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A fire history reconstruction of the western San Juan Mountains using alluvial sediment and tree-ring methods. Of . Erica Bigio LTRR and Geosciences University of Arizona. L arge and severe wildfires of the past 10 – 20 years:.

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  1. A fire history reconstruction of the western San Juan Mountains using alluvial sediment and tree-ring methods Of Erica Bigio LTRR and Geosciences University of Arizona

  2. Large and severe wildfires of the past 10 – 20 years: • Increasing trend in area burned across the western US over past 40 years • linked to earlier snowmelt, higher spring/summer temps • large areas of high-severity fire within burned areas Westerling et al., 2006

  3. Photo: Craig Allen Post-Fire Geomorphic Responses In mountainous areas, increased erosion on hillslopes and channel erosion in valleys Depending on the extent of high-severity burned area, runoff may lead to flooding and debris flow events at the watershed scale. Photo: John Moody

  4. In the Southwest, high-severity burned areas are larger when compared with historical conditions in ponderosa pine and mixed-conifer forests: Today 1500 – 1900 AD dense forest today Pre- 19th century open forest

  5. Fire history methods: Tree rings: Tree-Rings Alluvial fans: Lake sediments: 5,000 years 500 years 10,000 years

  6. Approach: Combine tree-ring and alluvial sediment methods Sediment exposure Fire-scarred tree Age-structure plot

  7. Study Area: Missionary Ridge Fire (2002); Durango, Colorado • Flooding and debris flows occurred throughout the burned area • created natural exposures of older sediment deposits in tributary stream • channels and alluvial fans Cannon and others, 2003

  8. Alluvial sediment accumulates in the valley bottoms and fans

  9. Central research question: • How have fire regimes have variedin ponderosa pine and mixed-conifer forests of the western San Juan Mountains over the past ~ 3,000 years, and how do fire regimes relate to climate variability over this time period? • Alluvial records may show broader range of fire regime changes than observed in the past 500 years • Specifically, the Medieval Climate Anomaly (AD 900 – 1300) • Evidence of crown fire activity during this time at alluvial fan fire history sites in the Northern Rockies and southwestern NM.

  10. N Sediment exposure Fire-scarred tree Age-structure plot Tree-ring and alluvial sediment methods combined within four study sites Additional sediment samples taken where available One study site published previously (Bigio et al., 2010)

  11. Locations sampled: Combined with tree-ring methods

  12. Upper Haflin Canyon

  13. Debris flow unit Burned soil surface

  14. Charcoal deposited with fine-grained sediments within clast-supported deposits

  15. Sediment characteristics (end members): Debris flow deposits are evidence of high-severity fires Fine-grained fluvial deposits indicates low-energy deposition and are used to infer low-severity fires

  16. Summary of Sediment Samples:

  17. All 14C dates from multiple locations within watersheds and fan surfaces • Increased fire-related sedimentation events from AD 900 – 1280 •  Medieval Climate Anomaly (AD 900 – 1300) • Increased fire-related sedimentation events from AD 1470 – 1650

  18. Medieval Warm Period Events (AD 900 – 1280):

  19. Locations of sedimentation events during the Medieval Period:

  20. Events from ~1400 - present:

  21. Locations of events from ~1400 - present:

  22. Example of recent charcoal deposit 263 +/-44

  23. Example of fine-grained sediment with abundant charcoal: 263 +/- 44 1485 – 1954 AD

  24. Fire-scarred trees Age-structure plots

  25. Haflin Canyon Fire Scar Chronology Widespread fire years defined by 25% of recording trees scarred by an event Most fire-scarred trees located on south-facing slopes

  26. Haflin Canyon North and South Facing Plot #3 North-facing plot South-facing plot

  27. Establishment Ages of all plots and fire scar data combined

  28. Conclusions: Increased fire activity during the Medieval Warm Period: Sediment characteristics indicate debris flow events (high- severity fire) and fine-grained deposition (low-severity fire) Multiple locations within a one watershed (Steven’s Creek), suggests a widespread fire event, including high-severity fire.  not observed throughout the study area High-severity burned area during the MWP may not have exceeded the extent of high-severity burned area observed during the Missionary Ridge Fire Localized fire activity observed in sediment record from 1470 – 1650 AD: Not as widespread as events observed in the MWP at the watershed scale  though synchronous throughout the study area

  29. Thanks to a great field and lab crew! Support from: • JFSP GRIN award PEO Scholar award LTRR/College of Science Awards • GSA graduate student award • NSF IGERT in Archaeological Sciences

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