Forest soil paludification and mid-Holocene retreat of jack pine in easternmost North America: Evidence for a climatic shift from fire-prone to peat-prone conditions

Abstract

Soil paludification is the main ecosystem process initiating the formation and development of most peatlands in the Northern Hemisphere. Sandy podzolic soils developed on coarse deposits are among a large variety of substrates overgrown by thick peat layers, particularly along the amphi-atlantic coasts of North America and Europe. Whether the podzolic soils beneath peat layers are remnants of former dry environments allowing the morphogenetic development of forest soils or the progressive outcome of natural succession towards full peatland growth is still debated. We have explored a part of this dual facet in documenting the interface between buried podzol profiles and basal peat. Two paludified sites located at the center and at the edge of an extensive plateau bog along the Gulf of St Lawrence were documented based on radiocarbon-dated tree and charcoal macrofossils. Paleosols beneath thick and relatively old (> 4000 cal. yr BP) peat were composed of slightly cemented, placic B horizons whereas those under thinner and younger (< 2500 cal. yr BP) peat were made of heavily cemented, ortstein B horizons. Forest soil paludification and peat growth at both sites commenced with the cessation of fire occurrence as evidenced by charcoal fragments in the paleosol matrix beneath Sphagnum peat layers devoid of charcoal fragments. Botanically identified charcoal fragments include several tree species, in particular jack pine (Pinus banksiana) presently absent from this part of the continent. The retreat of the species likely occurred after 5500 cal. BP with the cessation of fire occurrence, an indication of a shift in maritime Québec from dry to wetter conditions initiating peat growth and peatland expansion. It is concluded that the genesis and development of podzols with different degrees of soil cementation (placic and ortstein horizons) preceded the inception and development of the plateau bog which have been facilitated by wetter climatic conditions inimical to fire activity. The progressive lateral growth of large ombrotrophic peatlands during the Holocene is also an additional, possible factor influencing the natural occurrence and spread of fire.