Post-fire mulching for runoff and erosion mitigation: Part I: Effectiveness at reducing hillslope erosion rates

Mulch treatments often are used to mitigate post-fire increases in runoff and erosion rates but the comparative effectiveness of various mulches is not well established. The ability of mulch treatments to reduce sediment yields from natural rainfall and resulting overland flow was measured using hillslope plots on areas burned at high severity following four wildfires in the western United States. Wheat straw mulch, wood strand mulch, and hydromulch were evaluated along with untreated control plots on multiple fires for 4 to 7 years after burning. Needle cast from fire-killed conifer trees was evaluated in an area of moderate burn severity at one fire, and seeding with genetically native seed was tested, with and without hydromulch, at another fire. Rainfall, ground cover, and soil water repellency were measured in each treatment site at all 4 fires. Mean sediment yields on the control plots ranged from 0.3 to 7.5 Mg ha− 1 in the first post-fire year, from 0.03 to 0.6 Mg ha− 1 in the second, and from 0 to 0.4 Mg ha− 1 in the third and fourth post-fire years. Assuming a linear fit between sediment yield and rainfall intensity, storms with equivalent rainfall intensities produced nearly an order of magnitude less sediment on the control plots in the second post-fire year as compared to the first post-fire year. Large storms (at least a 2-year return period, 10-min maximum rainfall intensity) produced sediment on all fires in all years where they occurred; however, sediment yields produced by large storms that occurred in the first post-fire year were larger than the sediment yields from equivalent storms that occurred in later years at the same fire.Sediment yields decreased as ground cover increased and all the mulch treatments increased total ground cover to more than 60% immediately after application. However, the longevity of the mulches varied, so that the contribution of the treatment mulch to total ground cover varied by mulch type over time. The wood strand mulch was the most long-lived of the mulch treatments and was observed in ground cover assessments throughout the study period (4 and 7 years) at two fires. The wheat straw mulch decreased nearly twice as fast as the wood strand mulch, and no hydromulch was detected after the first post-fire year on either fire where it was tested.Mulch treatment effectiveness varied when data were analyzed separately for each fire. Wood strand mulch reduced sediment yields at both fires where it was tested, wheat straw mulch reduced sediment yields at 2 of the 4 fires where it was applied, and the hydromulch tested at 2 fires did not reduce sediment yields on either. When data were normalized and analyzed by treatment across all fires, wood strand mulch reduced sediment yields for the first four post-fire years, but wheat straw mulch and hydromulch did not significantly reduce sediment yields in any post-fire year. The greater variability in the combined data resulted in fewer statistically significant treatment effects being observed as compared to the individual fire analyses. We believe the fire-specific results provide the more accurate representation of potential post-fire mulch treatment effectiveness.

► Rainfall and site variability resulted in widely divergent post-fire erosion rates.
► Sediment yields were related to rain intensity, time since fire, and ground cover.
► Wood strand mulch reduced sediment yields for up to four post-fire years.
► Straw mulch reduced first post-fire year sediment yields at two of four fires.
► Hydromulch did not reduce post-fire sediment yields in any years.