Over the past half century, the synergistic impacts of fire suppression and outbreaks of the southern pine beetle have caused heavy mortality in yellow pine forests throughout the southern Appalachian Mountains.  Over the past decade, prescribed burning has been increasingly used to reintroduce native fire regimes and restore these fire-dependent communities.  However, the relationship between burn severity, changes in stand structure, and yellow pine regeneration are poorly understood.  The Fire Effects Monitoring Program in Great Smoky Mountains National Park has been collecting data within prescribed burns for over a decade with the goal of better understanding the relationship between fire and the perpetuation of yellow pine forests.  Using these data, we address three important questions: (1) Are there threshold effects on stand structure (understory density, overstory density, shrub cover, duff depth, and total fuel load) that must be achieved with prescribed burning to facilitate the regeneration of yellow pine seedlings? (2)  Do changes to stand structure that favor the regeneration of yellow pine also promote the cover of herbaceous species? (3) How do the effects of fire on stand structure and pine regeneration relate to drought severity as represented by two commonly used indices (KBDI and PDSI)?

Results/Conclusions Our results show that fire-induced changes in stand structure exhibited strong relationships with yellow pine regeneration.  We observed drastic increases in pine regeneration on plots that experienced >60% reduction in total fuels, >80% reduction in understory density, and >50% reduction in overstory density.  Post-fire duff depths of 3 cm or less and total shrub cover of 10% or less were also highly correlated with increased pine regeneration.  Similar to pine seedling regeneration, post-fire cover of herbaceous species increased greatly on plots that exhibited a >80% reduction in understory density with a post-fire duff depth of 3 cm or less.  Pine seedling density was significantly greater on plots with high values for KBDI, as were reductions in total fuel load, understory density, and overstory density.  PDSI exhibited weak relationships with seedling density and changes in stand structure.  The results of our study suggest that there are clear thresholds of forest structural change that must be reached to insure the successful regeneration of yellow pine.  Pre-burn KBDI values are a strong predictor of whether these post-burn structural conditions are likely to be achieved.