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. Ice-rich permafrost is commonly impervious to the infiltration of water, and, as a consequence, soil water is confined to the near surface. This creates an abundance of saturated soils and impressive biological productivity of vegetation despite very low annual precipitation.

On the North Slope of Alaska, permafrost is continuous and underlying virtually the entire landscape. In relatively warmer subarctic regions, such as Interior Alaska, permafrost is discontinuous and found in locally cold settings (e.g., north-facing slopes and low-lying, poorly drained valley bottoms). In areas of Interior Alaska where permafrost is present, temperatures of upper permafrost layers range from -0.5°C to -2.0°C and are near the point of thawing. Over the past century, the mean annual air temperature in Interior Alaska has increased 0.016°C year-1, resulting in a gradual warming of permafrost.

Regions underlain by permafrost account for 16% of the land surface yet store nearly 50% of the world’s reactive soil carbon. 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 documents significant losses of soil carbon with permafrost thaw that, over decadal time scales, could make permafrost a large biospheric carbon source in a warming world.

Permafrost has a dominant control on watershed hydrology and biological processes in Interior Alaska as it forms an impermeable barrier that restricts subsurface flows to the shallow active layer of soils. In regions with discontinuous permafrost, north facing slopes and valley bottoms are commonly underlain with permafrost whereas warmer south facing slopes are not. Consequently, ground water flowing through north versus south facing slopes travels along different subsurface flow paths, which has important implications for stream discharge.

Permafrost presence and summer thaw depth 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 with white spruce (Picea glauca), birch (Betula papyrifera), and aspen (Populus tremuloides) on south-facing slopes with thin organic soils and black spruce (Picea mariana) on north-facing slopes and in low-lying areas with thick organic soils.