Decades of fire suppression following extensive timber harvesting have left much of the forest in the intermountain western United States exceedingly dense, and forest restoration techniques (i.e., thinning and prescribed fire) are increasingly being used in an attempt to mitigate the effects of severe wildfire, to enhance tree growth and regeneration, and to stimulate soil nutrient cycling. While many of the short-term effects of forest restoration have been established, the long-term effects on soil biogeochemical and ecosystem processes are largely unknown. We assessed the effects of commonly used forest restoration treatments (thinning, burning, and thinning + burning) on nutrient cycling and other ecosystem processes 11 yr after restoration treatments were implemented in a ponderosa pine (Pinus ponderosa var. scopulorum)/Douglas fir (Pseudotsuga menziesii var. glauca) forest at the Lubrecht Fire and Fire Surrogates Study (FFS) site in western Montana, USA. Despite short-term (<3 yr) increases in soil inorganic nitrogen (N) pools and N cycling rates following prescribed fire, long-term soil N pools and N mineralization rates showed only subtle differences from untreated control plots. Similarly, despite a persistent positive correlation between fuels consumed in prescribed burns and several metrics of N cycling, variability in inorganic N pools decreased significantly since treatments were implemented, indicating a decline in N spatial heterogeneity through time. However, rates of net nitrification remain significantly higher in a thin + burn treatment relative to other treatments. Short-term declines in forest floor carbon (C) pools have persisted in the thin + burn treatment, but there were no significant long-term differences among treatments in extractable soil phosphorus (P). Finally, despite some short-term differences, long-term foliar nutrient concentrations, litter decomposition rates, and rates of free-living N fixation in the experimental plots were not different from control plots, suggesting nutrient cycles and ecosystem processes in temperate coniferous forests are resilient to disturbance following long periods of fire suppression. Overall, this study provides forest managers and policymakers valuable information showing that the effects of these commonly used restoration prescriptions on soil nutrient cycling are ephemeral and that use of repeated treatments (i.e., frequent fire) will be necessary to ensure continued restoration success.

Inferring habitat requirements of rare plants can be challenging when the few remaining populations occur in sites with divergent successional states. In island-like rock outcrop systems within forest landscapes, edaphic conditions are assumed to modulate successional patterns, but changes to disturbance regimes in the landscape matrix could alter ecotone microenvironments over time. We used demographic surveys and controlled experiments with a dispersal-limited and endangered ecotonal plant, Astragalus bibullatus, to test the hypothesis that woody encroachment from the forest matrix threatens rare plants in globally imperiled limestone cedar glades. Tree canopy cover was more important than edaphic conditions or ground-layer structure for explaining variation in demographic structure. As tree canopy cover increased, stem densities, flowering, and seed production declined. Over three-years, per plant inflorescence production, fruit production, and fruit set were markedly greater in open microhabitats than in edge or closed microhabitats. In contrast, seedling densities peaked in edge and closed microhabitats. Seedling recruitment and seed production were spatially decoupled across a canopy cover gradient, suggesting that shaded sites historically had lower tree canopy cover. Shade reduced growth rates and biomass of seedlings and adults under nonlimiting moisture conditions. Although A. bibullatus can persist in degraded ecotones using multiple demographic strategies, growth, flowering and seed production depend on open microhabitats. Our results demonstrate that woody encroachment effects in temperate grasslands extend to island-like rock outcrop systems with unique edaphic conditions. Conservation and recovery programs with rare ecotonal plants should integrate restoration of historical disturbance regimes in the landscape matrix.

Successful recovery of populations of endangered plant species requires conservation of existing populations as well as the creation of new populations through reintroduction. However, the ecological requirements of many rare plant species are poorly understood, and many reintroduced populations are unable to survive long term. Effective reintroduction of rare plants, such as the federally endangered Astragalus bibullatus, may depend on developing a greater understanding of the symbiotic relationships that these rare species form with the soil microbiome, as well as determining whether these are species-and site-specific. We inoculated seedlings of A. bibullatus, its more widespread congener A. tennesseensis, and the common grass Schizachyrium scoparium, with soil biota collected from five glade sites where A. bibullatus is historically present (HP) and four glade sites where the species is historically absent (HA). We examined the impacts of soil microbes from HP and HA glade sites as well as from each species on the growth, arbuscular mycorrhizal fungi (AMF) colonization, and formation of root nodules of A. bibullatus and its congener A. tennesseensis. Astragalus bibullatus grew significantly larger when grown in soil from HP glade sites compared to its growth in soil from HA glade sites. Astragalus bibullatus also formed significantly more root nodules when grown in HP soil, but no difference was detected in AMF colonization based on glade history. Our findings suggest that the successful establishment of rare plant species may depend on species-specific associations with soil mutualists such as nitrogenfixing rhizobial bacteria and should consider whether essential microbes are present.

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