Technical Thematic Report No. 9. - Trends in permafrost conditions and ecology in northern Canada
Implicatons of Changes in Permafrost Conditions
Permafrost is an important feature of the northern Canadian landscape and has impacts on the biophysical environment. Important inter-relationships exist between permafrost conditions, hydrological processes, soil conditions, and vegetation (Jorgenson et al., 2001; Hinzman et al., 2005). Permafrost and the ice-rich soil associated with it essentially provide the physical foundation for vegetation communities and ecosystems. Changes in permafrost conditions resulting from natural processes, climate change, or human activity can therefore have implications for both aquatic and terrestrial ecosystems.
A number of recent publications (for example Woo et al., 1992; Brown et al., 2004; Smith and Burgess, 2004; Mackenzie River Basin Board, 2004; ACIA, 2005) provide reviews of the linkages between permafrost conditions, hydrology, and vegetation including the implications of warming and thawing permafrost for aquatic and terrestrial ecosystems. Large quantities of moisture in permafrost are locked up as ground ice with only a thin layer (often less than one metre thick) of the overlying ground, the active layer, thawing and refreezing on an annual basis. Where permafrost is present, moisture and gas exchanges and biological processes are largely restricted to the seasonally-thawed active layer. Frozen ground plays an important role in northern hydrology through its influence on infiltration, runoff, and groundwater storage and flow (Woo et al., 1992).
Frozen ground and active layer thickness can influence rooting zone depth and soil moisture conditions which are important for vegetation succession and growth and also indirectly affect the hydrologic cycle through the influence on evapotranspiration (Woo et al., 1992; Hinzman et al., 2005).
Changes in the surface energy balance resulting from, for example, changes to vegetation cover due to natural processes (such as fire) or human activity (such as clearing for infrastructure construction) or changes in climate (air temperature and precipitation) can result in increases in ground surface temperature, and warming and thaw of permafrost (for example Mackay, 1995; Burgess and Smith, 2003; Smith et al., 2008). Ground surface settlement may occur as ice-rich permafrost thaws, a process referred to as thermokarst development (Jorgenson et al., 2008). Since ground ice conditions vary spatially, differential settlement may occur resulting in irregular topography. The impact of thermokarst development depends on ground ice and drainage conditions. Ponding (thermokarst ponds) may occur where settlement of ice-rich terrain occurs if drainage is poor.
In subarctic and boreal regions, flooding of tree roots may occur where drainage is poor, resulting in a change in the ecosystem structure as forests are replaced by wet sedge meadows, bogs, and thermokarst ponds and lakes (Jorgenson et al., 2001; Hinzman et al., 2005; Jorgenson and Osterkamp, 2005). The change in subsurface conditions and shift in ecosystem will be accompanied by changes in biological productivity, biomass, gas exchange, nutrient cycling, vegetation patterns and biodiversity (Racine et al., 1998; Lloyd et al., 2003; Lantz et al., 2009). In peatland areas, frozen peat plateaus that are normally forested may be replaced by ponds or sedge wetlands as ice-rich peat and underlying mineral soil thaws and collapses (Burgess and Tarnocai, 1997; Smith et al., 2008). Forested peatlands may therefore become fens (Aylsworth and Kettles, 2000; Christensen et al., 2004; Hinzman et al., 2005). The overall result of thermokarst development may be a new ecosystem that favours aquatic birds and other species instead of a forested ecosystem that supported land-based birds and mammals (Hinzman et al., 2005). Thermokarst processes, including expansion of lakes due to thaw slumping (Kokelj et al., 2009a), have also been found to alter the chemistry of tundra lakes which may have implications for aquatic ecosystems (Kokelj et al., 2009b).
Frozen peatlands store significant amounts of carbon. Climate warming in permafrost regions can therefore affect the carbon cycle through changes in greenhouse gas sources and sinks associated with thawing or burning of permafrost-affected peatlands (for example Robinson and Moore, 2000).
As thaw deepens over time, infiltration of water into the ground is less limited and may increase. Depending on precipitation and soil conditions, and therefore drainage characteristics, the upper layers of the soil may become drier which may impact ecosystem dynamics (Yoshikawa and Hinzman, 2003). These drier conditions may make vegetation more susceptible to forest fires (Hinzman et al., 2004; Hinzman et al., 2005).
