Climate Drivers and Landscape Response: Holocene Fire, Vegetation, and Erosion at City Of Rocks National Reserve, Idaho

Climate exerts primary control over vegetation and fire occurrence but landscape structure, vegetation type, and density determine fire pattern, frequency and severity (i.e., fire regime), and the nature of fire-related geomorphic response. To explore these relationships, we compare alluvial charcoal records of fire and fire-related sedimentation with a woodrat midden reconstruction of vegetation at the northern migration front for single-leaf pinyon and Utah juniper at City of Rocks National Reserve (CIRO), south-central Idaho. Radiocarbon ages from 37 charcoal macrofossils sampled from discrete fire-related deposits indicate five episodes of increased fire activity over the past ~11 ka. 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). Fire activity increased as pinyon-juniper expanded (850-700 and 550-400 cal yr BP), and fire peaks during this interval correspond to decadal droughts. No fires were recorded during extended wetter conditions (~9500-7200 cal yr BP) and fires were also infrequent during an interval of dry but relatively stable climate (~6700-4700 cal yr BP), suggesting a fire regime shift from a moisture-limited system to a fuel-limited system likely occurred during the mid-Holocene. Characteristics of Holocene fire-related deposits also provide information about past fire severity and landscape characteristics. Gently sloping terrain (mean slope \u3c16°) and clay-poor colluvium at CIRO make debris flow development unlikely; rather, sediment-rich, low-volume sheetfloods from unburned basins dominate the modern response to storm events. Alluvial stratigraphic sections also record small sheetflooding events ~6500-2500 cal yr BP, which account for only 4% of measured alluvial stratigraphic thickness. This suggests a prolonged interval of minimal erosion, when drier, warmer mid-Holocene climate and low vegetation densities suppressed both severe fires and colluvial storage of sediment needed for debris flow development. However, our record indicates large fire-related debris flows were common during early and late Holocene. After ~4000 cal yr BP, higher vegetation densities (inferred from midden radiocarbon ages) re-stabilized hillslopes and increased colluvial storage, as indicated by post ~2200 cal yr BP soil horizon development. This, combined with frequent fires of expanding pinyon-juniper woodlands, likely triggered episodic post-wildfire debris flows

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