Soils in Arctic and boreal ecosystems store twice as much carbon as the atmosphere, a portion of which may be released as high-latitude soils warm. Some of the uncertainty in the timing and magnitude of the permafrost–climate feedback stems from complex interactions between ecosystem properties and soil thermal dynamics. Terrestrial ecosystems fundamentally regulate the response of permafrost to climate change by influencing surface energy partitioning and the thermal properties of soil itself. Here we review how Arctic and boreal ecosystem processes influence thermal dynamics in permafrost soil and how these linkages may evolve in response to climate change. While many of the ecosystem characteristics and processes affecting soil thermal dynamics have been examined individually (e.g., vegetation, soil moisture, and soil structure), interactions among these processes are less understood. Changes in ecosystem type and vegetation characteristics will alter spatial patterns of interactions between climate and permafrost. In addition to shrub expansion, other vegetation responses to changes in climate and rapidly changing disturbance regimes will affect ecosystem surface energy partitioning in ways that are important for permafrost. Lastly, changes in vegetation and ecosystem distribution will lead to regional and global biophysical and biogeochemical climate feedbacks that may compound or offset local impacts on permafrost soils. Consequently, accurate prediction of the permafrost carbon climate feedback will require detailed understanding of changes in terrestrial ecosystem distribution and function, which depend on the net effects of multiple feedback processes operating across scales in space and time.
Permafrost, or perennially frozen ground, underlies approximately 24 % of Northern Hemisphere land masses, primarily in Arctic and boreal regions (Brown et al., 1998). Soils in permafrost ecosystems have a seasonally thawed active layer that develops each summer. Organic carbon and nutrients in the active layer are seasonally subjected to mineralization, uptake by plants and microbes, and lateral hydrological transport. Carbon and nutrients locked in perennially frozen ground are considerably less active, often remaining isolated from global biogeochemical cycles for millennia (Froese et al., 2008). However, increases in temperature, associated with recent climatic change are warming soils in many high-latitude regions (Romanovsky et al., 2010), introducing permafrost carbon and nutrients to modern biogeochemical cycles (Schuur et al., 2015). Microbial activity may release some carbon and nutrients to the atmosphere in the form of carbon dioxide, methane, and nitrous oxide, greenhouse gases that contribute to further warming (e.g., Abbott and Jones, 2015; Koven et al., 2011; Voigt et al., 2017). While the magnitude of this permafrost-climate feedback remains uncertain, it is considered one of the largest terrestrial feedbacks of climate change, potentially enhancing human-induced emissions by 22 %–40 % by the end of the century (Comyn-Platt et al., 2018; Schuur et al., 2013, 2015).
A major source of uncertainty in estimating the timing and magnitude of the permafrost–climate feedback is the complexity of the soil thermal response of permafrost ecosystems to atmospheric warming. Permafrost soil temperature and its response to climatic change are highly variable across space and time (Jorgenson et al., 2010), owing to multiple biophysical interactions that modulate soil thermal regimes across Arctic and boreal regions (Romanovsky et al., 2010). Moving northward, permafrost temperature and active layer thickness generally decrease, while permafrost thickness and spatial extent increase. In more northern locations, the areal distribution of permafrost may be continuous (> 90 % areal extent), whereas at lower latitudes discontinuous, sporadic, and isolated permafrost (> 50 %–90%, 10 %–50 %, and < 10 % areal extent, respectively) (Brown et al., 1998) have large areas that are not perennially frozen. This general latitudinal gradient is interrupted by considerable local variability in active layer and permafrost thickness and temperature due to differences in local climate, vegetation, soil properties, hydrology, topography, and snow characteristics. These factors can increase or decrease the responsiveness of permafrost soil temperatures to climate, mediating a high degree of spatial and temporal variability in the relationship between air and permafrost soil temperatures (Jorgenson et al., 2010; Shur and Jorgenson, 2007). Understanding how ecosystem characteristics influence local and regional permafrost temperature is critical to interpreting variability in rates of recent permafrost temperature increases (Romanovsky et al., 2010), and to predicting the magnitude and timing of the permafrost–climate feedback. However, links between permafrost and climate could fundamentally change as Arctic and boreal vegetation (e.g., Pearson et al., 2013) and disturbance regimes (e.g., Kasischke and Turetsky, 2006) respond to climate change.
In this paper, we review how ecosystem structural and functional properties influence permafrost soil thermal dynamics in Arctic and boreal regions. We focus on how ecosystem responses to a changing climate alter the thermal balance of permafrost soils (energy moving into and out of permafrost soil) and how these thermal dynamics translate into seasonal and interannual temperature shifts. Our objectives are to (1) identify and review the key mechanisms by which terrestrial ecosystem structure and function influence permafrost soil thermal dynamics; (2) characterize changes in these ecosystem properties associated with altered climate and disturbance regimes; (3) identify and characterize potential feedbacks and uncertainties arising from multiple opposing processes operating across spatial and temporal scales; and (4) identify key challenges and research questions that could improve understanding of how continued climate-mediated ecosystem changes will affect soil thermal dynamics in the permafrost zone.
