Forest, Wildlife, and Climate News

High Erosion Rates After Wildfires Affect Forest Rehabilitation

Colorado State News Release, August 10, 2009 [here]

FORT COLLINS - For nearly 10 years, researchers at Colorado State University have investigated wildfires and the resulting effects on flooding and erosion. These include intensive field studies prior to and after the Hayman wildfire in Colorado, which burned more than 100,000 acres in 2002, and rainfall simulation studies in the laboratory. Newly published results in the Soil Science Society of America Journal indicate that soil erosion rates after forest fires are largely dependent on the amount of ground cover rather than the amount of soil water repellency, and this has important implications for designing effective post-fire rehabilitation treatments.

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.

The number, extent and severity of wildfires are projected to increase as a result of land-use and climate change. After large fires, millions of dollars are often spent on a variety of treatments to reduce flooding and erosion. The effectiveness of these treatments is uncertain, and land managers must better understand the underlying causes of the increase in post-fire runoff and erosion to make informed decisions about possible treatment options.

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.

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.

This work is funded by the Joint Fire Science Program through the Department of Interior, the Department of Agriculture, and the U.S.D.A. Forest Service.

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Isaac J. Larsen, Lee H. MacDonald, Ethan Brown, Daniella Rough, Matthew J. Welsh, Joseph H. Pietraszek, Zamir Libohova, Juan de Dios Benavides-Solorio and Keelin Schaffrath. 2009. Causes of Post-Fire Runoff and Erosion: Water Repellency, Cover, or Soil Sealing? Soil Sci Soc Am J 73:1393-1407 (2009).

Abstract: Few studies have attempted to isolate the various factors that may cause the observed increases in peak flows and erosion after high-severity wildfires. This study evaluated the effects of burning by: (i) comparing soil water repellency, surface cover, and sediment yields from severely burned hillslopes, unburned hillslopes, and hillslopes where the surface cover was removed by raking; and (ii) conducting rainfall simulations to compare runoff, erosion, and surface sealing from two soils with varying ash cover. The fire-enhanced soil water repellency was only stronger on the burned hillslopes than the unburned hillslopes in the first summer after burning. For the first 5 yr after burning, the mean sediment yield from the burned hillslopes was 32 Mg ha–1, whereas the unburned hillslopes generated almost no sediment. Sediment yields from the raked and burned hillslopes were indistinguishable when they had comparable surface cover, rainfall erosivity, and soil water repellency values. The rainfall simulations on ash-covered plots generated only 21 to 49% as much runoff and 42 to 67% as much sediment as the plots with no ash cover. Soil thin sections showed that the bare plots rapidly developed a structural soil seal. Successive simulations quickly eroded the ash cover and increased runoff and sediment yields to the levels observed from the bare plots. The results indicate that: (i) post-fire sediment yields were primarily due to the loss of surface cover rather than fire-enhanced soil water repellency; (ii) surface cover is important because it inhibits soil sealing; and (iii) ash temporarily prevents soil sealing and reduces post-fire runoff and sediment yields.