Ecosystem changes across a gradient of permafrost degradation in subarctic Québec (Tasiapik Valley, Nunavik, Canada)

Abstract

Permafrost thaw, tundra shrubification, and changes in snow cover properties are documented impacts of climate warming, particularly in subarctic regions where discontinuous permafrost is disappearing. To obtain some insight into those changes, permafrost, active layer thickness, vegetation, snow cover, ground temperature, soil profiles, and carbon content were surveyed in an integrated approach in six field plots along a chronosequence of permafrost thaw on an ice-rich silty soil. Historical air photographs and dendrochronology provided the chronological context. Comparison of the plots reveals a positive feedback effect between thaw settlement, increased snow cover thickness, shrub growth, increase in soil temperature, and the process of permafrost decay. By the end of the sequence permafrost was no longer sustainable. Along the estimated 90 year duration of the chronosequence, the originally centimeter-thin pedogenic horizons under mosses and lichens increased to a thickness of nearly 65 cm under shrubs and trees. Snow cover increased from negligible to over 2 m. The thickness of soil organic layers and soil organic matter content increased manyfold, likely a result of the increased productivity in the shrub-dominated landscape. The results of this study strongly suggest that permafrost ecosystems in the subarctic are being replaced under climate warming by shrub and forest ecosystems enriched in carbon on more evolved soils.

A Conceptual Model for Anticipating the Impact of Landscape Evolution on Groundwater Recharge in Degrading Permafrost Environments

“…Pelletier et al (2018) proposes a conceptual model where the different forms of vegetation within the Tasiapik Valley represent a 90‐year chronosequence for the greening of the tundra in the North. In their model, lichens and herbs are the most “immature” landscape, which has not been substantially altered from its prethawed state.…”

“…This greening, occurring primarily as a result of climate warming, has involved both an increase in the extent of shrubs (e.g., Betula nana or dwarf birch), as well as the expansion of black spruce forests ( Picea mariana ; Truchon‐Savard et al, 2019). In this study, the classification system from Pelletier et al (2018) is used, in an effort to link the chronosequence of landscape evolution to changing patterns of recharge. This system includes six distinct categories of land cover: lichens and herbs, lichens and low shrubs, low shrubs and lichen, medium shrubs, tall shrubs, and trees.…”

“…In the peaty, wetland‐dominated landscapes of the boreal region of Canada, the formation of thermokarst has resulted in the lowering of permafrost plateaus, subsequently expanding the extent of wetlands and altering the timing, volume, and geochemistry of fluid fluxes within drainage areas (Hayashi et al, 2004; Quinton et al, 2011). While the lowering of plateaus in the boreal region of Canada has rendered the existing plant life susceptible to inundation, further north, in the discontinuous permafrost region, the formation of thermokarst has helped to promote the expansion of shrubs into drier, mineral‐soil‐dominated landscapes that were previously tundra (Baltzer et al, 2014; Pelletier et al, 2018). Though shrubification in these regions is most closely linked to warming temperatures, recent work has shown that this process preferentially occurs in lower, wetter regions, such as the hollows formed by thermokarst (Lemay et al, 2018; Pelletier et al, 2018; Tremblay et al, 2012).…”

“…While the lowering of plateaus in the boreal region of Canada has rendered the existing plant life susceptible to inundation, further north, in the discontinuous permafrost region, the formation of thermokarst has helped to promote the expansion of shrubs into drier, mineral‐soil‐dominated landscapes that were previously tundra (Baltzer et al, 2014; Pelletier et al, 2018). Though shrubification in these regions is most closely linked to warming temperatures, recent work has shown that this process preferentially occurs in lower, wetter regions, such as the hollows formed by thermokarst (Lemay et al, 2018; Pelletier et al, 2018; Tremblay et al, 2012). While previous research in permafrost‐dominated landscapes has investigated the relationships between geomorphological and hydrological changes, the effects of long‐term ecological change on catchment‐scale hydrology in these areas, particularly how ecological shifts influence groundwater recharge, are not well understood (Connon et al, 2015; Quinton et al, 2011; Walvoord & Kurylyk, 2016).…”

“…The objective of the study presented herein is to assess how the progression of this chronosequence can influence groundwater recharge in northern catchments. To meet this objective, the water table fluctuation (WTF) method described by Sophocleous (1991) is used to compute groundwater recharge under five different kinds of land cover within the Tasiapik Valley, each corresponding to a landscape stage in the chronosequence of Pelletier et al (2018). Recharge values are then combined with the preexisting chronosequence, allowing for the creation of a new conceptual model that describes how the landscape and hydrology of the Tasiapik Valley could change over the period of 2020–2100 in response to continued climate warming.…”