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

摘要

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