Mixed-severity Fires in the Southern Appalachians
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2018-10-30
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DellaRocco, Thomas. Master of Natural Resources Assessment and Analysis Mixed-severity Fires in Southern Appalachians
Warmer, drier conditions and extended growing seasons are intensifying forest disturbance regimes, particularly wildfire. In the southern Appalachians, historical wildfires were primarily low severity and promoted growth of understory vegetation. Fires also promoted the dominance of fire-tolerant species with thick bark in the overstory, such as oaks (Quercus spp.) and hickories (Carya spp.) over mesic, fire-sensitive species, such as maples (Acer rubrum L., Acer saccharum Marsh.), tulip-poplar (Liriodendron tulipifera L.) and birch (Betula lenta L., Betula alleghaniensis Britton). Fire exclusion over the last century has contributed to a gradual shift toward mesic species dominance, which has likely altered the forest understory. Further, these mesophytic species might experience mortality in the overstory in the projected future conditions that include longer dry periods and more frequent and intense wildfires. In fall 2016, the southern Appalachians experienced exceptional drought and multiple wildfires, some of which were novel and included moderate and high severity burned areas. We investigated the impacts of fire severity on forest understory, seedling, and groundlayer composition. 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 burned watersheds within two large wildfire complexes and compared those responses to adjacent, unburned watersheds. Across the burned watersheds, we assigned three burn severity classes, based on tree and evergreen shrub mortality, forest floor depth, mineral soil exposure and bole scorch height. We did not find a positive effect of fire on sapling density or richness in any of the burn severity classes. However, oak saplings occurred in similar densities to maples in higher burn severity plots. Mountain laurel and rhododendron shrubs had lower densities in burned areas. Additionally, seedling densities were higher in burned areas, particularly mesophytic seedlings such as tulip-poplar and birch. The groundlayer cover and richness was lower in areas affected by higher severity fires.
