PS 28-140 - Duff and fine root consumption in Southern Appalachian Mountain surface fuels under simulated wildfire and prescribed fire scenarios

Abstract: A 2016 outbreak of wildfires in the Southern Appalachian Mountains resulted in delayed mortality of overstory trees, particularly in oak-hickory dominated stands. The consumption (by fire) of fine roots in the deep organic horizons of the forest floor has been implicated as one mechanism by which this delayed mortality may occur, suggesting that fire after long-term exclusion may threaten oak-hickory stands. It is unclear whether this risk exists when these ecosystems are subjected to prescribed fires, which are conducted when fuel moisture levels are higher than levels when wildfires typically occur. We collected surface fuels (wood, leaf litter, and duff) from a mixed oak-pine stand in the vicinity of Rock Mountain in northern Georgia, and conducted experimental fires in a controlled combustion facility. To simulate conditions prevalent during wildfires and prescribed fires we manipulated fuel moisture in the duff (wildfire = low duff moisture and prescribed fire = high duff moisture), and constructed fuel beds typical of these forests using the collected woody, litter, and duff materials. Following the experimental fires, we physically fractionated the remaining material using sieves to separate coarse and fine particulate organic matter, and a flotation technique to separate fine organic matter from mineral particles. Preliminary results showed that organic matter in the duff, including fine roots, was available as fuel in the wildfire scenario whereas this material was largely conserved in the prescribed fire scenario. On average, wildfire-burned fuel beds contained only 18.2% of the fine particulate organic matter that prescribed-burned fuel beds contained. Patterns of fine root consumption were even more pronounced, with 98+% consumption in the wildfire scenario, relative to prescribed fire. These results corroborate observations of wildfire effects from the field, and suggest that management approaches that gradually decrease the mass of forest floor organic horizons (such as repeated prescribed fires) will be necessary to mitigate risks associated with future wildfires.