Holocene Fire Occurrence and Alluvial Responses at the Leading Edge of Pinyon–Juniper Migration in the Northern Great Basin, USA

Fire and vegetation records at the City of Rocks National Reserve (CIRO), south-central Idaho, display the interaction of changing climate, fire and vegetation along the migrating front of single-leaf pinyon (. Pinus monophylla) and Utah juniper (. Juniperus osteosperma). Radiocarbon dating of alluvial charcoal reconstructed local fire occurrence and geomorphic response, and fossil woodrat (. Neotoma) middens revealed pinyon and juniper arrivals. Fire peaks occurred ~. 10,700-9500, 7200-6700, 2400-2000, 850-700, and 550-400. cal. yr. BP, whereas ~. 9500-7200, 6700-4700 and ~. 1500-1000. cal. yr. BP are fire-free. Wetter climates and denser vegetation fueled episodic fires and debris flows during the early and late Holocene, whereas drier climates and reduced vegetation caused frequent sheetflooding during the mid-Holocene. Increased fires during the wetter and more variable late Holocene suggest variable climate and adequate fuels augment fires at CIRO. Utah juniper and single-leaf pinyon colonized CIRO by 3800 and 2800. cal. yr. BP, respectively, though pinyon did not expand broadly until ~. 700. cal. yr. BP. Increased fire-related deposition coincided with regional droughts and pinyon infilling ~. 850-700 and 550-400. cal. yr. BP. Early and late Holocene vegetation change probably played a major role in accelerated fire activity, which may be sustained into the future due to pinyon-juniper densification and cheatgrass invasion.