Warmer, drier conditions and extended growing seasons are intensifying forest disturbance regimes, particularly wildfire. In the Southern Appalachians, wildfires were historically low severity and promoted dominance of fire-tolerant species with thick bark, primarily Quercus over mesic, fire-sensitive species, such as Acer. Fire suppression over the last century has contributed to a gradual shift toward mesic species dominance, which might be vulnerable to the projected future conditions that include longer dry periods and more frequent and intense wildfires. As such, mixed-severity fires might become more frequent with future climate. In fall 2016, the southern Appalachians experienced severe drought and multiple wildfires, some of which included moderate and high severity areas. We investigated the impacts of fire severity on forest community composition and structure. We measured the understory (woody species <5 cm dbh, >0.5 m tall), woody seedlings (<0.5 m tall), and ground-layer (herbaceous species), across paired burned and unburned watersheds, within two wildfires. Across the burned watersheds, we assigned five burn severity classes, based on tree and evergreen shrub mortality, forest floor depth, mineral soil exposure and bole scorch height.
Results/Conclusions
Initial results indicate an effect of burn severity on forest understory and ground-layer community composition and density. Unburned watersheds had the highest understory density (1.52 ±0.18 stems m-2). Within burned watersheds, the greatest understory tree density (0.59 ± 0.17 stems m-2) was in high burn severity plots. Ground-layer cover in low burn severity plots (55±27%) exceeded unburned watersheds (32± 4%). Such threshold effects were also evident in seedling density, which was greater in burned (34.15±5.76 stems m-2) than unburned watersheds (21.75±4.54 stems m-2) overall, but peaked in low-moderate severity plots (84.25±31.80 stems m-2). Unburned watersheds had the greatest species richness (13.25±5.08 species plot-1). High and low severity burned plots had the greatest species richness across burned watersheds, 7.08±4.79 and 7.33±3.20 species plot-1, respectively. We also found initial evidence the fires affect community composition. Quercus and Acer sapling densities were similar across unburned (0.11±0.04 vs. 0.13±0.03 stems m-2) and burned (0.05±0.01 vs. 0.07±0.01 stems m-2) watersheds. However, within burned watersheds, Quercus saplings (0.15±0.04 stems m-2) were more abundant than Acer saplings (0.10±0.03 stems m-2) in high burn severity plots. Our results suggest low severity fire promotes understory and ground-layer recruitment and growth, while high severity fire impedes recovery.