Soil paludification andSphagnumbog initiation: the influence of indurated podzolic soil and fire

Abstract

The initiation and growth of boreal peatlands developed on well‐drained, sandy landforms are closely associated with podzolic soil paludification processes. The origin ofSphagnumbogs extending on large deltaic plains was examined to test the hypothesis of the dual impact of indurated (ortstein) podzols and fire on forest soil paludification and concurrent peatland initiation and expansion. Mineral soil, basal organic matter and peat monoliths were sampled for soil and macrofossil analyses along an 800‐m toposequence starting from a mixed‐wood boreal forest to aSphagnumbog (Lebel bog, eastern Quebec, Canada), and ending at a peat dome in the thickest section of the peatland. Mineral soils along the toposequence are ortstein humo‐ferric podzols distributed in the forest environment and beneathSphagnumpeat in the bog, except at the peat dome. Initial peatland growth occurredc. 6000 cal. aBP. Soil paludification coincided with the cessation of fire occurrence as recorded in the organic and mineral layers precedingSphagnumexpansion. Unlike most temperate and boreal raised bogs, theSphagnumbog developed directly from a forest environment without passing through a transitional fen stage. Conifer forests regenerated successively after several fires between 4200 and 1600 cal. aBPbefore bog expansion. Pre‐bog forests were composed of fire‐prone black spruce (Picea mariana) and jack pine (Pinus banksiana) trees, and ericaceous species. Given the distribution and thickness of ortstein horizons progressively decreasing and disappearing towards the peatland dome, growth and expansion of theSphagnumbog was not caused by soil induration processes, which could have potentially impeded vertical and horizontal drainage. The development of indurated podzols outside and several hundred metres inside the peatland preceded the initiation and expansion of theSphagnumbog. Cessation of fire activity appears to be a key factor facilitating the lateral expansion of theSphagnumbog under wet soil conditions.