The Effects of Forest Fires on Runoff Rates: The Role of Duff Removal and Surface Sealing by Vegetative Ash, Western Montana
Increases in runoff and erosion following a forest fire are often attributed to the removal of the surface duff and litter, the formation water repellent soils, and surface sealing. There has been considerable work on the influence of water repellency on post-fire runoff, however few studies have addressed the hydrologic importance of litter and duff consumption and surface sealing. To further understand these processes, six controlled burns were conducted within Lubrecht Experimental Forest, Western Montana. Three of the study sites were situated on a granitic sandy loam soil formed from quartz monzonite, with Lodgepole pine (Pinus contorta) and Douglas fir (Pseudotoga menziesii) as the dominant vegetation. The other three sites were located in the northwestern part of Lubrecht forest, where the dominant tree species is Ponderosa pine (Pinus Ponderosa) with sporadic western larch (Larix occidentails); soils are formed out of Belt series metasedimentary rocks and consist of gravelly silt loams that contain 50% angular rock fragments. A rainfall simulator was used to measure overland flow associated with pre- and post-burn treatments within replicated 0.5 m2 plots. Rainfall simulations were also conducted on adjacent unburned plots, before and after the manual removal of the duff and litter layer. To further examine the role of ash in post-fire environments, micromorphological assessment of soil profiles were conducted via thin section analysis. Within granitic soils, burning resulted in a decrease in mean infiltration capacity from 9.1 to 3.5 cm.hr-1, while burning had no effect on infiltration in Belt series soils (2.3 and 2.3 cm.hr-1). The manual removal of duff and litter within Granitic soils resulted in a mean infiltration reduction from 6.4 to 4.0 cm.hr 1, while Belt series sites exhibited no significant change (2.3 and 1.6 cm.hr 1). Decreases in infiltration associated with both treatment types are attributed to surface sealing. The larger infiltration decrease due to the burning treatment is the result of a thin ash layer, which provided an additional source of fine material that contributed to surface sealing. The sealing effect was minimized within the Belt series sites due to the fact that the infiltration capacity was already relatively low prior to treatment. The ash layer deposited on the soil surface following the fire treatment was divided into black and white components, where white ash pertained to the completely combusted calcium portion, and black ash was defined as small char fragments (diameter < 1 mm). Micromorphologic observations within Granitic soils indicate that ash may contribute to infiltration reduction following fire in two ways: 1) by white ash filling pore spaces, and 2) by black ash becoming oriented parallel to the soil surface creating a thin layer of low porosity and permeability within the upper few millimeters of the soil.
Advisor:Dr. Scott Woods; Dr. Marc Hendrix; Dr. Tom DeLuca
School:The University of Montana
School Location:USA - Montana
Source Type:Master's Thesis
Keywords:college of forestry and conservation
Date of Publication:01/18/2008