In semiarid ecosystems, past fire regimes are poorly understood due to limited fire scars and few lakes containing charcoal sediments. Past fire and fire-related sedimentation can be reconstructed using charcoal preserved in alluvial and fluvial sediments, which we compare with woodrat midden reconstructions of vegetation at the migration front for single-leaf pinyon and Utah juniper at City of Rocks National Reserve (CIRO), Idaho. Radiocarbon ages from 37 charcoal samples indicate five episodes of increased fire activity. Frequent fires burned following deglaciation (10,700-9500 cal yr BP), and later during prolonged drought (7200-6700 cal yr BP). A moderate fire interval (2400-2000 cal yr BP) followed arrivals of Utah juniper (~3800 cal yr BP) and single-leaf pinyon (~2800 cal yr BP). Frequent fires burned as pinyon-juniper expanded (850-700 and 500-400 cal yr BP) and correspond to decadal droughts. No fires were recorded during extended wetter climate (9500-7200 cal yr BP) and fires were infrequent during dry but relatively stable climate (6700-4700 cal yr BP). Fire-related erosion also changed during the Holocene at CIRO. Debris flows were common during early and late Holocene, but infrequent ~6500-2500 cal yr BP, when only ~4% of measured alluvium deposited. This suggests prolonged minimal erosion, when drier, warmer mid-Holocene climate and low vegetation densities suppressed fire and colluvial storage for debris flow development. After ~4000 cal yr BP, higher vegetation densities (inferred from midden radiocarbon ages) re-stabilized hillslopes and increased colluvial storage. This, combined with frequent fires of expanding pinyon-juniper woodlands, likely triggered episodic post-wildfire debris flows.
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