Forest Soil Disturbance: Implications of Factors Contributing to the Wildland Fire Nexus

Abstract

Core Ideas Wildfire is a major cause of forest soil disturbance. Fire severity, scale of fire, slope, infiltration rates, and post‐fire rainfall are major factors. Soil management has limited potential for mitigating negative effects. Fire area and severity are on the increase. Climate change is causing wildfires to be hotter, windier, drier, and larger. Wildfires and prescribed fires cause a range of impacts on forest soils depending on the interactions of a nexus of fire severity, scale of fire, slope, infiltration rates, and post‐fire rainfall. These factors determine the degree of impact on forest soils and subsequently the need for post‐fire soil management. Fire is a useful tool in landscape management but it can be benign or set off serious deteriorations in soil quality that lead to long‐term desertification. If parts of the nexus are absent or not inherently risky, forest soil impacts can be relatively minor or nonexistent. A low severity prescribed fire on a small landscape unit with minimal fuel loading, slopes less than 10%, and no water repellency is unlikely to damage soil condition and functions with all but heavy rainfall. On the other hand, a high severity wildfire in a substantial area of heavy fuels with slopes >100% and water repellency may undergo serious soil damage with even moderate rainfall. Soil management is not likely to be needed in the former case but virtually impossible in the latter scenario. This paper examines thresholds in the nexus factors which can raise the risks of wildland fire from low and moderate to high. It documents the interactions of the fire nexus using several case histories in North America and elsewhere to demonstrate different degrees of soil impact.