Permafrost soil thermal regimes can be characterized by four seasonal phases
annually. In spring, soil thaw onset occurs as day length increases energy
inputs and air temperatures, and snow melts. Thaw onset occurs fairly
rapidly, typically over a period of several days to weeks. During the
summer, thaw period soils accumulate energy resulting in deepening of the
active layer and warming of both frozen and unfrozen material. In autumn,
soil freeze-back occurs as day length and air temperatures decrease. The
length of the freeze-back period varies widely, from days to several months,
and is heavily dependent on soil moisture content. Finally, the winter
freezing period is characterized by energy losses to the atmosphere and
declining soil temperatures until day length increases available energy in
the spring and the annual cycle begins again. The permafrost soil thermal
regime is complex because it varies with depth, and the four phases are
connected. Key metrics used to characterize the soil thermal regime include
the length of the freeze-back and summer thaw periods, mean annual
temperature, the annual amplitude of mean temperature, and the ratio of air
to soil freezing/thawing degree days (i.e.,
Soil thermal dynamics in the permafrost zone are governed by
ground-atmosphere energy exchange and internal energy transfers associated
with phase changes of water and temperature gradients within the soil. The
simplified thermal balance at the ground surface is the difference between
net radiation (
Unlike lower-latitude ecosystems where
Key ecosystem controls on surface energy partitioning in relation to
permafrost soil thermal dynamics (energy fluxes are indicated by orange
arrows). Net radiation (
Vegetation canopies attenuate incoming solar radiation (Juszak et al., 2014,
2016), thereby reducing radiation at the ground surface and subsequently
Whereas increases in tree and shrub cover reduce solar radiation at the
ground surface, the increased canopy stature and complexity generally reduces
canopy albedo, leading to an overall increase of the canopy
Snow covers much of the Arctic and boreal regions for long periods each year and
is a critical driver of ground temperature (Goodrich, 1982; Stieglitz, 2003).
Deep and/or low-density snow has low
In tundra, shrub canopies trap blowing snow, leading to localized deepening of snow cover and higher winter soil temperatures (Domine et al., 2015; Liston et al., 2002; Marsh et al., 2010; Myers-Smith and Hik, 2013; Sturm et al., 2001, 2005). However, shrub canopies can bend in winter under the snowpack potentially leading to different amounts of snow trapping in years with heavy wet snow vs. dry snow in early winter (Marsh et al., 2010; Ménard et al., 2014). Even buried vegetation can lead to turbulent airflow that transports snow in complex patterns (Filhol and Sturm, 2015), which creates spatially variable ground temperatures in a given year. In some cases vegetation-snow interactions can also have a negative effect on winter ground temperature, leading to soil cooling. In northeast Siberia, large graminoid tussocks exposed above the snowpack in early winter create gaps in the insulating snow layer, which leads to lower ground temperatures, earlier active layer freezing and cooling of surface permafrost (Kholodov et al., 2012).
In the boreal forest, the presence of trees reduces the wind regime and snow redistribution (Baldocchi et al., 2000). While there is less wind-distribution in boreal forests than in the tundra, tree composition and density affect snow distribution and depth through interception of snow by the canopy branches and subsequent evaporation and sublimation. This results in lower snow inputs in dense forests and areas of shallow snow underneath individual trees (Rasmus et al., 2011). This winter effect of tree density on snow cover may, in part, explain the negative relationship found between larch stand density and ground thaw (Webb et al., 2017) and is consistent with the effects of winter warming experiments on summertime active layer dynamics (e.g., Natali et al., 2011). However, at the treeline or areas with patchy tree cover, forests can trap blowing snow, leading to decreased heat loss from soil in winter (Roy-Léveillée et al., 2014)
Tall-statured vegetation canopies that protrude above the snowpack decrease
land surface albedo. While the accompanying increases in
Across the annual cycle, the net effect of vegetation canopies on soil
thermal regimes remains unclear. Relatively few studies have simultaneously
examined the role of summer energy partitioning and winter snow trapping on
Ground cover in permafrost ecosystems may include bare soil, plant litter,
lichens, and mosses. Unlike vascular plant canopies, moss and lichen are in
close thermal contact with the underlying soil layers so heat can be
transferred from the vegetation into the soil (and vice versa) via conduction
(e.g., O'Donnell et al., 2009a; Yi et al., 2009). During the growing season,
differences in albedo and LE are the primary causes of variability in
Soil
Moisture content influences the thermal dynamics of soil and moss in a
variety of important ways. Linear increases in
Liquid water and water vapor can also warm soils through non-conductive heat transfer (Hinkel and Outcalt, 1994; i.e., water movement; Kane et al., 2001). Here, the timing and source of water is important. For example, infiltration of snowmelt in spring does not deliver substantial heat to the soil because the water temperature is very close to freezing (Hinkel et al., 2001) and near-surface soil horizons are mostly frozen. Alternatively, condensation of water vapor in frozen soils can lead to fairly rapid temperature increases during spring melt (Hinkel and Outcalt, 1994). Heat delivery from groundwater flow has been implicated as a cause for permafrost degradation in areas of discontinuous permafrost in interior Alaska (Jorgenson et al., 2010). The hydraulic properties of soil horizons are especially important in this regard. Unsaturated peat and organic-soil horizons with large interconnected pore spaces generally promote non-conductive transport of heat in soils unless the substrate is dry enough that it absorbs water.
