Active-Layer Hydrology in Nonsorted Circle Ecosystems of the Arctic Tundra

The interdependency of biological and physical components of the arctic tundra system can yield strongly nonlinear processes with potential thresholds for ecosystem shifts. Hydrology critically infl uences the ecosystem dynamics in the Arctic, even though the tundra appears to have a relatively simple hydrologic system, often consisting of only a saturated active layer underlain by permafrost. Disturbances associated with the freezing and thawing of the active layer (cryoturbation) are key hydrological processes that shape the arctic tundra. In most cases, water is assumed to be present in excess, and its sources are, therefore, mostly ignored. One type of land surface formation generated by active layer freezing and thawing is the nonsorted circle. Nonsorted circles (sometimes referred to as frost boils) are approximately 1 to 3 m in diameter and generally have little vegetation on them due to excessive soil expansion from ice accumulation during winter (Washburn, 1956). Nonsorted circles are, however, typically surrounded by dense vegetation, which acts as an insulator against winter cooling, leading to preferential formation of ice within the circles. The water needed for the observed ice formation within the circle is thought to migrate horizontally from vegetation-covered soil, outside of the circle, as a result of increased tension within the circle during freezing. Thus, moisture migrates laterally as a result of horizontal differences in insulation at the surface, particularly in fi ne-textured soils. The detailed processes governing the formation and persistence of nonsorted circles and their effects on soil–plant–water relationships are still an area of active research (Peterson and Krantz, 2003). Several modeling studies have focused on ice lens formation within nonsorted circles that causes substantial soil expansion (frost heave) (Miller, 1980; Fowler and Krantz, 1994). Yet these and other models account for only a single dimension, which is insuffi cient to capture the hydrological dynamics of a nonsorted circle (Boike et al., 2002). Nicolsky et al. (2004) developed a two-dimensional model that accurately estimates frost heave for a single nonsorted circle, assuming an open system with an unlimited supply of water. However, even in a relatively saturated arctic tundra system, the available water is limited; therefore, for accurate results, the supply of water generating frost heave must be present within the domain of the simulation. Ippisch (2003) developed a three-dimensional model that also captures the dynamics of a single nonsorted circle; however, the details focus more on gas and solute fl ow than on the rheology of the soil. The modeling of these systems could be improved to include coupled Active-Layer Hydrology in Nonsorted Circle Ecosystems of the Arctic Tundra