The Role of Hummocks in Re-establishing Black Spruce Forest Following Permafrost Thaw

Kristine M. Haynes
Jessica Smart
Brenden Disher
Olivia Carpino
William L. Quinton

Northwestern Canada's discontinuous permafrost landscape is transitioning rapidly due to permafrost thaw, with the conversion of elevated, forested peat plateaus to low‐lying, treeless wetlands. Increasing hydrological connectivity leads to partial drainage of previously‐isolated bogs, which have been observed to subsequently develop hummock microtopography. However, the role of microtopographic features in the future trajectory of the transitioning landscape is unclear, including their potential controls on tree re‐establishment. In order to understand the role of hummocks in landscape change, research was conducted at the Scotty Creek Research Station, Northwest Territories, to measure hummock and black spruce tree physical characteristics, and assess tree and hummock spatial coverage in peat plateaus, collapse scar bogs and the advanced transitional feature known as treed bogs. Canopy coverage in all landforms and wetland hummock areal coverage was assessed using a LiDAR (Light Detection and Ranging) canopy gap fraction model and multispectral imagery. Hummocks, which are not underlain by permafrost but contain seasonal ice, support the establishment of black spruce trees due to favourable soil moisture conditions. Hummock flank moisture in treed bogs is intermediate between those of dry peat plateaus and inundated collapse scar bogs. Black spruce trees on peat plateaus and in treed bogs are significantly taller and of greater circumference than those in collapse scar bogs. The spatial distribution of hummocks and canopy coverage of black spruce trees in treed bogs collectively suggest that these features may play an important role in the advanced stages of permafrost thaw‐driven transition of the discontinuous permafrost landscape.