Late‐Holocene Development of Subarctic Ombrotrophic Peatlands: Allogenic and Autogenic Succession

Ombrotrophic peatlands that developed on islands in Clearwater Lake, a large subarctic lake in northern Quebec, provide evidence of long—term community stability and successional change. Repeated alternations of regeneration cycles of Sphagnum cushions and black spruce (Picea mariana [Mill.]BSP.) over the last 5050 ¹ ⁴C years of peatland development suggest climate, self—building mechanisms, and cyclic—autogenic succession as the dominant active factors operating in the absence of external disturbance such as fire. During the last several thousand years, self—perpetuating vegetation cycles have occurred at variable periodicity and have been controlled primarily by local ecological conditions whose existence is marked by differential peat accumulation rates. Water supply for peat production, in the absence of water table in most sites, was controlled by snow availability, precipitation, and atmospheric humidity from the large lake. Allogenic succession, illustrated by the community shifts from Sphagnum girgensohnii—Picea to S. russowii—Picea, S. fuscum—Picea, and lichen—heath, has been initiated by climate, and more particularly by continuous peat accumulation enhancing, as time passed, the ecological gradient from moist, snow—protected conditions to snowless, exposed conditions. In contrast, absence of such changes at some sites characterized by a uniform, self—regenerated S. fuscum—Picea assemblage suggests that autogenic succession has been an active driving mechanism in community persistence over several thousand years. Long—term trends in peatland development toward topographic equilibrium and late Holocene cooling best explain the gradual phasing—out of the vegetation cycles. Natural deforestation over the last 2000 ¹ ⁴C years may have triggered a negative feedback process by changing the winter snow environment and the associated thermal soil regime. The ensuing peat fossilization, permafrost aggradation, and ice—wedge development at the peatland surface are all events consistent with the occurrence of this process. Present growth of the peatland pproceeds mostly along the margins, where levelling of the topographic profile is not yet completed.