ALLUVIAL FAN RECORDS OF FIRE AND GEOMORPHIC RESPONSE IN RANGELAND ECOSYSTEMS

While numerous fire histories exist for western U.S. forests, few records exist for sage (Artemisia spp) and grass-dominated rangelands, which comprise ~80% of the landscape in the western U.S. Existing fire histories for rangelands rarely extend prior to the pre-settlement period (>400 yr); hence it is difficult to tease apart effects of recent warming and local drying, invasive species, and land use on fire regimes. Estimates of pre-settlement fire return intervals vary from 11– 200+ yr; introduction of cheatgrass (Bromus tectorum) is believed to have decreased fire return intervals while conversion of sagebrush steppe to juniper woodlands may have increased fire return intervals. In forested ecosystems, severe, stand-replacing fires have been linked to large episodic sedimentation events (e.g. debris flows) on alluvial fans which contribute significantly to long-term (10³ yr) erosion rates.

Because fires in sagebrush tend to be stand-replacing, do these fires exert a similar control on erosion rates? We are developing a late Holocene record of geomorphic response to fires in the sagebrush rangelands of the Danskin Mountains, Idaho, through ¹⁴C dating of charcoal samples in fire-related deposits and stratigraphic analysis of fire-related deposits in alluvial fans. OSL dating of interfingered main-channel overbank flood deposits with alluvial fan sediments also provides chronological control and basal ages for alluvial fan deposits. Fieldwork is focused on steep (typically >30 º) rangeland areas where alluvial fans are found at the mouths of low order channels (95% of mapped fan basins are <1 km²). Fans accumulate sediment from the small contributing basins, including fire-related (charcoal rich) debris and hyperconcentrated-flow deposits. Modern vegetation distributions for the study area mapped from aerial photographs are 76% grass and shrub, 17% aspen (Populous tremuloides), 1% Douglas fir (Pseudotsuga menziesii) and 4% riparian species. Preliminary OSL ages from one fan provide an age of 7,356 ± 1,332 yr for overbank fine sands at the base of exposed interfingered fan and floodplain deposits. This deposit is overlain by 3 distinct fire related debris flows and charcoal-rich overbank flood deposits. Future ¹⁴C dates will provide further information about fire-related sedimentation events.