Fire mitigation and exotic species invasions in the Front Range: Responses of soil and vegetation to woodchip amendments in thinned forest, across a gradient of experimentally-manipulated nutrient availability

Mechanical thinning for fire mitigation has become increasingly widespread in recent years throughout the Western United States. A common practice in fire mitigation procedures is the conversion of slash into chipped mulch (referred to as "woodchips") that is spread on-site. However, relatively few studies have investigated the effects of stand-scale applications of woodchip mulch on subsequent patterns of understory vegetation establishment and soil nutrient dynamics.;In this multi-year project, I investigated the effects of woodchip amendments on soil properties and determined whether those responses varied across an experimentally-induced gradient of soil nutrient availability. Additionally, I examined patterns of understory plant distribution in response to woodchip amendments and/or variations in soil nutrient availability. Finally, I determined whether plant responses to woodchip and nutrient manipulations differed between a conventionally-thinned site, which was characterized by an absence of residual understory vegetation after thinning, and a site thinned with low-impact techniques, where a pre-existing understory community of predominantly native plants was preserved.;Nutrient manipulations affected native and non-native plants, as groups, differently, although those responses varied by site. At the conventionally-thinned site, where non-native species richness was high, the absolute cover of native plants was unaffected by increases or reductions in soil nitrogen availability. There, the native plants tolerated low levels of nitrogen availability better than non-natives, but were more constrained in their response to fertilization. In contrast, at the low-impact thinning site, where non-native species richness was low, native plants exhibited a significant increase in absolute cover in response to fertilizer applications.;I found that woodchip amendments applied to a standard depth of 7.5 cm did not alter soil pH, but did increase average soil moisture content. Woodchips did not induce net nitrogen immobilization at ambient levels of soil fertility but were associated with increased availability of inorganic soil nitrogen by the third year after application. When applied directly to freshly-disturbed soil at the conventionally-thinned site, woodchips affected the trajectory of vegetation establishment, but when woodchips were applied over the top of an existing understory community of native plants at the low-impact site, the patterns of plant distribution did not change. Thus, woodchip amendments appeared to inhibit seedling germination and/or survivorship more than they inhibited the survival of existing native, perennial vegetation. In conclusion, where plant management concerns co-occur with fire mitigation concerns, the decision to apply woodchip mulch may be contingent on the level of soil and understory disturbance associated with planned thinning operations, as well as the level of non-native propagule pressure in the area.