Question

Reference: Varner J, Dearing MD (2014) The Importance of Biologically Relevant Microclimates in Habitat Suitability Assessments

Answer must be a few sentences long.

Q7. The authors found that removing moss from a surface increased surface temperature while adding moss to a surface decreased surface temperature. Using your knowledge of soil temperature energy balance, why do you think this occurred?

Answer 1

The mass of the permafrost carbon (C) stock is estimated to be almost twice that of the atmosphere, totalling ca. 1300 Pg (Hugelius et al., 2014). As permafrost thaws an increasing amount of previously frozen C is exposed to microbial decomposition and hence can be transferred to the atmosphere and hydrosphere (Zimov et al., 2006; Schuur et al., 2009; Schaefer et al., 2011). This transfer is of major concern given that high latitudes are predicted to experience the fastest rate of warming compared to the rest of the globe (IPCC, 2013). Observations over recent decades demonstrate that permafrost is warming, thinning and shrinking in area (Romanovsky et al., 2010). Therefore, to accurately predict the release of carbon from thawing permafrost and its feedback to climate, it is essential to fully understand the controls on permafrost thaw.

In the early stages of permafrost degradation, thickening of the active layer (the seasonally thawed soil layer above permafrost in which biological activity takes place) is thought to be the dominant process (Schuur et al., 2008). Although climatic warming is important in increasing active‐layer thickness (ALT), the strength of the relationship between air temperature and ALT varies substantially between different regions and may be strongly influenced by factors such as vegetation cover and edaphic properties (Jorgenson et al., 2010; Shiklomanov et al., 2010; Shiklomanov & Nelson, 2013). As a result of the surface offset (the difference between air temperature and near‐surface ground temperature) provided by ground cover and surface conditions, permafrost can persist in areas where the mean annual air temperature (MAAT) is as high as +2 °C, or degrade in areas where MAAT is −20 °C (Jorgenson et al., 2010). Therefore, along latitudinal gradients, increasing vegetation cover southward may compensate for greater summer warmth, weakening the relationship between MAAT and ALT (Walker et al., 2003). At finer scales, within catchments or hill slopes, ecosystem characteristics may play the dominant role in driving ALT (Jorgenson et al., 2010).

Several vegetation characteristics can influence soil temperature and hence ALT. Increasing leaf area reduces the amount of radiation reaching the soil, which should act to reduce ALT and hence protect permafrost (Marsh et al., 2010). However, with increasing stem density, particularly in shrubby species, vegetation can trap more snow, which insulates the ground and reduces heat loss in winter, potentially increasing ALT in the subsequent thaw season (Sturm et al., 2001). Experimental removal of shrub or dwarf shrub and nontussock sedge cover in Siberian and Alaskan tundra has been shown to increase ALT considerably (Kade & Walker, 2008; Blok et al.,2010). In the boreal region, the tree canopy leaf area performs a similar shading role to that of the understory, but evergreen canopies also trap snow aloft and reduce snow cover on the ground. This trapping may increase conductive heat loss from the ground in winter, which may decrease ALTs and so protect permafrost (Yi et al., 2007).

Mosses are another important component of high latitude vegetation (Street et al., 2012, 2013), and have been largely neglected in coupled C‐climate models (Turetsky et al., 2007, 2012). Mosses strongly dampen temperature fluctuations in the soil, largely because their open structure makes them effective insulators. However, their thermal conductivity is strongly influenced by their moisture content (Gornall et al., 2007; O'Donnell et al., 2009). In summer, a dry moss layer minimizes downward heat conduction, whereas when wet during the shoulder seasons, and when frozen in winter, the higher thermal conductivity increases upward heat conduction (Burn & Smith, 1988). Both processes keep the ground cool, thus reducing ALTs. Because the thermal properties of mosses can be explained solely by their physical properties (such as mat thickness and moisture content), this should simplify their inclusion in processed‐based models (Soudzilovskaia et al., 2013).

The thickness of the soil organic layer beneath the moss layer performs a similarly important role in determining ALT (Johnson et al., 2013). The low bulk density of organic relative to mineral soils means organic soils can present more varied and extreme air and water contents, leading to a much greater range of thermal conductivities and specific heat capacities. Moisture content plays a major modifying role in the thermal properties of the soil organic layer, as it does for moss (O'Donnell et al., 2009), and can also influence ALT by nonconductive heat transfer through movement of liquid water and vapour (Hinkel & Outcalt, 1994; Kane et al., 2001). ALT monitoring in Canada has revealed that within‐site variation is much reduced at sites with homogeneous thin organic layers, but where large variations in organic layer thickness or its water content exist, ALT is much more variable (Smith et al., 2009).

Our goal here is to better understand the magnitude of effects and relative importance of the multiple vegetation and edaphic characteristics that influence ALT. Such an understanding is particularly important given that climate change is likely to have contrasting impacts on different ecosystem characteristics. For instance, the tree line will (overall) move poleward with climate warming while tundra shrub cover is also predicted to increase (Grace, 2002; Jia et al., 2009; Forbes et al., 2010). Greater shrub cover, however, is likely to reduce moss cover and may over time result in thinner organic layers (Walker et al., 2006). Furthermore, fire activity in boreal forest and tundra is likely to increase in the future, causing further changes to vegetation structure and organic layer thickness and strongly influencing soil moisture (Stocks et al., 1998 Kelly et al., 2013). Additionally, these factors affecting ALT are likely to be of particular importance within the discontinuous and sporadic permafrost regions, which are typically dominated by boreal forests, as these areas have relatively warm (−0.2 °C) and thin permafrost, which may be particularly vulnerable to thaw (Smith et al., 2005; Baltzer et al., 2014).

Here, we aim to quantify the influence and importance of vegetation and soil characteristics in driving ALT in boreal forests, which cover over 50% of permafrost regions globally (Osterkamp et al., 2000). Our study includes four field sites within the discontinuous permafrost zone in the Northwest Territories, Canada. These incorporate different fire histories, substrates and tree canopies (deciduous or evergreen) that capture three representative and contrasting boreal forest cover types [black spruce (Picea mariana) at two sites of differing canopy density, a burned black spruce site and a paper birch (Betula papyrifera) site]. We employed a stratified sampling strategy to encompass the full range of variation in vegetation and edaphic characteristics within each site to produce the most detailed fine scale survey of the links between ALT, vegetation and soil characteristics to date. Specifically, we hypothesized that (i) increasing canopy and understory LAI would decrease ALT; (ii) taller understory vegetation would increase ALT, (iii) increasingly thick moss and soil organic layers would decrease ALT and (iv) increasing soil moisture would increase ALT. In addition to addressing these hypotheses, our approach allowed us to determine the relative importance of these different drivers of ALT, and how they interact to determine ALTs