The relative importance of non-conductive heat transfer on permafrost thermal
dynamics is difficult to determine. Observations of elevated soil
temperature, active layer thickness, and thermal erosion in areas with poorly
drained or inundated soils (e.g., Curasi et al., 2016; Jorgenson et al., 2010;
Woo, 1990) suggest the effects of soil moisture on
In wet soils the large latent heat content of soil moisture can delay
freezing of the active layer (i.e., extend the freeze-back duration;
Romanovsky and Osterkamp, 2000). The period during which soil active layer
temperatures remain constant near 0
Across the seasonal cycle soil and ground cover thermal properties interact
to affect the thermal regime in complex ways that vary across ecosystem
types. For example, a comparison of wet and dry microsites within tundra
ecosystems found warmer surface soils in dry microsites due to lower heat
capacity; however, deeper soil layers in the dry microsite remained cooler
because of lower thermal conductivity of dry surface soils (Göckede
et al., 2017). In wet microsites greater soil moisture lengthened the fall
freeze-back period meaning that soils were warmer than dry microsites;
however, once soils froze, temperatures in the wet microsites dropped rapidly
and became cooler than dry microsites because of higher
The mechanisms described in the previous sections are relatively well
understood individually and at seasonal timescales. When considered in
concert, the net effect of specific processes on annual ground temperatures
and thermal regimes is often unclear. This is particularly true when
ecological processes co-vary or have opposing effects on permafrost soil
thermal dynamics. For example, is the effect of canopy shading mitigated by
LW enhancement, or amplified by reductions in soil
Further complexity is added when processes are considered across the annual
cycle. The extent to which vegetation canopy effects on snow-distribution
impact growing season soil moisture, either via direct moisture inputs or
affects on growing season length, has not been thoroughly investigated. A
study examining interannual variability in snow cover found that
growing season energy partitioning was similar in a wet-fen after winters
with above- and below-average snowfall (Stiegler et al., 2016b). However, in
a nearby dry heath, below average snowfall resulted in earlier snowmelt and
reduced soil moisture during the lengthened growing season, which in turn
suppressed LE and
Disentangling the relative impacts of multiple ecosystem characteristics on
Vegetation productivity and community composition are changing in response to longer and warmer growing seasons associated with amplified climate warming across the Arctic and boreal regions. Relationships between air temperature and soil thermal regimes vary with ecosystem properties and will thus evolve as ecosystems respond to climate change. Ecosystem structural and functional characteristics that influence soil thermal dynamics may be altered directly by ecosystem responses to climate change, or indirectly by climatic alteration of disturbance processes that in turn modify ecosystems (e.g., O'Donnell et al., 2011a). In this section, we outline key ecosystem changes arising from direct and indirect climate responses (summarized in Fig. 2), and describe how these changes are likely to affect permafrost soil thermal regimes via impacts on processes described above.
Summary of key drivers of ecosystem change, and the associated
ecosystem responses observed (solid lines) or hypothesized (dashed lines) in
permafrost ecosystems. Arrows
(
In tundra ecosystems, increases in vegetation productivity inferred from satellite observations (Beck and Goetz, 2011; Jia et al., 2003) have been linked to shrub expansion and accelerated annual growth at locations throughout the Arctic (Forbes et al., 2010; Frost and Epstein, 2014; Macias-Fauria et al., 2012; Tape et al., 2006). However, warming experiments indicate that productivity increases may occur without shifts in the dominant vegetation type (Elmendorf et al., 2012b; Walker et al., 2006), and dendroecological observations illustrate that shrub responses to temperature are moderated by moisture and nutrient availability and are highly heterogeneous in space and time (Ackerman et al., 2017; Myers-Smith et al., 2015; Zamin and Grogan, 2012). Despite the high degree of heterogeneity in tundra vegetation responses to warming (Elmendorf et al., 2012a), there are several consistent changes that include increased vegetation height, increased litter production, decreased moss cover (Elmendorf et al., 2012b), and increased graminoid cover in lowland permafrost features (Johansson et al., 2006; Malhotra and Roulet, 2015; Malmer et al., 2005). However, reductions in greenness in some regions (referred to as “browning”) driven by, for example, reduced summer warmth index (Bhatt et al., 2013) or acute “browning events” from disturbances such as winter frost droughts (Bjerke et al., 2014; Phoenix and Bjerke, 2016) add complexity to predicting vegetation change and hence subsequent impacts on permafrost.
