Higher severity fire increases the long-term competitiveness of pyrophytes in an upland oak-pine forest, Kentucky, USA

Abstract Background In eastern North American forests and woodlands, formerly dominant oaks ( Quercus spp. L.) and pines ( Pinus spp. L.) are experiencing widespread regeneration challenges. These challenges are due to past land use and fire exclusion facilitating positive feedbacks that reduce the effectiveness of low-severity prescribed fire. High-severity fires (wildfires and potentially prescribed burns) offer promise to overcome these feedbacks and assist in ecosystem restoration. In 2010, a 670-ha mixed-severity wildfire burned in the Daniel Boone National Forest, Kentucky, USA, providing a rare opportunity to track oak-pine community recovery across a wide fire severity spectrum for 12 years. We examined the effects of burn severity on species diversity, forest structure, community assemblage, stem recruitment into the midstory (2–10 cm diameter at breast height [DBH]) and canopy (10 + cm DBH), and non-native invasive plant (NNIP) populations, and compared these effects to previous results published after 6 years post-fire. Results The relative importance of midstory pyrophyte species (e.g., Quercus spp. and Pinus spp.) was positively related to field-measured burn severity, while mesophytes (e.g., Acer rubrum , Betula lenta ) displayed a weaker negative relationship. After 12 years, higher burn severity yielded greater midstory recruitment of pyrophytes; however, this was only observed for yellow pines ( P. echinata Mill., P. rigida Mill., P. virginiana Mill.) and non-oak-pine pyrophytes. Oaks’ and mesophytes’ midstory recruitment displayed no relationship with burn severity. Increased burn severity strongly increased invasive species presence, particularly Chinese silvergrass ( Miscanthus sinensis Andersson). Conclusions Six years after fire, oak midstory recruitment was positively related to burn severity. By year 12, higher burn severity was no longer associated with oak midstory recruitment but was associated with increased recruitment and importance of other pyrophytic species, particularly yellow pines. Our results also suggest that a single high-severity fire can increase the long-term competitive status of pyrophytes relative to mesophytes. As such, higher-severity fire may be an important tool in restoring fire-adapted communities where low-severity fire lacks the ability to produce meaningful forest structure and community changes. However, NNIP proliferation in high-severity burn areas can undermine the restoration benefits we observed.