Boreal peatlands are frequently underlain by permafrost, which is thawing rapidly. A common ecological response to thaw is the conversion of raised forested plateaus to treeless wetlands, but unexplained spatial variation in responses, combined with a lack of stand-level data, make it difficult to predict future trajectories of boreal forest composition and structure. We sought to characterize patterns and identify drivers of forest structure, composition, mortality and recruitment in a boreal peatland experiencing permafrost thaw. To do this, we established a large (10 ha) permanent forest plot (completed in 2014), located in the Northwest Territories, Canada, that includes 40,584 mapped and measured trees. In 2018, we conducted a comprehensive mortality and recruitment recensus. We also measured frost table depth, soil moisture, soil humification and organic layer thickness within the plot between 2012 and 2018, and used habitat association tests to link these variables to forest characteristics and dynamics. Forest composition and structure varied markedly throughout the plot and were strongly governed by patterns in permafrost presence and organic layer thickness. Overall, there was a net loss of trees from the plot at a rate of 0.7% year(-1). Mortality of black spruce, the dominant tree species, was more than double that of recruitment and was strongly associated with permafrost thaw. In contrast, recruitment of larch was over four times greater than mortality, and occurred primarily in low-lying, permafrost-free wetlands with mineral soil near the surface. Synthesis. The trends in tree demography and underlying drivers suggest that spruce-dominated permafrost plateaus will be converted into larch-dominated wetlands as permafrost thaw progresses in boreal peatlands, particularly in areas where mineral soil is near the surface. In the longer term, thaw could increase the hydrologic connectivity of the landscape, resulting in widespread drainage and re-vegetation by spruce, but we did not find evidence that this is occurring yet. Given the increasing rates of permafrost thaw, and positive feedbacks between thaw and forest change, we predict that larch abundance will continue to increase in boreal peatlands over the coming decades, leading to shifts in ecosystem function, wildlife habitat, albedo and snow dynamics.

The acceleration of permafrost thaw due to warming, wetting, and disturbance is altering circumpolar landscapes. The effect of thaw is largely determined by ground ice content in near-surface permafrost, making the characterization and prediction of ground ice content critical. Here we evaluate the spatial and stratigraphic variation of near-surface ground ice characteristics in the dominant forest types in the North Slave region near Yellowknife, Northwest Territories, Canada. Physical variation in the permafrost was assessed through cryostructure, soil properties, and volumetric ice content, and relationships between these parameters were determined. Near-surface ground ice characteristics were contrasted between forest types. In black spruce forests the top of the permafrost was ice-rich and characterized by lenticular and ataxitic cryostructures, indicating the presence of an intermediate layer. Most white spruce/birch forests showed similar patterns; however, an increase in the active layer thickness and permafrost thaw at some sites have eradicated the transition zone, and the large ice lenses encountered at depth reflect segregated ground ice developed during initial downward aggradation of permafrost. Our findings indicate that white spruce/birch terrain will be less sensitive than black spruce forests to near-surface permafrost thaw. However, if permafrost thaws completely, white spruce/birch terrain will probably be transformed into wetland-thaw lake complexes due to high ground ice content at depth.

Boreal and arctic regions are predicted to warm faster and more strongly than temperate latitudes. Peatlands in these regions contain large stocks of soil carbon in frozen soil and these may effect a strong positive feedback on climate change. We modelled the predicted effects of climate change and wildfire on permafrost in organic soils using a peatland-specific soil thermal model to simulate soil temperatures. We evaluated the model at a lowland black spruce site in Alaska and a sedge-dominated Canadian arctic fen. We estimated the response of soil temperatures and the active layer thickness (AcLTh) under several climate change scenarios. With surface soil temperatures increased by 4.4 degrees C-5.4 degrees C, soil temperatures at 100 cm depth increased by 3.6 degrees C-4.3 degrees C, the AcLTh increased by 12-30 cm, the zone of partially thawed soil increased, and the number of thaw days increased by 17-26 %. Wildfire caused AcLTh to increase by 26-48 % in the year following fire; AcLTh differences in 2091-2100 were significant (8 cm) at one site. By 2100, climate change effects on AcLTh were larger than wildfire effects suggesting that persistent temperature increases will have a more substantial effect on permafrost than the transient effects of disturbance.

