EFFECTS OF NORTHERN SIERRA FIRE MANAGEMENT STRATEGIES ON POST-FIRE SOIL PROPERTIES

Over 100 years of fire suppression in the United States has led to frequent and intense wildfires, especially in the west. These wildfires cause changes in soil properties that lead to increased flooding and debris flows. For example, infiltration may decrease as a result of pore spaces becoming clogged with ash and debris. In addition, hydrophobic layers that repel water from the burned soils may form under certain conditions. Recent changes to forest management strategies are working to reestablish natural fire patterns in many regions. These strategies use regular prescribed burns that mimic natural fires to limit the available fuel load, minimizing the potential for large wildfires. However, methods for prescribed burns vary greatly and depend on the types of vegetation and the forest history.

Our study investigates two sites in California's northern Sierra Nevada that differ in the type of vegetation (old growth vs. forest with a logging and grazing history) and in the style of prescribed burns (high-intensity pile burns vs. low-intensity broadcast burns). Both sites were treated with prescribed burns during Fall 2006. Data on infiltration (using mini disk infiltrometers) and hydrophobicity (using the water drop penetration test) of the soils from the surface to 6 inches depth in the two study areas were collected prior to the prescribed burns, as well as approximately one year post-fire. Both sites exhibited high hydrophobicity at the soil surface and low infiltration rates prior to the prescribed burn treatments. However, preliminary results indicate that the two sites respond quite differently to their respective prescribed burn strategies. The old growth site, with primarily pile burns exhibit post-fire hydrophobicity at depth, rather than near the surface, as was observed in the site treated by broadcast burn. Both fire management strategies resulted in the development of hydrophobic layers of the soils that will hinder groundwater infiltration. Further study is required to determine the long-term impacts on soil properties results from these management strategies.