Mulch Treatment Effectiveness

Agricultural Straw

o After the Cerro Grande Fire in New Mexico the application of straw mulch with seed reduced mean annual sediment yields by 70 percent in the first post-fire year and 95 percent in the second post-fire year; however, precipitation was below normal during the two study years (Dean 2001).

o Wagenbrenner and others (2006) reported that straw mulch immediately increased the mean ground cover to nearly 80 percent and facilitated vegetative regrowth after the 2000 Bobcat Fire in Colorado. Mulching did not reduce sediment yields in the year of the fire. Yet, in post-fire years one, two, and three, there was over a 95 percent reduction in mean sediment yield on the straw mulch treated plots as compared to the untreated control plots.

o Using paired swale plots installed on the 2002 Hayman Fire, Rough (2007) measured reductions in sediment yields of 94 percent in post-fire year one and 90 percent in post-fire year two on the straw mulch swale compared to the untreated control swale.

o Immediately after the 2002 Indian Fire, one set of paired swales was used to compare sediment yields from a straw mulch treated plot and an untreated control plot after a high intensity (I₁₀= 4.6 in h^-1 [117 mm h^-1]) rain event. The straw mulch reduced the sediment yield by 81 percent compared to the control (Riechers and others 2008).

o Rainfall simulations (1 h at I₁₀=3.3 in h^-1 [83 mm h^-1]) were done on small (5.3 ft² [0.5 m²]) hillslope plots after the 2002 Fox Creek Fire. In post-fire year one, the 10 wheat straw mulched plots had 86 percent less sediment compared to the 10 control plots (Groen and Woods 2008).

Wood-based Mulches

o Immediately after the 2002 Indian Fire, one set of paired swales was used to compare sediment yields from a wood chip mulched plot and an untreated control plot after three post-fire erosion-causing summer rain events. The wood chip mulch reduced the sediment yield by about 95 percent compared to the control for the first two smaller rain events; the effectiveness decreased for the third, high intensity (I₁₀=4.6 in h^-1 [117 mm h^-1]) rain event when the wood chip mulch reduced sediment yields by less than 68 percent.

o Wood strands, a manufactured wood mulch product, have been tested in two laboratory rainfall/overland flow simulation studies using screened forest soils in a rectangular plot placed at a 30 percent slope. In the first study, several sizes of wood strands were compared to equal cover amounts of agricultural straw, and showed that wheat straw and two sizes of wood strands were equally effective in reducing erosion by over 98 percent. Compared to the untreated controls, wood strand materials reduced sediment yield by at least 70 percent for all treatment combinations.

o Wood shreds were further tested using the same laboratory rainfall and overland flow simulations on the burned surface soil and ash collected following the 2006 Tripod Fire in north-central Washington. By controlling the proportion of "fine" shreds (shreds less than 1 in [2.5 cm]) in the mulch blend, three blends of shreds were evaluated for runoff and sediment concentration reduction. All the blends reduced runoff amounts, but the blend with all fines removed was most effective for both runoff and sediment yield reduction under conditions of rainfall and rainfall plus concentrated flow with no difference between 50 and 70 percent ground cover (Foltz and Wagenbrenner 2010).

Natural Burned Needle Cast Mulch

o Conifer forests burned at low and moderate severity often have trees that are charred and partially consumed by fire, leaving dead needles in the canopy. Pannkuk and Robichaud (2003) tested the effectiveness of 50 percent ground cover of Douglas fir and ponderosa pine needle cast. The short flat Douglas fir needles laid directly on the ground for its full length and reduced interrill erosion by 80 percent compared to a 60 percent reduction with ponderosa pine needles.

Hydromulch

o Aerially-applied hydromulch reduced sediment yields from swale plots by 95 percent in post-fire year one and 50 percent in post-fire year two as compared to control plots. However, ground-based application of hydromulch did not significantly reduce sediment yields in either year (Rough 2007).

o The aerial hydromulch treatment at the 2002 Hayman Fire is being evaluated using a paired watershed study, and was found less effective in reducing erosion during the first and second post-fire years than was reported by Rough (2007). The sediment yield from the hydromulch treated watershed was only 18 percent less in the first post-fire year and 27 percent less in the second post-fire year as compared to the sediment yields from the control watershed.

o In 2003, after the Cedar Fire in southern California, hillslopes burned at high soil burn severity were treated with aerial hydromulch using two configurations–application over 100 percent of the treatment area (H100) and application of 100 ft (30 m) wide contour strips of hydromulch such that 50 percent of the area was treated (H50). In the year of the fire, the H100 watershed reduced the sediment yield by 53 percent compared to the control. However, the H50 watershed had a greater rainfall and a larger sediment yield than either the control or the H100 watersheds. In post-fire year one, the control and the H100 watersheds had more than three times greater rainfall at greater intensities than the previous year, and the H50 watershed, again, had greater rainfall amount and intensity than either of these. These greater rain amounts resulted in greater sediment yields in all three watersheds, but both the H100 and H50 watersheds reduced the sediment yield (43 and 37 percent, respectively) compared to the control (Hubbert, unpublished report 2007).