Organic soil horizons function as important controls on the thermal state of near-surface soil and permafrost in high-latitude ecosystems. The thermal conductivity of organic horizons is typically lower than mineral soils and is closely linked to moisture content, bulk density and water phase. In this study, we examined the relationship between thermal conductivity and soil moisture for different moss and organic horizon types in black spruce ecosystems of interior Alaska. We sampled organic horizons from feather moss-dominated and Sphagnum-dominated stands and divided horizons into live moss and fibrous and amorphous organic matter. Thermal conductivity measurements were made across a range of moisture contents using the transient line heat source method. Our findings indicate a strong positive and linear relationship between thawed thermal conductivity (K-t) and volumetric water content. We observed similar regression parameters (beta or slope) across moss types and organic horizons types and small differences in beta(0) (y intercept) across organic horizon types. Live Sphagnum spp. had a higher range of K-t than did live feather moss because of the field capacity (laboratory based) of live Sphagnum spp. In northern regions, the thermal properties of organic soil horizons play a critical role in mediating the effects of climate warming on permafrost conditions. Findings from this study could improve model parameterization of thermal properties in organic horizons and enhance our understanding of future permafrost and ecosystem dynamics.

Boreal forests at high latitudes are climate-sensitive ecosystems that respond directly to environmental forcing by changing their position according to latitude or by changing their abundance at local and regional scales. South of the arctic treeline, external forcing (warming, cooling, drought, fire) necessarily results in the changing abundance of the impacted forests; in particular, the deforestation of well-drained sites through fire is the most important factor. In this study, we examined the changing abundance of wetland forests located at the arctic treeline (northern Quebec, Canada) during the last 1500 years, a period of known contrasting climatic conditions. Black spruce (Picea mariana) trees submerged in small lakes and peatland ponds and soil-peat stratigraphy were used concurrently to reconstruct the millennial-long developmental sequence of wetland stands associated with moisture changes and fire disturbance. Changing lake levels from AD 300 to the present were identified based on radiocarbon-dated submerged paleosols and tree-rinc, cross-dating of submerged trees distributed in three wetlands from the same watershed. Dead and living trees in a standing position below and above present water level of a small lake (LE Lake) showed direct evidence of past water levels from the 12th century to the present day. Submerged subfossil trees from another lake (LB Lake) and two peatland ponds (PB Peatland) also responded synchronously to changes in soil moisture during the last 1500 years. Regional-scale catastrophic flooding around AD 1150, inferred from paleosol and subfossil tree data, eliminated riparian peat and wetland trees growing at least since AD 300. Also, the coincidence of events such as the mass mortality of wetland spruce and post-fire deforestation of a small hill surrounding LE Lake during the late 1500s suggests the impact of local-scale flooding, probably attributable to greater snow transportation and accumulation on the lake surface after fire disturbance. Massive tree mortality climaxed at ca. 1750, when all wetland trees at LB Lake and PB Peatland died because of permafrost disturbance and soil upthrusting. Lower water levels from AD 300 to 1750 were associated with drier conditions, possibly caused by greater evaporation and/or reduced snow accumulation. Permafrost development in shallow waters occurred during the Little Ice Age, after 1600. It is concluded that the climate at the eastern Canadian treeline was warmer and drier from AD 300 to the onset of the Little Ice Age and promoted tree establishment. The highest water levels were recorded recently (19th and 20th centuries), causing lake and peatland expansion. Any future Moisture changes at these subarctic latitudes will result in important spatial rearrangements of wetland ecosystems.