Using Aerial Hydromulch in Post-fire Chaparral in Southern California: Effectiveness and Consequences

High severity wildfire can make landscapes susceptible to accelerated erosion that may retard resource recovery. High levels of erosion can also threaten life, property, and infrastructure in downstream human communities. Land managers often use mitigation measures on the burned hillside slopes to control post-fire sediment fluxes both as the first step in post-fire restoration and to protect off-site human developments. Aerial hydromulch, a slurry of paper or wood fiber with tackifiers and other amendments that dries to a permeable crust, is a relatively new erosion control treatment that has not been rigorously field-tested in wildland settings. Concerns have been raised over the ability of aerial hydromulch to reduce hillslope erosion as well as its potential for negative effects on post-fire ecosystem recovery. Since 2007 we have measured sediment fluxes and vegetation development on plots treated operationally with aerial hydromulch and compared them to untreated controls after three separate wildfires in southern California. These study plots, located on steep slopes with coarse upland soils previously covered with dense mixed chaparral vegetation, were monitored with silt fences to trap eroded sediment. Meter-square quadrats were used to measure ground and vegetation cover. Although dependent on rainfall and site characteristics, surface erosion on untreated plots generally attenuated sharply with years since burning. We found that aerial hydromulch did reduce bare ground on the treated plots and that this cover persisted through the first post-fire winter rainy season. For the initial year after a fire, aerial hydromulch reduced hillslope erosion from small and medium rainstorms, but not during an extremely high intensity rainfall event. Hydromulch had no effect on regrowing plant cover, shrub seedling density, or species richness. Thus, in chaparral ecosystems aerial hydromulch appears to be an effective post-fire erosion control measure that is environmentally benign with respect to vegetation.