ABSTRACT: Climate drivers and ecological triggers of permafrost formation and thaw in boreal permafrost peatlands.

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Discontinuous permafrost in boreal peatlands is strongly controlled by interactions among climate, hydrology, plant community succession, fire, and Picea mariana (black spruce) establishment. I synthesized data from several experimental, descriptive, and paleoecological studies from northern Manitoba, Canada, to determine how climate change and ecological processes control permafrost formation and thaw and the mass balance of permafrost in this region. In the modern environment, permafrost formation in climatically suitable zones (MAAT < 0°C) is triggered by P. mariana recruitment and subsequent impacts of the forest canopy on snow density. Experimental analyses of seed rain, seed germination, and seedling survival in thawed peatlands indicate that P. mariana recruitment is extremely poor, suggesting that spruce establishment is a major ecological constraint on the rate of new permafrost formation. Meanwhile, thaw rates in modern environments are accelerating as a result of significant temperature increases over the past half century. Over longer time scales, climate has been important for controlling peatland succession, fire, carbon accumulation, and permafrost. During the Holocene Thermal Maximum (~~6000-4000 BP), warmer conditions drove plant communities from wetter fens to drier forested bogs, resulting in greater fire severity and slower rates of carbon accumulation. Decreasing temperatures and increasing moisture from 2000-1000 BP increased the occurrence of wetter poor fen communities but with no apparent onset of permafrost. The Medieval Warm Period (~~1150-650 BP) appeared to drive succession back to drier, forested bog communities, setting the stage for modern permafrost development with the onset of cooler temperatures during the Little Ice Age (~~650-150 BP). Although succession to forested bogs increased fire severity over much of the Holocene, permafrost over the past 700 years has shown a remarkable resistance to thaw following severe fires, possibly as a result of cooler LIA temperatures stabilizing frozen soils. These results indicate the combined importance of climate change, peatland succession, fire, and spruce recruitment on permafrost dynamics. In the modern environment, rapid thaw combined with poor spruce recruitment suggest that the mass balance of permafrost in boreal peatlands is currently negative. Warmer conditions in the future may decrease further the stability of frozen soils and push peatland landscapes to drier bog communities with a greater likelihood of fire and thaw, thereby amplifying the loss of permafrost in these regions.

Bibliography of Canadian Geomorphology