Mulching for Postfire Rehabilitation
Mulch is any organic material spread over the soil surface that increases the ground cover and reduces raindrop impact and overland flow. Both wet mulch (hydromulch) and dry mulch (wheat straw, jute excelsior, rice straw, etc.) can be applied from the air or from the ground; however, mulches have only recently been used as a postfire rehabilitation treatment.
Mulch mixed with grass seed is frequently applied to postfire hillslopes to improve the germination of seeded grasses by increasing infiltration and enhancing soil moisture retention (Robichaud and others 2000). In the past, seed germination from grain or straw mulch was regarded as a bonus as this increased the cover on a site; however, the introduction of noxious weeds and other non-native plants are now considered drawbacks to the use of straw mulch and postfire rehabilitation projects usually require certified ‘weed-free’ straw for postfire rehabilitation efforts (Beyers 2004).
Due to the cost and logistics of mulching it is usually used to protect high value resources, such as reservoirs, water quality, habitat, roads, structures, and sensitive cultural resources, from upslope erosion. Both hydromulch and dry mulch were used to stabilize soils on the Cerro Grande Fire of 2000 and the Rodeo-Chediski and Hayman Fires of 2002. However, use of these treatments escalated the BAER treatment costs to $10 to $20 million per fire.
The use of helicopters to spread dry mulch is relatively new in postfire emergency rehabilitation. Straw bales suspended in cargo nets break apart as they fall and spread further upon impact, resulting in a fairly even distribution of straw mulch with approximately 70% ground cover when applied at a rate of 1 t ac-1 (2.4 Mg ha-1) (SDTDC 2003). Ground application of dry mulch is usually done by hand using ATVs to carry the straw from a staging area into the treatment area.
Straw mulch has been shown to reduce erosion rates after wildfires by 50-94% (Miles and others 1989; Bautista and others 1996; Faust 1998). In a comparative study continued for two rainy seasons after the 2000 Cerro Grande Fire in New Mexico, the plots treated with aerial seed and straw mulch yielded 70% less sediment than the control plots in the first year and 95% less in the second year. Ground cover transects showed that aerial seeding without added straw mulch provided no increase in ground cover relative to untreated plots (Dean 2001). In the second year after the 2000 Bobcat Fire in Colorado, Wagenbrenner (2003) reported sediment yields from mulched hillslopes that were significantly less than those from both untreated and seeded-only slopes.
The use of hydromulch for postfire rehabilitation is a new effort to take advantage of the general success of hydromulch as an erosion mitigation treatment on road cut and fill slopes and in construction site rehabilitation. There are numerous combinations of tackifier, polymers, bonded fiber, seeds, etc. used in hydromulch that, when mixed with water and applied to the soil surface, form a matrix that can reduce erosion by up to 97% in unburned applications (San Diego State University 2002) and foster plant growth.
Hydromulch has only been applied in a limited number of postfire situations. Ground application of hydromulch is done from spray trucks and is limited to an area 200 ft (60 m) of either side of a road. Large-scale application requires helicopters fitted with slurry tanks and access to a nearby staging area, making aerial application of hydromulch one of the most expensive hillslope treatments available. After the 2002 Hayman Fire in Colorado, 1500 acres(600 ha) of aerial hydromulching was applied to steep, inaccessible areas that drain directly to the South Platte River and the reservoir system that provides 90% of Denver’s municipal drinking water (Robichaud and others 2003). The effectiveness of hydromulch in reducing erosion after the Hayman Fire is currently under study.
In some burned areas, natural mulch may provide adequate ground cover making the ‘no treatment’ option a practical choice for those areas. After a wildfire, there is a mosaic of low, moderate, and high burn severity conditions within the burned area (DeBano and others 1998). Low and moderate burn severity areas produce less runoff and erosion than high severity burned areas (Benavides-Solorio and MacDonald 2001). In conifer forests, low and moderate severity burned sites often have trees that are lightly charred and partially consumed by fire, leaving dead needles in the canopy. These needles fall to the ground and provide a natural mulch ground cover. Pannkuk and Robichaud (2003) found a 60-80% reduction in interrill erosion and a 20-40% reduction in rill erosion due to a 50% ground cover of dead needles. Thus, prudent use of postfire rehabilitation treatments would exclude areas where needles, or other litter components are present to provide sufficient ground cover.
Encyclopedia ID: p293