Deepening of the active layer and breaching of the permafrost can facilitate drainage to the subsurface leading to drainage of wetlands, ponds, and lakes (Smith et al., 2005a). A number of studies have reported a drying trend in thermokarst lakes and other water bodies in various regions, such as the Old Crow Flats area of the Yukon (Labrecque et al., 2001), Alaska (Yoshikawa and Hinzman, 2003), and Siberia (Smith et al., 2005a). Where ground ice contents are high, thawing and erosion of drainage channels may result in catastrophic drainage of lakes, such as has occurred in northwest Canada (Marsh and Neumann, 2001; Marsh, 2008; Marsh et al., 2009).
Changes in permafrost causing transitions to drier conditions, such as shrub and forest tundra, can result in the loss of aquatic ecosystems. Plant communities that are unable to successfully colonize cold, poorly drained soils underlain by permafrost could expand under warmer drier conditions. Drier conditions associated with better drainage along thermokarst pond banks, for example, could support trees and large shrubs (Lloyd et al., 2003; Hinzman et al., 2004; Hinzman et al., 2005). It should also be noted that growth of shrubs and trees may, in turn, affect the ground thermal regime and permafrost conditions by catching snow which leads to warmer subsurface conditions in the winter and further promoting permafrost degradation (Smith, 1975).
In the polar desert of the high Arctic, the maintenance of a high water table is critical for the existence of patchy wetlands which provide hydrological and ecological conditions important to plants, insects, birds, and rodents (Woo and Young, 1998). A shallow active layer restricts drainage and maintains the high water table. Increases in active layer thickness resulting from warming of the ground will improve drainage and lower the water table. In addition, thawing of ice-rich permafrost beneath the wetland followed by slumping and erosion can lead to its demise and thermokarst processes may dissect the landscape leading to wetland drainage (Woo et al., 2006; Woo and Young, 2006). Loss of these wetlands can cause alterations in plant species and potential loss of wildlife habitat (Woo et al., 2006) especially for muskoxen which use the wetlands in summer.
Streamflow normally exhibits a quick response to snowmelt and rainfall events where permafrost is present as the active layer is easily saturated and most water travels to the stream as overland flow (Woo, 1976). Drainage basins in the permafrost region will therefore have a high runoff-to-rainfall ratio and once the precipitation or snowmelt event is over streamflow quickly declines as permafrost restricts groundwater flow (i.e. baseflow) to the stream (Kane et al., 1998; Lilly et al., 1998). As permafrost degrades and active layers thicken, subsurface flow will become more important leading to a more uniform distribution of streamflow throughout the year (Woo et al., 1992; Michel and Vaneverdingen, 1994; Hinzman et al., 2005). In many streams in the permafrost region there is often no or very little winter flow, but with permafrost degradation (particularly in the discontinuous permafrost zone), which may lead to talik (unfrozen zones) formation, winter base flow will increase which will sustain winter streamflow (Hinzman and Kane, 1992; Yoshikawa and Hinzman, 2003; Janowicz, 2008). Summer peak streamflows are also expected to decrease as permafrost degrades due to increased infiltration (and reduction in runoff) and subsurface flow (Yoshikawa and Hinzman, 2003; Hinzman et al., 2005).
These alterations in streamflow and water levels may lead to changes in aquatic ecosystems and fish habitat. In addition, the increased contribution of subsurface and groundwater flow to surface water bodies can lead to changes in water chemistry as dissolved load content increases, which may also affect fish and other aquatic life (Hinzman and Kane, 1992; Michel and Vaneverdingen, 1994; Hinzman et al., 2005; Frey and McClelland, 2009). Slope failures and erosion along rivers and streams associated with thaw of ice-rich permafrost and resulting loss of strength can result in increased siltation as well as damming of rivers and associated changes in the river course and possible upstream flooding (Aylsworth et al., 2000; Lamoureux and Lafreniere, 2009), all of which can effect aquatic habitats.
Loss of bearing strength, settlement of soils, and increased soil permeability that can accompany thawing of ice-rich permafrost also have important implications for northern infrastructure (for example Smith et al., 2001a; Couture et al., 2003). Of particular concern is the loss of integrity of containment structures, including, sumps and tailings ponds and piles and other waste storage sites, which often depend on the presence of permafrost to isolate contaminants from the surrounding environment. The inability to maintain frozen conditions can lead to increased soil permeability, loss of integrity of containment dams, and mobilization of contaminants which may have implications for both terrestrial and aquatic ecosystems (for example Dyke, 2001; Hayley and Horne, 2008; Furgal et al., 2008).
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