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Accueil du site → Master → Etats Unis → 2012 → Climate Drivers and Landscape Response : Holocene Fire, Vegetation, and Erosion at City Of Rocks National Reserve, Idaho

Boise State University (2012)

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

Weppner Kerrie N.

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

Auteur : Weppner Kerrie N.

Université de soutenance : Boise State University

Grade : Master of Science in Hydrologic Sciences 2012

Résumé _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 <16°) 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.

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Page publiée le 9 novembre 2012, mise à jour le 10 novembre 2018