Fire-driven landscape disequilibrium at the hillslope and range scale

The Oregon Coast Range often serves as a "poster" landscape for steady state because it exhibits consistent rates of rock uplift and denudation, has not been glaciated, and features a relatively uniform lithology. Widespread aggradation along Oregon Coast Range rivers during the Pleistocene-Holocene transition, however, corresponds with a period of increased fire frequency, likely reflecting accelerated sediment production driven by climatic variations. Due to the ubiquity of steep slopes in the region, post-fire rates of denudation are high via dry ravel and accelerated shallow landsliding following root strength decay. Given the observed four-fold variation in fire frequency over the last 10,000 years, is it possible that the appearance of the Oregon Coast Range has varied correspondingly? Specifically, how has slope morphology and the distribution of soil depth responded to changing ecological boundary conditions? Here, we use a process-based hillslope evolution model to explore how variations in fire frequency affect soil depth and slope morphology. Field-based evidence suggests that post-fire sediment flux rates exceed temporally-averaged rates by an order of magnitude and vary nonlinearly with gradient. Because transport on hillslopes depends on the availability of soil, we scale flux rates according to available soil depth such that flux approaches zero as bedrock emerges at the surface. Coupling our transport model with a calibrated soil production function (Heimsath et al., 2001), we use high-resolution topographic data obtained with airborne laser altimetry to simulate hillslope response to variations in fire frequency. Consistent with recent field observations, the model predicts that specific regions in the landscape are likely to experience soil stripping and bedrock emergence following fire. Surprisingly, hilltops are sensitive to fire-driven increases in flux rate and may undergo soil removal while sideslopes remain soil-mantled. In such a scenario, bedrock erosion processes, such as exfoliation and rockfall, may play a significant role in erosion and the modulation of local relief.