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Experimental Design

Our study spanned a gradient from tundra to low-density spruce stands in Denali National Park and Preserve (DNP) in Alaska, USA. DNP has a continental climate characterized by long cold winters and short warm summers. Influences of the Alaska Range create a cooler and wetter environment within DNP than much of Interior Alaska. DNP headquarters (63 deg 43 min N, 148 deg 58 min W) had a mean annual temperature of -2 degC and total annual precipitation of 40.4 cm between 1991 and 2020. Continuous permafrost is common throughout much of DNP, especially in the northern lowlands between watersheds. However, as average air temperatures increase in DNP, the existence of near-surface permafrost is expected to decrease as active layers thicken in the future.

Methods

FIELD: Site selection: Our study spanned a gradient from tundra to low-density spruce stands in Denali National Park and Preserve (DNP) in Alaska, USA. In the summer of 2013, the East Toklat Fire burned ~34,216 acres in DNP. This fire encompassed two National Park Service Inventory and Monitoring (NPS I&M) grids we call 'East Toklat (ET)' and 'Wigand Creek (WC)'. Each grid contained 25 plots arranged in a 2 x 2 km area, separated by 500 m. We resampled 48 monitoring plots; two plots were excluded. In addition to the 48 long-term monitoring plots resampled within the burn, 15 control plots representing unburned or pre-fire vegetation were established and sampled following NPS I&M protocols. In each plot, we measured slope, aspect, elevation, latitude, and longitude. Slope, aspect, and latitude were used to calculate equivalent latitude. Soils: Soil sampling occurred in July 2016. Four organic soil monoliths were collected at every plot. At an additional eight sampling points, located 1 m outside the perimeter of each plot, we measured soil organic layer (SOL) depth by making a small cut with a knife and identifying the interface between the organic and mineral soil horizons or frozen ground by visual inspection. At each of these points, we also measured active layer thickness from the SOL surface to frozen ground. To estimate burn depth we used the "tussock crown" (as per Mack et al. 2011) or "adventitious root" method (as per Boby et al. 2010 and Walker et al. 2018). To assess the C content of the collected SOL monoliths we followed standard protocols. We bisected the monoliths depth-wise using an electric carving knife and one half of the monolith was re-frozen for archival purposes. The monoliths were divided into 5 cm depth increments, with the last sample of variable depth depending on the location of the organic to mineral soil or frozen ground interface. Two measurements each of depth, height, and width were obtained from each increment and the wet weight was recorded. Samples from all monoliths were homogenized by hand and any rocks, sticks, or coarse roots greater than 2 mm in diameter were removed. Fine (less than 2 mm) and coarse (greater than 2 mm) organic fractions were weighed wet and dried at 60 degC for 48 hours to determine dry matter content. Rock volume was estimated by water displacement in a graduated cylinder. Fine organic fractions were ground and the percent C was determined using a Costech Elemental Analyzer calibrated with the NIST peach leaves standard. We determined the bulk density of fine organic fractions for each sample by dividing the dry weight by the sample volume excluding rock volume. We calculated C content by multiplying the sample depth by the bulk density and percent C of the sample. Trees: We measured the diameter at breast height (DBH) at the standard height of 1.4 m from the base for all trees equal to than 1.4 m in height and the basal diameter of all trees less than 1.4 m that were originally rooted in the plot and assessed tree combustion based on a ranking from 0 to 3. Stem counts and diameter measurements were used to calculate tree density (number stems m-2). Stem counts, diameter measurements, and published allometric equations were used to calculate aboveground biomass (kg dry matter m-2) of the total tree, bark, main branches, fine branches, and needles/leaves, for each tree species in each plot. Total tree biomass combusted was calculated for each tree from the assigned combustion class and affected biomass components (foliage, branches, and bark). We summed individual tree estimates and divided by the sample area to estimate pre-fire biomass and biomass combustion (kg dry matter m-2) on an area basis in each plot. We assumed a biomass C content of 50%. Understory vegetation: Vegetation community data was originally collected at the time of plot establishment in 2001 and 2002 and post-fire data was collected in July 2016. Along each transect in each plot, vegetation and substrate cover were measured using the line-point intercept (LPI) method. Seedling regeneration: Post-fire seedling regeneration was assessed within four, 4 m2 quadrats in each plot. One quadrat was placed along the transect in each of the four quadrants. Seed rain: Spruce seed rain was assessed adjacent to 12 plots (6 burned and 6 control plots). We established a 20 m north-south transect 16 m directly east of these plots. Natural seed rain was determined by randomly placing 10 seed traps along the transect. Seed traps were greenhouse flats (54 x 28 x 6 cm) lined with astroturf (to provide a rough surface with good drainage) and affixed to the ground with four 10 cm nails. Seed traps were placed during July 2016 and collected in August 2016, June 2017, August 2017, June 2018, and August 2018. Seeding experiment: Using the same 12 plots where we assessed seed rain, we also established a black spruce seeding experiment. Along the 20 m transects, we randomly established five blocks, each one consisting of four 0.5 x 0.5 m quadrats. Blocks and quadrats were marked by stringing yarn around wooden skewers outlining each quadrat. Half of our treatments received an artificial soil scarring by removing living vascular and non-vascular species in order to mimic the immediate post-fire environment. Within each block, we randomly assigned quadrats to one of four treatments: i) not seeded and not scarified (control), ii) not seeded and scarified, ii) seeded and not scarified, or iv) seeded and scarified. n August 2016, we seeded an estimated 250 black spruce seeds (0.187 g) in each of the 120 quadrats that were seeded (~30,000 seeds). Counts of germinated seeds were completed in August 2017 and August 2018. LAB: To assess the C content of the collected SOL monoliths we followed standard protocols. In the laboratory, we thawed soil monoliths at room temperature (~25 degC) for approximately 24 hours. We bisected the monoliths depth-wise using an electric carving knife and one half of the monolith was re-frozen for archival purposes. With the remaining half, all live moss or vascular plants were sliced off and discarded from monoliths collected in burned plots. For unburned monoliths, the dimensions of living material (green moss) were measured and retained for further analysis. The remainder of all monoliths were divided into 5 cm depth increments, with the last sample of variable depth depending on the location of the organic to mineral soil or frozen ground interface. Two measurements each of depth, height, and width were obtained from each increment and the wet weight was recorded. The live material from unburned stands and the 5 cm increment samples from all monoliths were homogenized by hand and any rocks, sticks, or coarse roots >2 mm in diameter were removed. Fine (< 2 mm) and coarse (>2 mm) organic fractions were weighed wet and dried at 60 degC for 48 hours to determine dry matter content. Rock volume was estimated by water displacement in a graduated cylinder. Fine organic fractions were ground and the percent C was determined using a Costech Elemental Analyzer calibrated with the NIST peach leaves standard. We determined the bulk density of fine organic fractions for each sample by dividing the dry weight by the sample volume excluding rock volume. We calculated C content by multiplying the sample depth by the bulk density and percent C of the sample.