Fire and fire surrogate effects on soil properties in a Sierra Nevada mixed-conifer forest

Many mixed-conifer forests of the Sierra Nevada historically experienced frequent, low-to moderate-severity fire regimes. More than a century of fire exclusion and past timber management practices in these forests have led to increased stand densities and fuel accumulation, with a corresponding risk of large, high severity wildfires. To reduce hazardous fuel accumulations and restore the health and natural processes of forest ecosystems, fuel management programs often employ thinning and prescribed fire treatments, both alone and in combination. Forest thinning operations are used to mechanically remove and manipulate fuels by harvesting, masticating, chipping, or pruning. Thinning can be used as a precision tool with which managers can modify stand structure by controlling exactly which fuels will be removed or rearranged. Reintroduction of fire is used to restore its ecological function as a disturbance agent, as well as a means to reduce hazardous fuel loads. Combinations of thinning and burning allow land managers to both control stand structure and reintroduce fire as a key ecosystem process. These fire and fire surrogate treatments are often designed to reduce fuels and create forest structures that resemble historic stand conditions. Despite widespread adoption of large-scale fuel treatments strategies, little is known about the ecological impacts these treatments will have on forest resources. In this dissertation, I present effects of fire and fire surrogate fuel treatments on soil chemical, biological, and physical properties. I demonstrate that the effects of thinning treatments were indistinguishable from the untreated controls, even though stand structure and fuel conditions were significantly altered. I also determine that fire and combined thinning and fire treatments elicited very similar responses in soil properties, and that these were often significantly different than the thinning treatment and untreated control. I conducted this research in an actively managed commercial forest, where legacy effects of past harvests are evident in both the vegetation and soil conditions. An existing network of skid trails not only increased site heterogeneity, but also influenced many effects of the fuel treatments. By controlling for the presence of skid trails, I have incorporated a novel approach to measuring fuel treatment effects.