A detailed methods supplement is available online (link here).

Field sites: Twenty MAT sites within the burn were accessed via helicopter from either Umiat or Toolik Lake in July and August of 2008 . Burn severity was mapped using the differenced Normalized burn Ratio (dNBR) method40 and sites 1-12 were randomly chosen to represent the range and frequency of dNBR values as described in Jones et al. (2009). The remaining sites, 13-20, were chosen randomly along hillslope transects from areas where collaborators had previously established eddy covariance towers and lake/stream monitoring.
 
To obtain empirical relationships between ecosystem structure and the element pools necessary for reconstruction of pre-fire soil C and N pools, 11 unburned MAT sites were also sampled . Samples from one unburned site (ARF109) were held back from analyses for testing the reconstruction method. Three unburned sites were adventitiously encountered within the burn perimeter, an additional site was located near the 2007 Kuparuk Fire (69.2974 °N, 150.3221 °W), and seven others were systematically selected along the Dalton Highway. The latter sites were randomly selected from a GIS database that included all MAT areas along the Dalton within the elevation and climate range of the burn scar and were allocated to span the same elevations as the burn scar. All sites were >300 m from the road to minimize the effects of dust and other disturbances41,42.
  
Field sampling—unburned sites: We quantified the relationship between E. vaginatum crowns and SOL depth or biomass, and characterized SOL bulk density and element concentration across the 11 unburned sites. Along a 50 m transect in each site, we measured the depth of thaw and SOL at 5 m intervals (random point) and directly adjacent to the tussock nearest to the random point (n=10 random and 10 tussock points). We measured at both random and tussock points to determine whether the relationship between soil organic matter depth and tussock crowns was related to distance to tussock. Thaw depth was measured by inserting a metal rod into the soil until it hit ice or rock (differentiated by the sound and texture of the hit), marking the surface of the green moss on the rod, removing it, and measuring the distance from the tip to the mark with a meter stick. Soil organic layer depth was measured by slicing a square pit with a serrated knife, removing a monolith of organic soil, exposing the surface of the mineral soil, and measuring the distance from the surface of the green moss to mineral soil on two sides of the pit. The two depth measurements per pit were averaged to yield one SOL depth measurement per point.
 
At each point where SOL depth was measured, we used two meter sticks attached at a sliding right angle and fitted with a tubular spirit level to measure the depth of the green moss below a plane parallel to the crown of the nearest E. vaginatum tussock . Use of the level ensured that the right angle was parallel to the crown and orthogonal to the ground. For the randomly located sample point, we also measured the distance to the nearest tussock using this apparatus. We measured the distance to, crown diameter of, and survivorship of the three next closest tussocks and used a nearest-neighbor method to estimate tussock density44. For each tussock, two crown diameter measurements were made (and averaged) at right angles by compressing the leaves and manually locating the sides of the crown.
 
To determine soil bulk density and element concentration, organic soil horizons were sampled volumetrically with a serrated knife. At 10 m intervals, a pit was dug and a 10 x 20 cm soil monolith was excised from the side of the pit, extending from the surface of the green moss to the surface of the mineral soil (roughly 5-30 cm depth depending on location). This monolith was wrapped in tinfoil to preserve structure, returned to the field station and frozen prior to shipping to the University of Florida (UF) for analyses of bulk density, moisture, C and N concentration, and C isotopes. All aboveground plant material attached to the surface of the soil monolith was included in the sampling. Tussocks were also harvested at 15 m intervals to develop allometric relationships between tussock diameter and combustible biomass (see below). Biomass was shaved from the live tussock with a serrated knife and returned to the field station, where it was dried at 70° C for 48 hours before weighing and shipping to UF for analyses of C and N concentration.
 
Field sampling—burned sites: Measurements in burned sites were similar to those in unburned sites except that measurements were made on the surface of the residual burned organic layer rather than the surface of the green moss and tussock leaves were not sampled.
 
Laboratory analyses: Approximately 155 soil monoliths comprising ~1000 individual 5 cm increment soil samples were collected in total from the 20 burned and 11 unburned sites. In the lab, each monolith was bisected depth-wise with an electric carving knife. One half of the monolith was processed for radiocarbon measurements, as described below, and re-frozen for archival purposes. In the remaining half, green moss and dwarf shrubs were sliced off and the remainder of the core was sliced into 5 cm depth intervals with the last sample of variable depth depending on the location of the organic/mineral interface. Samples were homogenized by hand and coarse organic materials (>2.5 cm twigs and roots) and rocks were removed. Coarse and fine organic fractions were weighed wet, dried at 70° C for 48 hours to determine dry matter content, then ground on a Wiley mill with a 40 mm sieve. Carbon and N content was measured on a Costech Elemental Analyzer (Costech Analytical, Los Angeles, California, USA) calibrated with the NIST peach leaves standard (SRM 1547, National Institute of Standards and Technology, Gaithersburg, MD, USA). The volume of each monolith layer was calculated as depth times area minus the volume of rocks. Bulk density, C or N pools were calculated for both fine and coarse organic fractions.

Literature Cited

40Key, C. H. and Benson, N. C., in Firemon: Fire effects monitoring and inventory system, edited by D. C. Lutes, R. E. Keane, J. F. Caratti et al. (USDA Forest Service, Rocky Mountain Monitoring and Inventory System, Ogden, UT, 2005), pp. 25-36.

41Walker, D. A. and Everett, K. R., Road dust and its environmental impact on Alaskan taiga and tundra. Arctic and Alpine Research 19, 479-489 (1987).

42Myers-Smith, I. H., Arnesen, B. K., Thompson, R. M., and Chapin, F. S., Cumulative impacts on Alaskan arctic tundra of a quarter century of road dust. Ecoscience 13 (4), 503-510 (2006).

44West, P. W., Tree and forest measurement, 2nd ed. (Springer-Verlag, Heidleberg, 2009).
 
 
 

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