Fine‐scale influences on thaw depth in a forested peat plateau landscape in the Northwest Territories, Canada: Vegetation trumps microtopography

Abstract

The influence of vegetation and microtopography on fine-scale variability of thaw depth is largely unknown but potentially important for improving modeling of ecosystem-permafrost interactions. To elucidate their influence, we measured tree density, shrub cover and cryptogam presence (lichen and bryophyte) on forested permafrost peat plateaus in the discontinuous permafrost zone in the southern Northwest Territories, Canada. Greater tree density was associated with shallower thaw depth (approximately one quarter of the variance), whereas shrub cover had a negligible influence on thaw depth. Cryptogam species influenced thaw depth, with greater thaw depth associated with Sphagnum than with Cladonia (a difference on the order of 10%). Greater thaw depth occurred beneath hummocks than beneath hollows (a difference also on the order of 10%). Together, canopy cover, cryptogam species and microforms contribute to a variation of roughly half the variance in thaw depth in the peat plateau landscape.

Landscape‐scale variations in near‐surface soil temperature and active‐layer thickness: Implications for high‐resolution permafrost mapping

“…These studies reveal large variations of Tnss at the landscape scale. In areas with fine‐grained sediments, summer thaw depth can be probed quickly at multiple locations 16–19 . Soil temperatures are seldom measured across active layer grids.…”

The role of hummocks in re‐establishing black spruce forest following permafrost thaw

“…It has been suggested that peat accumulation in the form of hummocks may promote the aggradation of permafrost due to the insulative properties of peat and the re‐establishment of black spruce; altering the energy dynamics of the landscape (Camill, 1999; Figure 9). Spatial variability in thaw depth was observed to be significantly impacted by hummock microforms and their associated vegetation resulting from thermal insulation facilitated by peat deposits (Higgins & Garon‐Labrecque, 2018). Carpino et al (2018) concluded that the return to a forest‐dominated landscape occurred in approximately 40 years (Figure 9).…”

“…Further monitoring of the hummock ice would be necessary to confirm if this ice persists interannually. The variability in thaw depth and potential promotion of ice formation due to the insulation afforded by hummocks (Higgins & Garon‐Labrecque, 2018) may have the potential to act as a positive feedback and further contribute to the vertical growth of hummock microtopography through expansion upon freezing (Figure 9); similar to the observed effects of ice formation on the development of mineral earth hummocks in the Canadian Arctic and Subarctic (Tarnocai & Zoltai, 1978) and peat plateaus at the southern boundary of the discontinuous permafrost zone (Zoltai, 1972). This potential positive feedback mechanism of ice formation on hummock growth requires further examination, particularly to assess its influence relative to the controls on NPP and organic matter accumulation known to drive hummock development in boreal peatlands (Belyea & Clymo, 2001).…”

“…Each transect was comprised of a linear series of points spaced at 5‐m intervals. The transects across plateaus were set back at least 5 m from the plateau edge to avoid the incorporation of dying black spruce trees along the transitioning plateau perimeters into this analysis, as suggested by Higgins and Garon‐Labrecque (2018). Ten living black spruce trees dispersed evenly along each belt transect were selected for measurements of tree height and circumference at the root‐crown interface (given the small nature of the trees in this environment).…”

Exploring a New Air-Surface Temperature Diurnal Wave Analysis Method for Permafrost Areas

“…The air-to-surface temperature dynamic and its relation to overall heat transfer involves a multivariate system. Many variables including vegetation (Smith, 1975), microtopography (Higgins and Garon-Labrecque, 2018), and snow cover (Zhang, 2005) affect the transfer of heat to and from the ground surface. Below ground, differences in soil conductivity play a role in the rate and extent of heat transferred and this is not only based on soil type but also moisture content (Gavril'ev, 1989).…”