Permafrost
Nearly one-fourth of exposed land surface in the northern hemisphere is contained within the permafrost zone (Figure 1). Globally, permafrost is found in higher latitudes and elevations, where mean annual soil temperature is below freezing, and is defined as ground that is continuously frozen for at least two consecutive years. In colder regions, such as the North Slope of Alaska, permafrost is continuous and underlying virtually the entire landscape. In relatively warmer subarctic regions, such as the boreal forest of interior Alaska, permafrost is discontinuous and found in locally cold settings (e.g., north-facing slopes and low-lying, poorly drained valley bottoms). In interior Alaska, temperature of the upper layers of the permafrost typically range from -0.5 to -2.0 °C and is near the point of thawing. Over the past century, the mean annual air temperature in interior Alaska has warmed 0.016°C year‑1 (Figure 2) (Hinzman et al. 2006) resulting in a gradual warming of permafrost (Osterkamp and Romanovsky 1999, Osterkamp 2005).
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Figure 1. Circumpolar distribution of permafrost in the Northern Hemisphere (from the International Permafrost Association) and the distribution of permafrost in Alaska (Ferrians 1998).
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Figure 2. Recent climatic changes in air temperature and precipitation at four stations in the circumarctic boreal forest (from Hinzman et al. 2006).
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Permafrost presence and the depth of summer thaw (the "active layer") are major controls on vegetation distribution and productivity in the boreal forest. In interior Alaska, discontinuous permafrost, topographic variations and varying surficial geology result in a mosaic of plant communities typified by white spruce (Picea glauca), birch (Betula papyrifera) and aspen (Populus tremuloides) stands on south-facing slopes with dry mineral soils with a thin organic horizon (2-10 cm organic layer), and black spruce (Picea mariana) stands on north-facing slopes and in low-lying areas with thick (50-100 cm) organic soils (Viereck et al. 1986).
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Figure 3. Conceptual diagram of the effect of permafrost thawing on climate. Permafrost C, once thawed, can enter ecosystems that have either predominantly oxic (oxygen present) or predominantly anoxic (oxygen limited) soil conditions. There is a gradient of water saturation on the landscape that ranges from fully oxic to fully anoxic, and ecosystems can become drier as permafrost thaws (shrinking lake area, drying wetland/peatlands), or wetter (thermokarst lakes). The soil oxygen status is a key determinant of the rate and form of C loss to the atmosphere. Decomposition in oxic soils releases primarily CO2, whereas anoxic decomposition produces both CH4 and CO2, but at a lower total emission rate. Fire releases mostly CO2, but also some CH4, and can burn upland and wetland ecosystems, although burning of organic soils at depth is restricted in wetter environments unless there is severe drought. These emissions of C through decomposition are offset by gross and net primary productivity (photosynthesis and net plant growth). Under some local conditions, it is possible that C will enter the permafrost pool (grey arrow), although this total amount is small relative to C that is expected to thaw from permafrost as a result of climate change. Abbreviations: C, carbon; CH4, methane; CO2, carbon dioxide; Ffire, carbon flux from fire; GPP, gross primary productivity; NPP, net primary productivity; Raut, autotrophic respiration; Rhet, heterotrophic respiration. From Schuur et al. (2008).
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Regions underlain by permafrost account for 16% of the land surface yet store nearly 50% of the world’s reactive soil carbon and has a central role in the global carbon cycle (Schuur et al. 2008, McGuire et al. 2009, Tarnocai et al. 2009). As permafrost thaws, the liberated soil carbon can be rapidly mineralized (Figure 3) (Zimov et al. 2006a, Zimov et al. 2006b, Schuur et al. 2008, Schuur et al. 2009) or exported from soils as dissolved organic and inorganic carbon in river flow (Striegl et al. 2005, Guo and Macdonald 2006, Neff et al. 2006, Petrone et al. 2006, Striegl et al. 2007). Recent data document significant losses of soil C with permafrost thaw that, over decadal time scales, overwhelms increased plant C uptake at rates that could make permafrost a large biospheric C source in a warmer world, similar in magnitude in the future to current C fluxes from land use change (Schuur et al. 2009).
Permafrost has a dominant control on watershed hydrology and biological processes in the boreal forest of interior Alaska. Permafrost forms an impermeable barrier and restricts subsurface flows to the shallow active layer of soils (the shallow soil layer above permafrost that freezes and thaws each year; Figure 3). Over summer, as depth of thaw increases, subsurface flowpaths through soils migrate deeper and can transition from organic to inorganic dominated soil horizons. In regions with discontinuous permafrost, such as interior Alaska, north facing slopes and valley bottoms are commonly underlain with permafrost, whereas warmer south facing slopes and ridge tops typically lack permafrost. Consequently, ground water flowing through north versus south facing slopes travels along different subsurface flowpaths, which has important implications for stream discharge (Bolton et al. 2000).
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Figure 4. Conceptual model of the influence of permafrost on watershed hydrology and biogeochemistry (modified from MacLean et al. 1999).
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References
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