Post-fire geomorphic response in steep, forested landscapes: Oregon Coast Range, USA Molly Jackson, Joshua J. Roering * Department of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, USA article info Article history: Received 9 December 2007 Received in revised form 19 April 2008 Accepted 7 May 2008 Available online xxx abstract The role of fire in shaping steep, forested landscapes depends on a suite of hydrologic, biologic, and geological characteristics, including the propensity for hydrophobic soil layers to promote runoff erosion during subsequent rainfall events. In the Oregon Coast Range, several studies postulate that fire primarily modulates sediment production via root reinforcement and shallow landslide susceptibility, although few studies have documented post-fire geomorphic response. Here, we describe field observations and topographic analyses for three sites in the central Oregon Coast Range that burned in 1999, 2002, and 2003. The fires generated strongly hydrophobic soil layers that did not promote runoff erosion because the continuity of the layers was interrupted by pervasive discontinuities that facilitated rapid infiltration. At each of our sites, fire generated significant colluvial transport via dry ravel, consistent with other field- based studies in the western United States. Fire-driven dry ravel accumulation in low-order valleys of our Sulphur Creek site equated to a slope-averaged landscape lowering of 2.5 mm. Given Holocene estimates of fire frequency, these results suggest that fire may contribute 10–20% of total denudation across steep, dissected portions of the Oregon Coast Range. In addition, we documented more rapid decline of root strength at our sites than has been observed after timber harvest, suggesting that root strength was compromised prior to fire or that intense heat damaged roots in the shallow subsurface. Given that fire frequencies in the Pacific Northwest are predicted to increase with continued climate change, our findings highlight the importance of fire-induced dry ravel and post-fire debris flow activity in controlling sediment delivery to channels. Ó 2008 Published by Elsevier Ltd. 1. Introduction Historical burns in vegetated, mountainous topography high- light the profound erosive potential of fire. Disturbance via fire can alter soil, bedrock, vegetation, and hydrologic properties and induce geomorphic processes distinct from those occurring between burns. These include enhanced hydrophobicity that promotes overland flow erosion (e.g., rilling and gullying), incin- eration of vegetation that initiates dry ravel (i.e., bouncing, rolling, and sliding) of loose soil clasts on steep slopes, reduction of root reinforcement that increases the likelihood of shallow landsliding, and rapid aggradation of stream channels that may be mobilized as debris torrents in subsequent rainfall events (Swanson, 1981; McNabb and Swanson, 1990; Wondzell and King, 2003). Such fire- related geomorphic processes differ among physiographic prov- inces due to varying geology, topography, climate, vegetation, and fire regimes (Shakesby and Doerr, 2006). Recent studies outlining post-fire response in diverse land- scapes emphasize the potential dangers of a universal approach for quantifying the contribution of fire to long-term sediment production and transport (e.g., Shakesby et al., 2007). Over the last 50yr, most research on post-fire erosion has been concentrated in the interior northwest, Rocky Mountain, California, and southwest regions of the United States (DeBano et al., 1977; Cannon et al., 2001; Moody and Martin, 2001; Wohlgemuth et al., 2001; Malmon et al., 2007; Martin, 2007), although a persistent body of literature from Australia and Mediterranean regions has recently been highlighted (e.g., Shakesby and Doerr, 2006; Shakesby et al., 2007). One of the more prominent characteristics that distinguishes post- fire geomorphic response is the extent and persistence of hydro- phobic layers that result from alteration and redistribution of organic compounds in near-surface soils. The properties of fire- related hydrophobic layers, which differ in depth and continuity, dictate the efficacy of overland flow erosion that ensues during subsequent storms. Early studies in mountainous chaparral- and scrub-mantled slopes of southern California revealed strong and continuous hydrophobic layers below the topsoil that spawned extensive post-fire overland flow and debris torrent activity during rainfall events in the ensuing winter months (Munns, 1920; Rice * Corresponding author. Tel.: þ1 541 346 5574; fax: þ1 541 346 4692. E-mail address: jroering@uoregon.edu (J.J. Roering). Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev ARTICLE IN PRESS 0277-3791/$ – see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.quascirev.2008.05.003 Quaternary Science Reviews xxx (2008) 1–16 Please cite this article in press as: Jackson, M., Joshua J. Roering, Post-fire geomorphic response in steep, forested landscapes: Oregon Coast Range, USA, Quaternary Science Reviews (2008), doi:10.1016/j.quascirev.2008.05.003