Below-ground vegetation dynamics are more difficult to study, but recent observations indicate that the below ground growing season length (period of unfrozen temperatures allowing for plant growth) can be greater than that above ground (Blume-Werry et al., 2015; Radville et al., 2016). These differences likely vary with depth due to effects related to the progression of soil freezing and thawing (Rydén and Kostov, 1980). Thus, rooting depth and lateral root distributions will influence the below-ground phenology differentially for deep-rooted (e.g., sedge) vs. shallow-rooted (e.g., shrub) species (Bardgett et al., 2014; Iversen et al., 2015), which may alter soil moisture via plant water uptake under future warming related vegetation changes. The changing above- and below-ground growth phenology of tundra plants (Blume-Werry et al., 2015; Iversen et al., 2015; Radville et al., 2016) could also favor the proliferation of certain functional groups or species creating potential feedbacks to vegetation change. In addition to below-ground phenology, total root production could also increase in response to warming (e.g., Xue et al., 2015). However, increased nutrient availability from warming could decrease root production relative to above-ground production (Keuper et al., 2012; Poorter et al., 2012). Improved understanding of interactions between root dynamics and soil moisture may help to understand thermal changes in permafrost soils during the summer thaw and fall freeze-back periods.
Determining the net effect of tundra vegetation productivity changes on soil
thermal regimes requires improved understanding of the magnitude and spatial
extent of changes in vegetation stature and rooting dynamics. Enhanced tundra
vegetation productivity may reduce summer soil temperatures via ground
shading and increase winter soil temperatures via effects on snow depth and
density. The effect of declining moss cover will depend on the balance
between reduced insulation (i.e.,
Boreal forest responses to climate in recent decades were generally more heterogeneous than those observed in tundra ecosystems, due to a variety of interacting factors including species differences in physiology, disturbance regimes, and successional dynamics. Initial satellite observations of boreal forest productivity increases (Myneni et al., 1997) have slowed or even reversed in recent decades (Beck and Goetz, 2011; Guay et al., 2014). Tree ring analyses confirm productivity declines associated with temperature induced drought stress in interior Alaska boreal forests (Barber et al., 2000; Juday et al., 2015; Walker and Johnstone, 2014; Walker et al., 2015), and have been used to corroborate satellite observations (Beck et al., 2011). Similarly, drought-induced mortality has been observed at the southern margins of Canadian boreal forests (Peng et al., 2011), where correspondence between satellite and tree ring records have also been observed (Berner et al., 2011). In Siberia, positive forest responses to air temperatures observed in tree rings and satellite observations near latitudinal tree lines give way to declines in tree growth further south (Berner et al., 2013; Lloyd et al., 2010). These results are in line with ecosystem-scale observations of suppressed transpiration under high vapor pressure deficits and low soil moisture conditions (Kropp et al., 2017; Lopez C et al., 2007). More generally, forests growing on continuous permafrost exhibit more widespread productivity increases (Loranty et al., 2016), suggesting that permafrost may buffer against drought stress. However, waterlogged soil resulting from permafrost thaw can also lead to unstable soils and forest mortality (Baltzer et al., 2014; Helbig et al., 2016a; Iijima et al., 2014).
The extent to which ongoing boreal forest productivity changes influence
permafrost soil thermal dynamics is not entirely clear. If forest canopy
cover changes with productivity (e.g., canopy infilling or increased leaf
area), then changes in ground shading and LW dynamics could alter ground
thermal regimes. Increases in forest cover have been observed in northern
Siberia (Frost and Epstein, 2014); however, it is unclear whether the cause
is climate warming or ecosystem recovery after a fire. Conversely, productivity
declines are more pronounced in high-density forests (Bunn and Goetz, 2006)
and, consequently, browning trends associated with mortality in southern
boreal forests (Peng et al., 2011) may increase radiation at the ground
surface. Additionally, if browning is indicative of drought stress,
vegetation may enhance the insulation of organic soils by further depleting
of soil moisture via plant water uptake (Fisher et al., 2016). Forest
mortality and declines in canopy cover in southern boreal forests as a
consequence of permafrost thaw (Helbig et al., 2016a) may feedback positively
to permafrost thaw. Functional changes (e.g., stomatal suppression of
transpiration in response to drought) occur more quickly than structural
changes, so boreal forest effects on soil moisture will likely be an
important driver of changes in soil thermal regimes. In addition there has
been relatively little work on how the effects of forest distribution on snow
cover alters
Wildfire is the dominant disturbance in boreal forests and is increasingly present in Arctic tundra.
