Slash pile burning is a common fuels disposal method in ponderosa pine restoration harvests, which aim to reduce the risk of catastrophic fire and restore historic forest structure and function. The impacts of high-intensity, long-duration fire (pile burning) on soil chemistry, arbuscular mycorrhizal (AM) fungi, and plant establishment are unclear. Our objectives were: (1) to characterize the impact of slash pile burning on soil nutrients, microbial activity, and mycorrhizal infection; and (2) to contrast slash pile scar restoration techniques. In May 2006, slash piles were burned in a ponderosa pine stand in Florence, Montana. Soil samples were collected from the center, edge, and outside of 45 scars, then characterized for available NH₄⁺, NO₃^-, potentially mineralizable N, as well as total C and N, PO₄^-3, microbial biomass, and mycorrhizal inoculum potential (MIP).

Results/Conclusions

One month following burning, soil NH₄⁺ concentrations were elevated in the center of the slash pile scar; however, NO₃^- remained constant due to mortality of nitrifying bacteria. Total C was greatest in unburned soils.  In contrast, fumigation-extracted microbial biomass showed no significant difference among treatments, indicating rapid recovery of the microbial community. No significant difference was found for PO₄^-3. In October 2006, scars were seeded with native graminoids or left unseeded, and divided into subplots assigned to one of five restoration treatments. Soil and microbial analyses were repeated, as well as a survey of plant cover, frequency, and density for native and non-native vegetation. In a greenhouse experiment, MIP was negligible in burned soil collected one year post burn. In unburned soil, however, MIP was significantly greater in weedy forbs over native grasses.  Evaluating above and belowground impacts of slash pile burning will enhance our understanding of soil response to high-intensity fire, AM fungi’s role in recently restored ponderosa pine ecosystems, and allow us to evaluate the potential of restoration techniques in the suppression of non-natives. Collectively, these data will help characterize the risks and benefits of slash pile burning as a management tool in ponderosa pine forest restoration.