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

Abstract Background In many eastern North American forests and woodlands, formerly dominant oaks (Quercus spp. L.) and pines (Pinus spp. L.) are experiencing widespread regeneration challenges and biodiversity loss. These changes are due to past land use and widespread 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 in Kentucky, USA, providing a rare opportunity to track oak-pine vegetation community recovery across a wide fire severity spectrum. 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); 0.79–3.94 in DBH) and canopy (10 + cm DBH; 3.94 + in DBH), and non-native invasive plant (NNIP) populations.Results After 12 years, burn severity was positively related to midstory recruitment of yellow pine (Pinus echinata Mill., P. rigidia Mill., P. virginiana Mill.) and unrelated to recruitment of oak and mesophytic species. Burn severity was negatively related to the relative importance of midstory mesophyte species and positively related to the relative importance of midstory pyrophyte species. Increased burn severity dramatically increased the probability of invasive species presence, particularly Chinese silvergrass (Miscanthus sinensis Andersson).Conclusions Earlier results found that up to six years post-fire, burn severity was significantly and positively related to oak sapling recruitment. Our results indicate that, 6 years later, 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 provide long-term recruitment benefits to yellow pine species and increase the competitive status of pyrophytes relative to mesophytes. To re-stimulate recruitment of oak into the midstory, it may be necessary to reintroduce fire between six and 12 years following a high-severity burn. This research demonstrates the benefits of higher-severity fire for restoring fire-adapted communities, though the proliferation of NNIPs severely undermines these benefits.