Herb abundance and diversity among fire severity classes in pine-oak forests of Great Smoky Mountains National Park

Fire suppression in forest ecosystems has changed fire regimes and modified woody plant composition, structure, and function throughout the US. Specific effects on the herbaceous plant communities are largely unknown. My study quantified herbaceous plant abundance and diversity of xeric pine-oak forests across four fire severity classes (“no burn”, low, moderate, high) in four different fires occurring within the last seven years in Great Smoky Mountains National Park (GRSM). Fire severity was determined using Landsat data and strip transects were used to sample vegetation. Herb and subshrub cover combined was low and averaged only 5.86% (ranging 0.025 to 28.25%) across all fire severities. Herb cover and richness were significantly greater in high severity areas. These areas also had low litter-duff depth and high canopy openness. Litter-duff depth and subshrub cover, which were negatively related to herbs, explained variation in herb cover, richness, and diversity, while greater canopy openness was an important factor for increased herb cover. Specifically, eudicot forbs, ferns, and graminoids were associated with high severity areas. Historically, the herb species Schizacyrium scoparium and Pteridium aquilinum were historically dominant or co-dominant with subshrubs in xeric pine-oak forests, but this was only the case in high severity areas in this study. In contrast to these responses, subshrub abundance and non-graminoid monocot presence were not related to fire severity. Reduced litter-duff depth, non-herb cover, and generally higher herb cover, richness, and diversity occurred in late growing season fire with a high Keetch-Byram Drought Index (KBDI) compared to early growing season fires with a low KBDI. These results indicate that higher severity fires maintain herbaceous communities in these ecosystems. However, high fire severity may not be favorable to several species of non-graminoid monocots. Finally, since subshrub and herb cover responded differently to fire severity, these plant groups should be considered separately in fire effects studies