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.

Keywords: Northwest Territories; active-layer thickness; boreal forest; discontinuous zone; permafrost; structural equation modelling.

Figure 1

Field sites. (a) Mosquito Spruce Burned (MSB), (b) Mosquito Spruce Unburned, (c) Boundary Creek Spruce (BS) and (d) Boundary Creek Birch (BB). Field site locations shown on map Fig. S1.

Figure 2

Boxplots of ALT and all vegetation and soil characteristics measured at each site. BB is Boundary Birch, BS is Boundary Spruce, MSB Mosquito Spruce Burned and MSU is Mosquito Spruce Unburned. Boxplots represent median, 1st and 3rd quartiles (line and box), whiskers represent maxima and minima and points represent outliers.

Figure 3

Partial residual plots for the main effects in the multiple regression model. Points represent active‐layer thickness (ALT) when all factors are held at their median values (partial residuals) and regression lines are derived from the multivariate tobit regression model. Partial residuals and regression lines (only presented where a significant main effect was found in the tobit model) have been back transformed to the original scale of ALT (exponential transformation with the base e).

Figure 4

Interaction plots with partial residuals derived from the multiple regression model. (a) Interaction between organic layer thickness (OLT) and slope; points represent partial residuals for OLT; dotted, solid and dashed lines show the relationship between active‐layer thickness (ALT) and OLT when slope is at its mean value (6.99°), one standard deviation (SD) above its mean value (10.5°) and one SD below its mean value (3.45°) respectively. (b) Interaction between tree canopy LAI (LAI_{T ree}) and deeper soil moisture (11–16 cm depth); points represent partial residuals for LAI_{T ree}; dotted, solid and dashed lines show the relationship between ALT and LAI_{T ree} when deeper soil moisture is at its mean value (0.33 m³ m⁻³), one SD above its mean value (0.55 m³ m⁻³) and one SD below its mean value (0.11 m³ m⁻³) respectively and (c) Interaction between understory canopy LAI (LAI_{U nderstory}) and deeper soil moisture; points represent partial residuals for LAI_{U nderstory}; dotted, solid and dashed lines show the relationship between ALT and LAI_{U nderstory} when deeper soil moisture is at its mean value (0.33 m³ m⁻³), one SD above its mean value and one SD below its mean value (0.11 m³ m⁻³) respectively. Significant stars in figure legends for individual relationships are as follows (*P < 0.05, **P < 0.01, ***P < 0.001).

Figure 5

Results of Bayesian structural equation model assessing the direct and indirect (soil moisture mediated) impact of vegetation and edaphic characteristics on ALT. Solid lines represent paths where the 95% highest density interval (HDI) for the coefficient did not include zero, whereas dashed lines included zero in the 95% HDI. The unstandardized path coefficient is shown on each path with the standardized coefficient in parentheses, with line thicknesses scaled in proportion to their standardized path coefficient. The curved grey arrow represents the covariance between the exogenous variables which is not displayed here to aid presentation. The overall posterior predictive P value for the model is 0.46 (with values close to 0.5 indicating close agreement with the data) and the model explained 70.5% of the variance in ALT. Convergence was achieved after 5907 iterations (convergence statistic <1.002).