The study area includes the floor and side-walls of the South Fork Flathead River valley in the Bob Marshall Wilderness, Montana, USA (47°38′N, 113°21′W). Upland forests of this area are composed primarily of Larix occidentalis Nutt., Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco, Pinus contorta Douglas ex Loudon var. latifolia Engelm. ex S. Watson, Abies lasiocarpa (Hook) Nutt. (Belote et al. 2015), and Picea engelmannii Parry ex Engelm., with limited areas of Pinus ponderosa Lawson & C. Lawson (Keane et al. 2006) Much of the study area burned in 2000 and 2003; the most recent prior fires occurred in the 1930s.
We located 10 sites in once-burned areas (five in 2000 fire only, and five in 2003 fire only), and 10 sites in twice-burned areas (five in 2013 reburn of 2003 fire, and five in 2011 reburn of 2003 fire). Sample sites were chosen randomly from patches within the initial fire that was classified as high-severity burn and at least 3 pixels × 3 pixels (90 m × 90 m) in area, using burn severity maps from the Monitoring Trends in Burn Severity program (mtbs.gov, accessed 2 April 2017).
Coarse woody debris and associated charcoal were sampled in August 2014 using the planar intercept method (Donato et al. 2009). Sampling transects were arranged in a 30 m × 30 m square oriented to the cardinal directions and along one interior diagonal of the square, for a total of 162.4 m of transect per site. We recorded diameter, species, decay class, and depth of char for each CWD piece that intersected the sampling plane.
Charcoal mass estimation involved first making the standard planar intercept CWD volume calculation for each CWD piece including the charred rind, as well as calculating the volume of the inner uncharred core by reducing the CWD piece radius by the measured char depth (Donato et al. 2009). The difference of these two cylinders is the volume of charcoal on the CWD piece. We calculated the total CWD volume using Equation 1 in Donato et al. (2009) and bias-corrected charcoal volume using Equations 1, 3, and 8 in Donato et al. (2009). We converted CWD volumes to mass estimates using species and decay class specific CWD densities (Bisbing et al. 2010), and estimated black C mass using Equation 4 in Donato et al. (2009). We tested for differences of black C mass (kg ha−1) and total CWD biomass (kg ha−1) between once-burned and twice-burned forests using two-sample Wilcoxon rank sum tests (W). Statistical analyses were performed in R version 3.1.2 (R Core Team 2014).