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. Nearly one-fourth of the earth's surface is influenced by permafrost. Ice-rich permafrost is commonly impervious to infiltration of water, and, as a consequence, soil water is confined to the near surface creating abundance of saturated soils and impressive biological productivity of vegetation despite very low annual precipitation.
On 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, where permafrost is present, temperatures of the upper layers of the permafrost are in the range of -0.5°C to -2.0°C
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 resulting in a gradual warming of permafrost.
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.
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 . As permafrost thaws, the liberated soil carbon can be rapidly mineralized or exported from soils as dissolved organic and inorganic carbon in river flow. 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.
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. 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.