Restoration and canopy type influence soil microflora in a ponderosa pine forest

In ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests of the western USA, fire exclusion by Euro-American settlers facilitated pine invasion of grass openings, increased forest floor detritus, and shifted the disturbance regime toward stand-replacing fires. We evaluated the impacts of two replicated ecological restoration treatments involving tree thinning alone (thinning restoration) and a combination of tree thinning, forest floor reduction, and prescribed burning (composite restoration) on soil microbial activity, biomass, and function approximately 8 yr after initial treatments. Microbial-N levels in the two restoration treatments were not significantly different from the control during either the dry or wet periods of the growing season. Soil respiration measured in situ was significantly higher in the restoration treatments than in the control only during the dry period, while soil enzyme activities were generally higher in the composite restoration treatment than in the thinning restoration or control treatments during the wet period. Community-level physiological profiles suggested differences in the physiological capacities of bacteria and fungi in the composite restoration treatment compared with the other treatments. We also compared microbial characteristics under different canopy types to evaluate the impacts of pine invasion and establishment in grass openings on soil microorganisms. Soil respiration rates (dry period only) and enzyme activities (wet period only) were higher in grass openings than under presettlement trees, with intermediate values found under postsettlement pines that have invaded grass areas. Taken together, our results suggest that restoration treatments have long-term impacts on the soil microflora in these forests.