Wildfire influences surface energy dynamics via impacts on vegetation and
surface soil properties, likely accelerating permafrost thaw (Brown et al.,
2015; Burn, 1998; Jafarov et al., 2013; Jones et al., 2015; O'Donnell et al.,
2011a; Viereck et al., 2008).Vegetation combustion and mortality increases
radiation at the ground surface. The combustion and charring of moss and
organic soil lowers albedo and increases
The magnitude of wildfire effects on soil temperature is closely linked to burn severity, as indicated by the degree of organic soil combustion and the post-fire organic horizon thickness (Kasischke and Johnstone, 2005). Post-fire recovery of the organic-soil horizon can allow recovery of soil temperature and active layer thickness to pre-fire conditions (Rocha et al., 2012). However, relatively warm discontinuous zone permafrost is often ecosystem-protected by vegetation and organic horizons (Shur and Jorgenson, 2007), thus loss or reduction of organic soil may result in the irreversible thaw or loss of permafrost (Jiang et al., 2015; Romanovsky et al., 2010). Site-based model simulations suggest that fire-driven change in organic-horizon thickness is the most important factor driving post-fire soil temperature and permafrost dynamics (Jiang et al., 2015).
Wildfire impacts on permafrost also vary spatially with ecosystems and topography. For instance, south-facing forest stands tend to burn more severely than north-facing stands (Kane et al., 2007). Further, poorly drained toe-slopes burn less severely than more moderately drained upslope landscapes. These topographic effects on burn severity can strongly influence the response of soil temperature and permafrost to fire (O'Donnell et al., 2009b). The loss of transpiration due to the combustion of trees may result in wetter soils in recently burned stands compared to unburned stands (O'Donnell et al., 2011a). However, other studies have documented drier soils in burned relative to unburned stands (Jorgenson et al., 2013), particularly at sites underlain by coarse-grained, hydrologically conductive soils. Post-fire thawing of permafrost can increase the hydraulic conductivity of mineral soils due to ice loss, leading to enhanced infiltration of soil water and soil drainage. Post-fire changes in soil moisture and drainage can function as either a positive or negative feedback to permafrost thaw (O'Donnell et al., 2011b). Recent evidence also indicates that mineral soil texture is an important control on post-fire permafrost dynamics (Nossov et al., 2013).
While the magnitude of fire effects on
Recent warming at high latitudes has increased the spatial extent, frequency, and severity of wildfires in North America (Rocha et al., 2012; Turetsky et al., 2011) to levels that are unprecedented in recent millennia (Hu et al., 2010; Kelly et al., 2013). Fire regimes in boreal forests in Eurasia remain poorly characterized (Kukavskaya et al., 2012), though several studies indicate that fire extent and frequency are likely increasing with climate warming (Kharuk et al., 2008, 2013; Ponomarev et al., 2016). Circumpolar wildfire in the boreal forest and Arctic tundra are projected to substantially increase by the end of the century due to direct climate forcing and ecosystem responses (Abbott et al., 2016). Recovery of soil thermal regimes and permafrost after fire is strongly influenced by ecosystem recovery, and recent studies have established links between burn severity and post-fire succession (Alexander et al., 2018; Johnstone et al., 2010). Consequently, in North America burn severity is likely the dominant factor controlling the effects of wildfire on permafrost soil thermal regimes both through direct influences on soil thermal regimes and indirectly through influences on post fire succession.
In boreal North America, low-severity fires in upland black spruce forest
typically foster self-replacing post-fire vegetation trajectories while
high-burn severity fosters a transition to deciduous dominated forests.
(Johnstone et al., 2010). In addition to changes in canopy effects on ground
shading, this transition also leads to reductions in post-fire accumulation
of the soil organic layer (Alexander and Mack, 2015). Observations of mean
annual soil temperatures that are 1–2
In Siberian larch forests, post-fire recovery is impacted by fire severity and seed dispersal (Fig. 3). High burn severity fires promote high rates of seedling recruitment and subsequent forest stand density (Alexander et al., 2018; Sofronov and Volokitina, 2010) when dispersal is not limited. However, as larch are not serotinous and seed rain varies from year to year, high burn severity does not guarantee succession to high-density forests. Recovery tends to be slow and highly variable (Alexander et al., 2012b; Berner et al., 2012). Wide ranges of post-fire moss accumulation and forest regrowth have been observed, though consequences for permafrost are unclear (Furayev et al., 2001). Observed declines in permafrost thaw depth with increasing canopy cover (Webb et al., 2017) support the notion of a link between fire severity and permafrost soil thermal dynamics. However, the combined effects of fire and climatic warming and drying could lead to widespread conversion of larch forests to steppe (Tchebakova et al., 2009), whereas declines in fire could result in increased cover of evergreen needleleaf species (Schulze et al., 2012). Thus the impacts of fire on permafrost in Siberia will depend on the combined effects of climate and fire severity.
