Vegetative and climatic controls on Holocene wildfire and erosion recorded in alluvial fans of the Middle Fork Salmon River, Idaho

摘要

The Middle Fork Salmon River watershed spans high-elevation mixed-conifer forests to lower-elevation shrub-steppe. In recent decades, runoff from severely burned hillslopes has generated large debris flows in steep tributary drainages. These flows incised alluvial fans along the mainstem river, where charcoal-rich debris-flow and sheetflood deposits preserve a record of latest Pleistocene to Holocene fires and geomorphic response. Through deposit sedimentology and C-14 dating of charcoal, we evaluate the processes and timing of fire-related sedimentation and the role of climate and vegetation change. Fire-related deposits compose similar to 66% of the total measured fan deposit thickness in more densely forested upper basins versus similar to 33% in shrub-steppe-dominated lower basins. Fires during the middle Holocene (similar to 8000-5000 cal. yr BP) mostly resulted in sheetflood deposition, similar to modern events in lower basins. Decreased vegetation density during this generally warmer and drier period likely resulted in lower-severity fires and more frequent but smaller fire-related sedimentation events. In contrast, thick fire-related debris-flow deposits of latest Pleistocene-early Holocene (similar to 13,500-8000 cal. yr BP) and late Holocene (<4000 cal. yr BP) age are inferred to represent higher-severity fires, although data in the former period are limited. Widespread fires occurred in both upper and lower basins within the Medieval Climatic Anomaly (1050-650 cal. yr BP) and the early Little Ice Age' ca. 550 cal. yr BP. We conclude that a generally cooler late Holocene climate and a shift to denser lodgepole pine forests in upper basins by similar to 2500 cal. yr BP provided fuel for severe fires during episodic droughts.