title:   The Long-term Effects of Wildfire Severity on Oak-pine Communities and their Microclimates
Creators: name: Culbert, Scott
Publisher: name: University of Kentucky Libraries
Year: 2023
Subjects: classificationCode: 1
subject: FOS: Natural sciences

description: In the eastern U.S., fire-dependent tree species have historically dominated upland forest communities, but are now experiencing widespread regeneration challenges as a result of 20th century fire suppression policies, and are being replaced by mesophytic species. Wildfires that contain areas of high burn severity may provide an important means of mitigating these challenges and facilitating fire-dependent species regeneration and recruitment into larger size classes. One mechanism by which high-severity fire can accomplish this is by modifying understory microclimate characteristics to be more conducive to these species’ growth. A wildfire within the Daniel Boone National Forest, Kentucky, USA, provided the opportunity to consider how wildfire may alter the regeneration of fire-dependent oak (Quercus spp.) and yellow pine (Pinus echinata Mill., P. rigidia Mill., P. viginiana Mill.) forests, and evaluate the long-term microclimate conditions following varying levels of burn severity. In this thesis, I examined the effects of burn severity on forest community structure and composition twelve years post-burn, and compared my results to those from previous measurement years. Likewise, I analyzed forest microclimate conditions across a range of burn severities throughout the 2022 growing season. Overall, I found that burn severity was negatively related to the importance of mesophytes (i.e. species known to facilitate the process of mesophication) and positively related to pyrophytes (i.e. species who positively benefit and are adapted to a frequent fire regime). At year 6, oak and pine recruitment of seedlings and saplings into larger size classes was positively related to burn severity, but by year twelve, only pine recruitment was related. This is likely due to the impact of burn severity on forest structure, and thus on understory microclimates, diminishing over time with forest regrowth. In this, I observed that the reduced canopy stem density that was caused by increased burn severity was still related to increased understory temperatures and vapor pressure deficits, but the midstory densification that has occurred increasingly with increased burn severity has largely obscured these effects. Based on this, burn severity is only partially related to microclimate conditions twelve years post-fire. At moderate burn severities where there was a meaningful amount of delayed canopy stem mortality that occurred, understory microclimates are experiencing more open growing conditions with greater light levels, greater temperatures, lower relative humidity, and greater vapor pressure deficits. Increased burn severity also increased the probability of non-native invasive plant (NNIP) presence, particularly Chinese silvergrass (Miscanthus sinensis Andersson). Overall, my results indicate that areas that experience increased burn severity can have lasting effects of increasing the competitive status of pyrophytes relative to mesophytes, and promoting yellow pine recruitment into canopy size classes. To bolster oak recruitment, understory microclimates may need to be modified between year six and year twelve to create more open growing conditions. Additional natural disturbance, prescribed fire, or a thinning treatment that reduces midstory stem densities may accomplish this. Creating microclimate conditions that promote the growth of fire-dependent species may be important for adapting eastern U.S. forests to climate change and help avoid degraded forest health in the future. However, any action that occurs should be coupled with NNIP monitoring and control efforts to mitigate their establishment and proliferation at high-severity burn sites.
description: © 2023 Scott Glenn Culbert