Impacts of fire on ecosystem structure in Siberian larch forests. A
firebreak near the town of Cherskii
In tundra ecosystems fire is becoming increasingly common (Rocha et al., 2012). Fire-induced transitions from graminoid- to shrub-dominated ecosystems have been observed in several instances (Jones et al., 2013; Landhäusser and Wein, 1993; Racine et al., 2004), while in others recovery of graminoid-dominated ecosystems has occurred, especially when fire leads to ponding (Barrett et al., 2012; Loranty et al., 2014b; Vavrek et al., 1999). If unusually large tundra fires with high burn severity (e.g., Jones et al., 2009) occur more regularly fire induced transitions from graminoid to shrub tundra may become more common (Jones et al., 2013; Lantz et al., 2013). A shift to shrub dominance could buffer permafrost soils from continued climate warming during summer (e.g Blok et al., 2010; Myers-Smith and Hik, 2013) or promote warmer soils in winter (Lantz et al., 2013; Myers-Smith and Hik, 2013) at the ecosystem-scale, depending on how topography and the spatial distribution of shrubs impact snow redistribution (Essery and Pomeroy, 2004; Ménard et al., 2014). In addition, there is evidence that thermal erosion as a consequence of fire may facilitate shrub transitions, especially in areas of ice-rich permafrost (Bret-Harte et al., 2013; Jones et al., 2013), and the associated changes in local hydrology and topography will also impact soil thermal regimes.
Across Arctic and boreal ecosystems increased fire extent and severity will
increase summer
Permafrost thaw can occur in two primary modes, depending on pre-thaw ground ice content. In terrain underlain by low ground ice content (typically < 20 % by volume), the soil profile can thaw from the top down without disturbing the surface in what is termed thaw-stable permafrost degradation (Jorgenson et al., 2001). Alternatively, in ice-rich terrain, when ground ice volume exceeds unfrozen soil pore space (usually > 60 %), permafrost thaw causes surface subsidence or collapse, termed thermokarst (Kokelj and Jorgenson, 2013). Thermokarst is the predominant disturbance in Arctic tundra and is an important disturbance in boreal forests underlain by permafrost (Lara et al., 2016). Recent evidence indicates increasing prevalence of thermokarst features during the last half-century (Jorgenson et al., 2006, 2013; Liljedahl et al., 2016; Mamet et al., 2017), though circum-Arctic prevalence and change of thermokarst extent are poorly constrained (Lantz and Kokelj, 2008; Olefeldt et al., 2016; Yoshikawa and Hinzman, 2003). Thermokarst features form over the course of weeks to decades, can involve centimeters to meters of ground surface displacement, and typically lead to dramatic changes in ecosystem vegetation and soil properties (e.g., Douglas et al., 2016; Osterkamp et al., 2000; Wagner et al., 2018). Thermokarst could affect 20 %–50 % of the permafrost zone by the end of the century, according to projections of permafrost degradation and the distribution of ground ice (Abbott and Jones, 2015; Slater and Lawrence, 2013; Zhang et al., 2000). Upland thermokarst in the discontinuous permafrost zone already impacts 12 % of the overall landscape in some areas and up to 35 % of some vegetation classes (Belshe et al., 2013).
Following initial thaw, hydrologic conditions play an important role in the subsequent evolution of thermokarst features because the high thermal conductivity of water can increase heat flux to the active layer and permafrost (Nauta et al., 2015). Lowland and upland thermokarst may have contrasting effects on surface hydrology, with lowland thermokarst initially increasing wetness (e.g., O'Donnell et al., 2012), but eventually leading to greater drainage if permafrost is completely degraded (Anthony et al., 2014). Upland thermokarst can either increase or decrease surface wetness, depending on soil conditions and local topography (Abbott and Jones, 2015; Abbott et al., 2015; Mu et al., 2017). Redistribution of water to thermokarst pits and gullies can lead to drying in adjacent areas that have not subsided (Osterkamp et al., 2009). In winter, increases in snow accumulation in thermokarst depressions insulates soils (Stieglitz, 2003).
