Permafrost

Permafrost is ground that is continuously frozen through at least two successive cold seasons and the intervening summer. Globally, permafrost is found in higher latitudes and elevations, where mean annual soil temperature is below freezing (Brown and Andrews 1982; Young 1989). Nearly one-fourth of the earth's surface is influenced by permafrost (Brown and Andrews 1982). Ice-rich permafrost is commonly impervious to infiltration of water (Young 1989), providing the basis for the "paradox of the arctic," i.e. the abundance of saturated soils and impressive biological productivity of vegetation despite very low annual precipitation. In colder regions such as the North Slope of Alaska, permafrost is "continuous," underlying virtually the entire landscape except beneath large water bodies. In the somewhat warmer subarctic regions such as the taiga of central Alaska, permafrost is discontinuous and is found in locally cold settings (e.g. north-facing slopes and low-lying, poorly drained locales). In interior Alaska, where permafrost is present, temperatures of the upper layers of the permafrost are in the range of -0.5C to -2.0C.

Permafrost presence and the depth of summer thaw (the "active layer") are major controls on vegetation distribution and productivity. In central 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). Soil carbon density is high in boreal ecosystems compared to other, warmer systems (Schlesinger 1977; Post et al. 1982). Soil carbon densities reflect the balance between input (organic matter production) and decomposition. In the cold and often water-saturated soils common at high latitudes, decomposition is much reduced and, while primary production is low, soil carbon still accumulates over very long time periods. Thus, there is a positive relationship between the amount of soil organic matter and the amount of permafrost in a watershed.

Permafrost also affects the hydrological regimes of subarctic streams: streams dominated by permafrost are more "flashy," i.e. more extreme, than those relatively permafrost-free (Carlson 1974; Dingman 1973). Snowmelt runoff is later and higher in a permafrost-dominated basin than snowmelt runoff from a permafrost-free basin. Likewise, peak stormflow discharge from a permafrost-dominated basin is much higher than in a non-permafrost stream (Figure 1), but during rain-free periods and in winter, flow is much lower (Haugen et al, 1982). This pattern is a result of the flow-paths of precipitation as it travels to the stream. On permafrost dominated north-facing slopes, precipitation enters the thick organic layer and flows above the permafrost to the stream. On permafrost-free south-facing slopes, precipitation enters the bedrock groundwater, and is released much more slowly to the stream. Differences in discharge result in different patterns of carbon and sediment flux from basins with differing amounts of permafrost, and likely result in different patterns of channel geomorphology and nutrient flux (see below). Thus, the synergistic effects of altered flowpath and organic carbon accumulation combine to make permafrost soils true wetland soils.

As mentioned above, central Alaska is the region of discontinuous permafrost. General circulation models all predict that global warming from increased concentrations of radiatively active gasses will occur earlier and to a larger extent at high latitudes. Because the temperature of permafrost in this area is so close to the melting point, we are at what might be called the "thermal ecotone," that is, the region in which small changes in temperature will have large consequences to ecosystems. Colder regions (i.e. colder than interior Alaska) must experience larger warming increments before permafrost melting occurs. Thus, we believe that interior Alaska is an ideal area to investigate and monitor the potential effects of climate change.

References Cited

Brown, J., and J. T. Andrews. 1982. Environmental and societal consequences of a possible CO2-induced climate change: influence of short-term climate fluctuations on permafrost terrain. U.S. Dept. Energy, Office of Energy Research, Washington, DC.
Carlson, R. F. 1974. Permafrost hydrology: an Alaskan's experience. Pages 51-57 in J. Demers (editor). Permafrost hydrology. Environment Canada, Ottawa.

Dingman, S.L. 1973. The water balance in Arctic and subarctic regions. Special Report 187, Cold Regions Research and Engineering Laboratory. Hanover, New Hampshire.

Haugen, R. K., C. W. Slaughter, K. E. Howe, and S. L. Dingman. 1982. Hydrology and climatology of the Caribou-Poker Creeks Research Watershed, Alaska. CRREL Rep. 82-26, Hanover, NH.

Morrissey, L.A., Strong, L.L., Card, D.H. 1986. Mapping permafrost in the boreal forest with thematic mapper satelite data. Photogrammetric Engineering and Remote Sensing 52: 1513-1520.

Post, W. M., W. R. Emanuel, P. J. Zinke, and A. G. Stangenberger. 1982. Soil carbon pools and world life zones. Nature 298: 156-159.

Rieger, S., C.E. Furbush, D.B. Schoephorster, H. Summerfield Jr., and L.C. Geiger. 1972. Soils of the Caribou-Poker Creeks Research Watershed, Interior Alaska. Technical Report #236. Corps of Engineers US CRREL, Hanover, NH. 14 pp.

Schlesinger, W. H. 1977. Carbon balance in terrestrial detritus. Annu. Rev. Ecol. Syst. 8: 51-81.

Viereck, L. A., K. Van Cleve, and C.T. Dyrness. 1986. Forest ecosystem distribution in the taiga environment. Pages 22-43 in K. Van Cleve, F. S. Chapin III, P. W. Flanagan, L. A. Viereck and C. T. Dyrness, eds, Forest ecosystems in the Alaskan taiga: a synthesis of structure and function. Springer-Verlag, New York.

Young, S. B. 1989. To the arctic: an introduction to the far northern world. John Wiley and Sons, New York.