Interpretation of the charcoal record in forest soils: forest fires and their production and deposition of macroscopic charcoal

Abstract

Traps were used to quantify charcoal production and transport during three experimental forest fires in Boreal Scandinavia. The traps were spatially arranged to collect charcoal particles inside burn areas, and outside burn areas at different distances (0.1–100 m) from the fire edge. The number of inside and outside traps was 280 and 424, respectively. Trap area was 48 cm 2. After the burn, trap content was sorted and sieved in two size-classes of charcoal particles, namely small (0.5–2.0 mm) and large (. 2.0 mm), and number and mass of particles were determined. The production and distribution of charcoal were highly variable at fine spatial scales inside burn areas. On average, inside traps contained 12.1 small and 10.1 large particles, and the average charcoal mass was 0.112 g per trap (corresponding to 235 kg ha-1). The largest size-class made up 94% of the mass. Outside traps contained 0.3 small and 0.1 large particles per trap, and 45% of the outside particles were distributed, 1 m from the fire edge. It is concluded that the occurrence of macroscopic charcoal ($ 0.5 mm) in forest soils provides a solid evidence for local fire influence, and that the presence of large charcoal particles can be used to distinguish between fire-prone and fire-free areas with high spatial precision. Absence of large particles must, however, be more carefully interpreted as 14% of the inside traps lacked macroscopic charcoal. We argue that the charcoal in Boreal forest soils should be less persistent than previously suggested because documented fire-return intervals result in an unrealistic charcoal accumulation presupposing high persistence.