Ecological responses to thermokarst formation can act as either positive or negative feedbacks to continued thaw, depending on how thermokarst formation affects vegetation and hydrology, including snow cover (Kokelj and Jorgenson, 2013). Active layer detachments in uplands remove vegetation and organic soil, increasing energy inputs to deeper soil layers. In upland tundra, shifts from graminoid- to shrub-dominated vegetation communities have been observed with thaw, though communities varied locally with microtopography created by thermokarst features themselves (Schuur et al., 2007). In boreal forests, thermokarst and permafrost thaw can cause transitions to wetlands or aquatic ecosystems (Jorgenson and Osterkamp, 2005); whereas, vegetation community shifts are more subtle in uplands (Jorgenson et al., 2013). Permafrost thaw may also lead to a more nutrient-rich environment (Harms et al., 2014; Keuper et al., 2012), but this depends on local soil properties. The succession of aquatic or terrestrial vegetation can curb thaw through negative feedbacks associated with canopy cover and organic soil accumulation and aggrade permafrost (Briggs et al., 2014). Hydrologic changes associated with thermokarst likely have a stronger influence on the soil thermal regime than associated ecosystem changes, in part because the former occur more rapidly than the latter. Under thaw stable conditions there is the possibility that enhanced vegetation productivity could lead to summer soil cooling; however, the effects on soil composition and moisture, and snow distribution will also affect the thermal regime and are as yet unclear.
A large portion of the circumpolar Arctic is grazed by reindeer and caribou
(both
The most extensive direct anthropogenic disturbances within the permafrost
zone occur in three regions that have experienced widespread hydrocarbon
exploration and extraction activities: the North Slope of Alaska, the
Mackenzie River delta in Canada, and northwestern Russia, including the Nenets
and Yamalo-Nenets Autonomous Okrugs. The types of terrestrial degradation
commonly associated with the petroleum industry have historically included
rutting from tracked vehicles; seismic survey trails; pipelines, drilling
pads and roads and the excavation of the gravel and sand quarries necessary
for their construction (Huntington et al., 2013; Walker et al., 1987). A
single pass of a vehicle over thawed ground can create ruts with increased
More recently, gravel roads and pads have become common; however, this elevated infrastructure causes other unanticipated impacts to the permafrost from accumulated dust, snow drifts, and roadside flooding (Auerbach et al., 1997; Raynolds et al., 2014; Walker and Everett, 1987, 1991). Over time, the warmer environments adjacent to roads have led to strips of earlier phenology and shrub vegetation and even trees along both sides of most roads and buried pipeline berms in the Low Arctic (Gill et al., 2014). Aeolian sand and dust associated with gravel roads or quarries can affect tundra vegetation and soils up to 1 km from the point source (Forbes, 1995; Myers-Smith et al., 2006). At present, there is concern that climate warming and infrastructure are combining to enhance melting of the top surface of ice-wedges, leading to more extensive ice-wedge thermokarst (Liljedahl et al., 2016; Raynolds et al., 2014) and cryogenic landslides (Leibman et al., 2014) in areas of intensive development. The proportion of permafrost ecosystems affected by anthropogenic disturbance is not well quantified, but it will continue to increase in coming decades.
Interactions between ecosystem scale microclimate feedbacks and regional or global climate feedbacks stemming from ecological change are complex and represent a key source of uncertainty related to understanding permafrost soil responses to continued climate warming. If changing ecosystem characteristics influencing permafrost thermal dynamics described above are widespread, the accompanying changes in land surface water and energy exchange will feed back to influence regional climate, and changes in greenhouse gas dynamics will feed back on global climate (Chapin III et al., 2000b). Therefore, ecosystem changes that alter local permafrost soil thermal dynamics may also lead to regional and global climate feedbacks that compound or offset ecosystem-scale effects (Table 1).
Key ecosystem changes, associated drivers, and feedback effects on local soil climate and regional to global climate.
The net biogeochemical climate effects of ecosystem change across permafrost
regions will be a balance of changes in
The net
The effects of thermokarst on greenhouse gas dynamics depend largely on
associated hydrological changes. With increased drainage and surface drying,
increased oxidation rates reduce carbon accumulation (Robinson and Moore,
2000) and enhance
The biophysical effects of ecosystem change arising from shifts in surface
energy partitioning have climate feedback effects at scales ranging from
local to regional and global. Whereas biogeochemical climate feedbacks will
influence global temperature in conjunction with many other carbon cycle
processes, biophysical feedbacks operating at local and regional scales are
likely to influence the spatial and temporal patterns of permafrost thaw with
continued warming. As described in the previous sections, changes in
vegetation composition and structure alter soil thermal dynamics via changes
in
Decadal ecosystem responses to climate inferred from “greening” or
“browning” trends are the most spatially pervasive change affecting
vegetation in the permafrost zone (Loranty et al., 2016). Increases in leaf
area and/or vegetation stature will generally reduce albedo, and these
effects are particularly pronounced during the spring and fall if enhanced
productivity leads to increased snow-masking by vegetation (Loranty et al.,
2014a; Sturm et al., 2005). Reductions in albedo will lead to sensible
heating of the atmosphere (Chapin III et al., 2005) that may counteract the
effects of canopy shading on
A second important but relatively unexplored feedback relates to evaporative cooling of the land surface associated with increases in LE (Helbig et al., 2016b; but see Swann et al., 2010). Productivity increases are likely accompanied through increases in evapotranspiration (Zhang et al., 2009), which have been shown to mitigate temperature increases at global scales by increased cloud cover that reduces incoming short-wave radiation reaching the Earth's surface (Zeng et al., 2017). During the growing season, this cooling could effectively reduce the degree of atmospheric sensible heating associated with increased albedo, and would be particularly important if there is no change in snow masking by vegetation (e.g., greening in tundra without shrub expansion or in closed canopy boreal forest). However, the extent to which latent cooling with enhanced productivity may offset sensible heating associated with albedo decreases is uncertain for several reasons. First, model experiments simulating shrub expansion, for example, utilize canopy parameterizations for deciduous boreal tree species, because Arctic shrub canopy physiology has not been thoroughly characterized (e.g., Bonfils et al., 2012). Second, existing observations indicate an increasing degree of stomatal control on evapotranspiration with vegetation stature (Eugster et al., 2000; Kasurinen et al., 2014), indicating that LE will not necessarily continue to increase with climate warming, which is supported by the emergence of browning trends. Additionally, climatic changes in Arctic hydrology are highly uncertain and likely to vary spatially (Francis et al., 2009), meaning that LE may be limited by hydrology in some places but not others. Lastly, disturbance processes will also alter surface energy dynamics through short-term direct impacts on ecosystem structure and long-term impacts on post-disturbance succession (as described above).
