The production of pyrogenic carbon (PyC; a continuum of organic carbon (C) ranging from partially charred biomass and charcoal to soot) is a widely acknowledged C sink, with the latest estimates indicating that~50% of the PyC produced by vegetation fires potentially sequesters C over centuries. Nevertheless, the quantitative importance of PyC in the global C balance remains contentious, and therefore, PyC is rarely considered in global C cycle and climate studies. Here we examine the robustness of existing evidence and identify the main research gaps in the production, fluxes and fate of PyC from vegetation fires. Much of the previous work on PyC production has focused on selected components of total PyC generated in vegetation fires, likely leading to underestimates. We suggest that global PyC production could be in the range of 116-385 Tg C yr À1 , that is~0.2-0.6% of the annual terrestrial net primary production.According to our estimations, atmospheric emissions of soot/black C might be a smaller fraction of total PyC (<2%) than previously reported. Research on the fate of PyC in the environment has mainly focused on its degradation pathways, and its accumulation and resilience either in situ (surface soils) or in ultimate sinks (marine sediments). Off-site transport, transformation and PyC storage in intermediate pools are often overlooked, which could explain the fate of a substantial fraction of the PyC mobilized annually. We propose new research directions addressing gaps in the global PyC cycle to fully understand the importance of the products of burning in global C cycle dynamics.

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 cm2. 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.

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This study aims to assess biomass and area growth of 600 thalli of the old forest lichen, Lobaria pulmonaria, transplanted to three successional boreal forest stands with (1) natural rainfall regime, (2) additional moistening during dry days, and (3) additional moistening with added nutrients. Mean biomass growth during 100 days varied from 8.3% in the dark young spruce forest to 23.1% in the clear-cut area, with the old forest in between (16.0%). Additional moistening did not enhance lichen growth, probably because the transplantation period was wet. Nutrient additions slightly increased area growth compared to artificial water additions only. Growth was determined by a combination of external (forest stand, site factors) and internal factors (chlorophyll content, biomass per area). Transplants acclimated to high light by increasing thickness and chlorophyll a/b-ratio. Some visible bleaching and a strong positive correlation between chlorophyll content per area and lichen growth in clear-cuts suggest some high light-induced chlorophyll degradation. We believe that biomass growth and natural occurrence of L. pulmonaria is controlled by a delicate balance between light availability and desiccation risk, and that the species is confined to old forests due to a physiological trade-off between growth potential and fatal desiccation damage, both of which increase with increasing light. The discrepancy between potential and realized ecological niches is probably caused by a long-term risk to be killed in open habitats by high light during long periods with no rain.