Publication Date

5-2009

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Major Advisor

D. Jennifer Pierce

Abstract

Because rangelands are characterized by more arid climates than forested regions, traditional methods of reconstructing fire histories (e.g. fire-scars from trees or charcoal preserved in lake sediments) are limited. Charcoal preserved in alluvial fans and stream terraces provide a means of reconstructing a record of fires and geomorphic responses in rangeland ecosystems.

This study used dated charcoal from fire-related deposits and deposit characteristics to determine a record of fire and geomorphic response. Twenty-five samples collected from terraces and alluvial fans within the Wood Creek study area indicate three major periods of fire-related activity. From 4,400 – 4,000 cal yr BP, firerelated sediment was preserved in floodplain deposits; charcoal from this period indicates that the fires burned hardwoods and conifers. From 2,000 – 1,400 cal yr BP, fires resulted in debris flows and sheetflooding events and burned primarily conifers. Between 650 and 300 cal yr BP, fires resulted initially in debris flows and later in floodplain deposition; identified charcoal indicates that fires burned primarily sagebrush and conifers.

Between two and three meters of incision has occurred during the last ~500 years, based on the position of fire-related floodplain deposits, and ~2.5 meters of incision has occurred since the deposition of a sandy flood deposit, dated using optically stimulated luminescence, approximately 90 years ago. Corresponding incision rates over these periods range from ~10 – 25 m/ka.

The study area may have been dominated by conifer forest prior to the Medieval Climatic Anomaly. Prior to ~1,300 cal yr BP, fire-related activity occurred at times similar to activity in the forested ecosystems of the Payette and Yellowstone National Park and sagebrush is rare. After ~600 cal yr BP, activity is similar to the record of inferred surface-fires in the Payette and sagebrush is common in charcoal samples. Inferred shifts in the forest-rangeland ecotone are consistent with those reported elsewhere in the Great Basin.

Sample ages were compared to a reconstruction of Palmer Drought Severity Index (PDSI) to investigate relationships between fire and climate. While fires in forested ecosystems are predominantly linked to droughts, ten of the fourteen most recent samples from Wood Creek indicate that fires occurred during times that were wetter than the record average PDSI. Eight of the fourteen indicate that conditions were likely wetter than indicated by an un-weighted average PDSI.

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