The effects of climatic change on permafrost thermal dynamics depend directly on terrestrial ecosystem properties, which mediate surface energy partitioning and soil thermal characteristics. Relationships between permafrost and climate vary spatially with ecosystem properties and processes, and these patterns vary through time on event to millennial timescales. The changing nature of permafrost thermal regimes will be driven by surface energy feedbacks operating on local-, regional-, and global-scales. Complex interactions among many of these feedbacks create uncertainty surrounding the timing and magnitude of the permafrost carbon feedback.
Continued ecosystem-scale research focused on several key process interactions will improve our understanding of ecological influences on soil thermal regimes. The influence of plant water use on spatial and temporal variability in soil moisture is unclear. Future work should seek to elucidate interactions between vegetation and soil moisture. The extent to which changes in decomposition rates and litter substrate quantity and quality alter the insulating effects of ground cover and the soil organic layer is also unclear and could benefit from continued research. More research on relationships between the spatial distribution of vegetation canopies and the insulative properties of snow is also needed, especially in boreal forests. Lastly, more studies should involve year-round data collection focused on understanding time-lags and the cumulative effects of seasonal processes. In particular the net thermal effects of canopy shading vs. snow-trapping, seasonally lagged effects of snow cover, and seasonally lagged effects of soil moisture could all be better understood through focused observational studies.
Improved process level understanding of ecosystem influences on soil thermal regimes will not be useful for predicting the fate of permafrost carbon unless the processes that control the timing, extent, and trajectories of ecosystem change are known. There has been a strong focus on graminoid–shrub transitions in tundra ecosystems, yet there are a number of other potential vegetation transitions, many mediated by disturbance, with equally important implications. Changes in boreal forest structure and function underlying productivity trends need to be elucidated. Continued work focused on understanding how changing fire regimes influence soils and post-fire succession is also important, especially in tundra and Siberian boreal forests. These changes are not spatially isolated, and compounding disturbances will likely become increasingly important to understand. In addition to vegetation changes, constraining the proportion of landscapes affected by drying vs. waterlogging associated with initial permafrost thaw is central to predicting both soil organic matter stocks and vegetation responses to climate warming. Whether precipitation increases or decreases with climate warming remains highly uncertain, and this will exert strong influence on vegetation and ecosystem responses to climate as well as disturbance mediated ecosystem changes.
Lastly, changes in ecosystem vegetation and soil characteristics that occur over sufficiently large spatial scales will affect soil thermal regimes via feedbacks to regional and global climate with the potential to amplify or attenuate local ecosystem-scale feedbacks. For example, could wetland expansion associated with widespread permafrost thaw lead to regional cooling through increased albedo, or might warming as a result of increased methane emissions offset this? Could increased evapotranspiration associated with enhanced vegetation productivity lead to surface cooling and cloud formation that cools soils in summer, or might the rise in atmospheric water vapor increase late summer precipitation and extend the fall freeze-back period? Complex feedback processes such as these will likely affect the trajectory of permafrost responses to climate. Continued efforts to understand the fate of permafrost in response to climate will require integrated analyses of processes affecting permafrost soil thermal regimes, changing circumpolar ecosystem distributions, and the net effects of resulting climate feedbacks operating across a range of spatial and temporal scales.
Data referenced in Fig. 3 are available at
MML conceived and led the study. All authors contributed to writing and editing the manuscript.
The authors declare that they have no conflict of interest.
This project benefited from input from members of the Permafrost Carbon
Network (