Holocene Records of Fire and Erosion Throughout a Range of Idaho Ecosystems: What can Erosional Response Tell Us About Fire Severity and Vegetation Change?

The question of how climate and climate change influence rates and processes of erosion has become more relevant in the face of ongoing and future anthropogenic warming. In particular, climate-driven increases in fire extent and severity can increase the likelihood and magnitude of post-fire debris flows and sediment-charged floods. We examine evidence of fire-related hillslope erosion and sedimentation in alluvial sediments over Holocene timescales throughout a range of ecosystems in Idaho, USA. Radiocarbondated fire-related deposits from five independent studies record forest-fires and firerelated erosion from sagebrush steppe, pinion-juniper, ponderosa pine, lodgepole pine and mixed conifer forest ecosystems indicate that sedimentation rates and processes on alluvial fans vary temporally with Holocene climate, and spatially with vegetation type. Despite variations in ecosystem type and associated fire regimes, all sites show similar broad-scale patterns. Importantly, modern and paleo variations in the type of fire-related deposit (debris flows vs. sheetfloods) appear to reflect the type and extent of vegetation on hillslopes, as well as fire severity.

Over millennial timescales, warmer and drier conditions are not always associated with increased fire activity. For example, limited fire-related deposition occurred at all sites during the generally warm, dry, and stable climate conditions of the mid-Holocene. At the City of Rocks, south-central Idaho, small sheetflooding events ~6500-2500 cal yr BP account for only 4% of all measured alluvial stratigraphic thickness over the past ~12 ka. This relatively fire-free and stable interval was preceded by fire peaks ~7.5-6 ka throughout a range of ecosystems, including the City of Rocks, the Middle Fork Salmon River and the South Fork Payette River. Interestingly, the geomorphic response during this ~7.5-6 ka fire interval is dominated by sheetflooding events, not debris flows. In contrast, early and late Holocene fire-related deposits are dominated by debris flows. Evidence from lake pollen records, plant macrofossils in alluvial deposits and vegetation reconstructions from woodrat middens indicates that expansion of pine species (lodgepole, ponderosa, and pinyon) during the late Holocene (~3 ka-present) corresponds with an increase in fire activity across a range of ecosystems. This vegetation change may be attributed to natural post-glacial migrations and/or generally cooler late Holocene climate conditions. Regardless of cause, likely increases in forest density, combined with variable climate conditions, fueled severe fires across a range of ecosystems. These late Holocene fires are associated with thick fire-related debris flow deposits. Medieval droughts correspond with major fire and debris flow peaks ~1000-800 cal yr BP; decadal to annual droughts during the generally cooler and wetter LIA also promote fire peaks ~500-300 cal yr BP.

In the Middle Fork Salmon River, fire-related debris flows in upper basin tributaries provide a significant proportion of the sediment to the larger Salmon River basin. Using a combination of modern sediment yields and reconstructed sediment yields from Holocene alluvial deposits, fire-related debris flows have supplied at least 83-262 Mgkm -2yr-1 of sediment to the main-stem river over the past 2 ka, and a minimum of ~30-101 Mg km-2yr-1 of sediment over the past ~6ka. This compares with an overall sediment yield (estimated from streambed cosmogenic samples) of ~260 +/-30 Mg km-2yr-1, suggesting that especially over the past ~2 ka, fire-related debris flows in upper basin tributaries have contributed a large proportion of the total sediment load. Modern observations of hillslope erosion indicate north-facing or moister slopes are characterized by dense vegetation, steeper slope angles, thicker soils, and episodic but large debris flows, while south-facing or drier slopes have sparse vegetation, thin soils, lower slope angles, and frequent small sheetfloods. Space for time substitutions indicate similar shifts in erosional response over Holocene timescales with climate-driven changes in vegetation type and density. We hypothesize that increased climate variability, where wet intervals of vegetation (fuel) production are followed by severe drought and fire, promote large debris flows. Dry (but stable?) climates or sparsely vegetated hillslopes are characterized more frequent sheetflooding and less severe fires. The question of whether cool/wet intervals or warm/dry intervals drive hillslope response is, therefore, very dependent on the timescale of observation. It is likely, however, that increased climate variability may drive erosional response over multiple timescales.