FIRE, EROSION, AND RESEEDING: RESULTS FROM THE 1988 "49’ER FIRE" OF GRASS VALLEY, CA

Aided by high winds and drought, the 1988 "49'er fire" in the Sierra foothills, northwest of Grass Valley, California, burned Ponderosa pine and oak trees including tree roots, exposing mineral soil and depositing white ash. To assess subsequent surface erosion on burned hillslopes devoid of vegetation and the effectiveness of a regional aerial reseeding campaign, a 3,890 m² hollow with an average channel slope of 25º was observed for 13 years. One hollow flank was seeded with predominantly Blando brome (Bromus hordeaceus) while the other was not seeded for control. Three sediment traps were installed to estimate erosion rates during the following winter. Seventy 15-cm long spikes, installed in a 7.5-m grid spacing, provide surface lowering estimates over a 13-year period. Denudation rates during the first winter following the fire were distinct with the unseeded, burned slope producing a minimum of 136 t km^-2 yr^-1 resulting in a surface-lowering rate of 0.12 mm yr^-1. The seeded hillslope, in contrast, produced a modest 62 t km^-2 yr^-1 and a lowering rate of 0.05 mm yr^-1 corresponding to 55% less sediment production. Using a metal detector to locate the 15-cm long spikes in 2002, measurements of post-fire surface erosion for the entire hollow spanning 13 years reveal a minimum, hollow-average surface-lowering rate of 0.39 mm yr^-1 and a denudation rate of 441 t km^-2 yr^-1. Surface erosion generally increased downslope with spikes absent from the lower portion of the hillslope. Local areas of aggradation cluster near the top of the hollow. Where the spikes are present, charcoal fragments were found at depths of 0 to 4 cm within the soil. The soil has a soft rupture resistance when dry and a penetration resistance of 0.02 MPa. Downslope where the spikes are absent, the soil retains no fire record, has a moderately hard rupture resistance when dry, and a penetration resistance of 0.3 MPa. Although lowering rates through the first winter after the fire differed markedly between the seeded and native portions of the hollow, the effectiveness of seeding was short-lived with no long-term statistical difference. In addition, the decade-scale, post-fire denudation rates exceed catchment-scale rates representative of thousands of years inferred from cosmogenic nuclide concentrations by Riebe et al. (2001) for the Sierra Nevada.