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The origin and structure of wooded permafrost mounds at the arctic treeline in eastern Canada

Background: Permafrost mounds covered by open or dense forests (or krummholz – stunted trees) correspond to the category of wooded palsas and wooded peat plateaux extensively distributed in wetlands of North America. Very few data exist on the origin and development of wooded palsas, in particular their inception during the Holocene and the structure and dynamics of their forest/krummholz cover over time. Aim: The main objective of this paper is to fill the gap of knowledge on the origin and structure of wooded palsas, with a focus on wooded palsas located at their range limit at the treeline in eastern Canada. Methods: Each of the nine sites studied was surveyed along a line transect to measure the topography, to record the vegetation cover, to infer the snow thickness and to measure the thickness of the active layer at the time of sampling. The size structure of the black spruce (Picea mariana) populations of three dense wooded palsas was described. Basal stem discs of living and dead Picea were recovered for age determination, except in one site where only dead stems were sampled. The minimum age of wooded palsas at each site was evaluated based on tree-ring dating of the oldest stems and radiocarbon dating of the organic layers. Results: The inception of wooded palsas occurred during two principal periods of permafrost expansion as shown by the combined tree-ring and radiocarbon dates obtained for each of the nine studied sites. Three wooded palsas formed between 750 and 1000 AD, whereas the inception of the other palsas occurred during the 1500s. The up-thrusting of the wetland surface associated with permafrost growth facilitated tree establishment more intensively during the first period than during the second period when full Little Ice Age conditions prevailed in wind-exposed peatlands. Conclusions: The maintenance of a permafrost mound under forest cover is a rare situation in northern environments, and wooded palsas in eastern Canada form peculiar ecosystems where forest and permafrost coexist; these have survived over the last 500 to 1000 years. It is believed that the inception of wooded palsas was facilitated during a prolonged period of snow-poor winter conditions. Tree establishment was possible due to well-drained soil conditions associated with palsa up-thrusting. Although thicker snowpack preventing deeper frost penetration into the soil is maintained by the development of a forest/krummholz cover, wooded palsas at the treeline can survive because of low annual temperatures, reduced solar radiation beneath the tree canopy, variable accumulation and duration of annual snowpack.