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A fire history reconstruction of the western San Juan Mountains using alluvial sediment and tree-ring methods

Erica BigioLTRR and GeosciencesUniversity of Arizona

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 areasWesterling et al., 2006

Photo: John Moody Photo: Craig Allen

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.

Post-Fire Geomorphic Responses

In the Southwest, high-severity burned areas are larger when compared with historical conditions in ponderosa pine and mixed-conifer forests:

1500 – 1900 AD Today

Pre- 19th century open forest dense forest today

10,000 years

Fire history methods:

5,000 years 500 years

Alluvial fans:Lake sediments: Tree-Rings

Sediment exposure Fire-scarred tree Age-structure plot

Approach: Combine tree-ring and alluvial sediment methods

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

Alluvial sediment accumulates in the valley bottoms and fans

Central research question:

1) How have fire regimes have varied in 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.

Sediment exposure Fire-scarred tree Age-structure plot

Tree-ring and alluvial sediment methods combined within four study sitesAdditional sediment samples taken where availableOne study site published previously (Bigio et al., 2010)

N

Locations sampled:

Watershed name Elevation (m) Burn Severity(MR fire)

Forest Type

Steven’s Creek 2200 – 2900 ~ 80% severe Ponderosa Pine (S)Mixed Conifer (N)

Haflin Canyon 2300 – 2900 100% severe Ponderosa Pine (S)Mixed Conifer (N)

Marina (Vallecito)

2400 – 2800 100% severe Mixed Conifer

Country Market(Vallecito) (prior work)

2400 – 2800 ~ 80% severe Mixed Conifer

Freed Canyon 2070 ~ 80% severe Ponderosa Pine (S)Mixed Conifer (N)

Woodard Canyon 2060 ~ 80% severe Ponderosa Pine (S)Mixed Conifer (N)

Shearer Creek 2400 ~ 60% severe Mixed Conifer

Combined with tree-ring methods

Upper Haflin Canyon

Debris flow unit

Burned soil surface

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

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

Sediment characteristics (end members):

Watershed Name Number of Exposures Number of Units 14C dates

Steven’s Creek 9 exposures 36 units 19 dates

Haflin Canyon 7 exposures 24 units 17 dates

Marina Drainage 2 exposures 9 units 10 dates

Country Market Drainage (prior work)

3 exposures 28 units 60 dates

Freed Canyon 3 exposures 12 units 3 dates

Woodard Canyon 2 exposures 6 units 3 dates

Shearer Creek 1 exposure 1 unit 1 date

Summary of Sediment Samples:

Increased fire-related sedimentation events from AD 900 – 1280 Medieval Climate Anomaly (AD 900 – 1300)

Increased fire-related sedimentation events from AD 1470 – 1650

All 14C dates from multiple locations within watersheds and fan surfaces

Medieval Warm Period Events (AD 900 – 1280):

Locations of sedimentation events during the Medieval Period:

Events from ~1400 - present:

Locations of events from ~1400 - present:

263 +/-44

Example of recent charcoal deposit

263 +/- 441485 – 1954 AD

Example of fine-grained sediment with abundant charcoal:

Fire-scarred trees

Age-structure plots

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

North-facing plot

South-facing plot

Haflin Canyon North and South Facing Plot #3

Establishment Ages of all plots and fire scar data combined

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

Support from:

JFSP GRIN awardPEO Scholar awardLTRR/College of Science AwardsGSA graduate student awardNSF IGERT in Archaeological Sciences

Thanks to a great field and lab crew!