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Post-fire Treatment Effectiveness for Hillslope Stabilization

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Performance Characteristics of Mulches

Ground cover–The amount of bare soil exposed, or stated conversely, the amount of ground cover, is related to watershed response and to the treatment effectiveness of post-fire hillslope mulching treatments whose basic functional feature is coverage of bare soil. Generally, post-fire mulch treatments need to provide a uniform distribution of mulch material at 60 to 80 percent ground cover to reduce hillslope erosion. However, Foltz and Wagenbrenner (2010) reported that a 50 percent cover of wood shreds significantly reduced sediment yields nearly as well as 70 percent cover.

Mulch effects on post-fire re-vegetation–Optimizing the thickness of post-fire mulch is a balance between soil protection and suppression of vegetative recovery and establishment of seeded species. Thick layers of mulch can prevent sunlight from reaching the soil surface and/or physically obstruct seedling emergence. Beyers and others (2006) reported that none of the studied post-fire treatment mulches (wood chips, hydromulch, and rice straw mulch) increased vegetation cover, and wood chip mulching inhibited vegetation recovery more than other treatments while providing the most total ground cover and greatest reduction in erosion for several years. Debats and others (unpublished report 2008) found that 100 percent hydromulch coverage reduced initial plant density on post-fire hillslopes in southern California chaparral. They compared their findings with Hubbert and others (unpublished report 2005) who reported no apparent vegetation suppression due to the 51 percent coverage of hydromulch on similarly burned landscapes following the 2003 Cedar Fire in southern California.

Longevity–The amount of time mulch remains in place on a hillslope will impact treatment effectiveness. Residence time of mulches varies depending on the type, size, and amount of the mulch material applied. Woody mulches are decay resistant. In one study, wood strands were visible 7 years after application. In contrast, hydromulch generally decays within a few months to a year. The rapid decay of hydromulch may result in much less ground cover than deemed necessary for hillslope stabilization in the first and second post-fire years.

Wind redistribution–The light weight of agricultural straw mulch makes it susceptible to strong winds that can blow it off-site leaving unprotected bare soil and deep mulch piles that inhibit seed germination. Wind displacement can be minimized by increasing the mulching rate (>1.5 t ac-1 [3 Mg ha1]), pushing the straw mulch into the soil (crimping), adding a tackifier to "glue" the mulch strands to one another and the soil, or felling trees on top of the mulch at a right angle to the prevailing winds to hold it in place. Wood mulches have greater resistance to displacement by wind and provide greater wind erosion reduction than straw mulch. Hydromulch can resist wind displacement during the first 6 to 12 months after application, but likely loses this capacity as the tackifier degrades.

Strand Length–Generally larger mulch particles (analogous to forest floor litter) function primarily to reduce sediment yield, while the smaller mulch particles (analogous to forest floor humus) primarily absorb rainfall to reduce runoff. In studies, long-stranded mulches (for example, agricultural straws, wood shreds, ponderosa pine needles, etc.) have been observed forming "mini-debris dams" as mulch fibers become interlocked along flow paths. These mulch clumps contort overland flow paths, slow the flow velocity, and hold sediment on the hillslope. In addition, long fiber mulches require greater shear force to displace them compared to shorter-fiber mulches. Hydromulches tend to have thin short fibers and depend on the formation of a smooth mat and/or soil adherence for effectiveness.

The information on this web page has been excerpted from the following publication:

Robichaud, Peter R.; Ashmun, Louise E.; Sims, Bruce D. 2010. Post-fire treatment effectiveness for hillslope stabilization. Gen Tech. Rep. RMRS-GTR-240. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 62 p.

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