The influence of vegetation and soil characteristics on active‐layer thickness of permafrost soils in boreal forest

Abstract

Carbon release from thawing permafrost soils could significantly exacerbate global warming as the active‐layer deepens, exposing more carbon to decay. Plant community and soil properties provide a major control on this by influencing the maximum depth of thaw each summer (active‐layer thickness; ALT), but a quantitative understanding of the relative importance of plant and soil characteristics, and their interactions in determine ALTs, is currently lacking. To address this, we undertook an extensive survey of multiple vegetation and edaphic characteristics and ALTs across multiple plots in four field sites within boreal forest in the discontinuous permafrost zone (NWT, Canada). Our sites included mature black spruce, burned black spruce and paper birch, allowing us to determine vegetation and edaphic drivers that emerge as the most important and broadly applicable across these key vegetation and disturbance gradients, as well as providing insight into site‐specific differences. Across sites, the most important vegetation characteristics limiting thaw (shallower ALTs) were tree leaf area index (LAI), moss layer thickness and understory LAI in that order. Thicker soil organic layers also reduced ALTs, though were less influential than moss thickness. Surface moisture (0–6 cm) promoted increased ALTs, whereas deeper soil moisture (11–16 cm) acted to modify the impact of the vegetation, in particular increasing the importance of understory or tree canopy shading in reducing thaw. These direct and indirect effects of moisture indicate that future changes in precipitation and evapotranspiration may have large influences on ALTs. Our work also suggests that forest fires cause greater ALTs by simultaneously decreasing multiple ecosystem characteristics which otherwise protect permafrost. Given that vegetation and edaphic characteristics have such clear and large influences on ALTs, our data provide a key benchmark against which to evaluate process models used to predict future impacts of climate warming on permafrost degradation and subsequent feedback to climate.

Improved moving window cross‐spectral analysis for resolving large temporal seismic velocity changes in permafrost

“…This site is~50 km north of Fairbanks, Alaska, in a zone of discontinuous permafrost [Jorgenson et al, 2008], and contains~21 m of eolian loess and alluvial deposits over schist bedrock [Schwering and Sirles, 2015]. The northwest corner of the array contained black spruce trees usually associated with shallow permafrost [Fisher et al, 2016]. The northwest corner of the array contained black spruce trees usually associated with shallow permafrost [Fisher et al, 2016].…”

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

“…When vegetation and organic material are removed, the depth of thaw may be meters greater than at similar intact vegetated sites . Greater vegetation cover as indicated by greater optically measured leaf area index (LAI) can be associated with shallower thaw depth . Shrub cover has been linked both to greater thaw depth attributed to greater snow accumulation and to shallower thaw depth associated with increased shading .…”

Variability in above- and belowground carbon stocks in a Siberian larch watershed

“…With increased temperatures, boreal forests are susceptible to insect invasions (Berg et al, 2006;Kurz et al, 2008), moisture stress (Beck et al, 2011;Trahan and Schubert, 2016;Walker et al, 2015), tree line advance and retrogression (Lloyd, 2005;Pearson et al, 2013), and more frequent forest fires (Kasischke and Turetsky, 2006;Rogers et al, 2015;Soja et al, 2007), which all have the potential to alter C cycling significantly in the region. Importantly, climatechange-driven alterations in forest cover, composition, and structure will influence regional energy balance through impacts on surface albedo, evapotranspiration, and ground insulation, which will in turn affect ground thaw and soil C cycling (Chapin et al, 2005;Euskirchen et al, 2009;Fisher et al, 2016;Jean and Payette, 2014;Loranty et al, 2014).…”