Post-fire mulching for runoff and erosion mitigation. Part I: Effectiveness at reducing hillslope erosion rates
Robichaud, Peter R.; Lewis, Sarah A.; Wagenbrenner, Joseph W.; Ashmun, Louise E.; Brown, Robert E.
Post-fire mulching for runoff and erosion mitigation.
Part I: Effectiveness at reducing hillslope erosion rates.
Catena 105: 75–92.
Keywords: Post-fire rehabilitation, Wheat straw mulch, Wood strand mulch, Hydromulch, Seeding, Needle cast
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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, stormswith equivalent rainfall intensities produced nearly an order ofmagnitude 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 fireswhere itwas tested, wheat strawmulch 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 strandmulch reduced sediment yields for
the first four post-fire years, butwheat strawmulch 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.
Moscow FSL publication no. 2013a