Wildfire alters belowground and surface wood decomposition on two national forests in Montana, USA

Wildfires can drastically alter belowground processes such as organic matter (OM) decomposition. We used wood stakes of two different tree species, trembling aspen (Populus tremuloides Michx.) and loblolly pine (Pinus taeda L.), placed at three soil locations (soil surface, forest floor–mineral soil interface, mineral soil), as an index of the long-term (5-year) effects of wildfire on OM decomposition in two forest stands after high-severity wildfire and in an unburned control. Wood stake mass loss was generally higher for aspen, especially in the mineral soil of burned plots, than in control plots after 5 years. Soil surface and interface (unburned stands only) stakes showed few significant differences for either species on the Bitterroot National Forest. On the Gallatin National Forest, both pine and aspen stakes had significantly greater mass loss at the interface (unburned stand) at the end of 5 years, and also decayed significantly faster at the 10–20-cm depth in the wildfire area. Using wood stakes as an index of soil microsite properties in burned and unburned plots, we show that fire increased both OM decomposition and mineral soil microsite variability. These results strengthen our understanding of soil-surface and belowground ecosystem responses to wildfire.