charcoal records. The record analyzed here can inform decision makers in setting policies consistent with the long-term fire regime of the Methow Valley.