Erosion after wildfires affect forest rehabilitation
Wildfires greatly increase hillslope- and watershed-scale runoff and sediment yields, according to research conducted by a group from Colorado State University and published in the Soil Science Society of America Journal. Land use and climate change have increased, or are projected to increase, the size and frequency of fires in many wildland environments.
These issues are of concern because severe wildfires can increase stream flows and erosion rates by 10 to 100 times relative to undisturbed forests. The increases in water flow can cause downstream flooding, degrade public water supplies, fill reservoirs and hurt fish habitats.
Issac Larsen, former CSU researcher, was first author on the study and is now a doctorate student at the University of Washington. The team of Colorado State University researchers led by Professor Lee MacDonald also included Ethan Brown, Daniella Rough, Matthew J. Welsh, Joseph H. Pietraszek, Zamir Libohova, Juan de Dios Benavides-Solorio and Keelin Schaffrath. The study was published in the Soil Science Society of America Journal.
The researchers say that large increases in runoff and sediment yields after high-severity fires have been attributed to several factors, including: soil water repellency; loss of surface cover; soil sealing by sediment particles; and soil sealing by ash particles. The problem is that the relative contribution of each factor to the observed increases in post-fire runoff and sediment yields is largely unknown. And this lack of understanding hampers the ability to predict post-fire sediment yields and design effective post-fire rehabilitation treatments.
Severe wildfires consume all surface-level vegetation in a forest as well as the forest floor’s protective layer of dead and decomposing leaves and pine needles. The burning of this organic matter causes soil particles to become unbound from one another. Raindrops falling on this bare, disaggregated soil can form a thin surface seal that can greatly increase the amount of overland flow and channel erosion. The more publicized increase in how soil repels water is less important than previously believed. These results have important implications for which treatments are most likely to successfully reduce post-fire erosion.
“This work helps demonstrate that the amount of surface cover, along with the amount and intensity of rainfall, is the primary control on post-fire runoff and erosion. This conclusion is strongly supported by our other studies, which show that the most effective post-fire treatments are those that immediately provide surface cover and thereby reduce soil sealing. Seeding and other treatments that do not immediately provide surface cover are ineffective in reducing runoff and erosion, and may increase erosion rates if the soil surface is extensively disturbed. Additional manipulative and long-term studies are needed to confirm these results in areas with different soil, climatic, and vegetative conditions,” said Lee MacDonald, professor in CSU’s Department of Forest, Rangeland and Watershed Stewardship in the Warner College of Natural Resources in a recent press release from Colorado State.
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