WETLANDS AND WILLOW RINGS IN THE NORTHERN PRAIRIES

The northern glaciated plains are dotted with millions of small wetlands (“potholes” or “sloughs”) generally surrounded by a ring of willows or other water-loving vegetation. Field studies have documented the complex interactions between groundwater and surface water and vegetation within and around the wetlands. The exchange of water between the riparian edge, (the “willow ring”) and the centre of the wetland varies from season to season and year to year, and the trees in the willow rings themselves are transient, in response to wet and dry climatic cycles. Processes that influence the exchange of water include snow trapping in the willow ring, infiltration through the unfrozen soil beneath the snow banks, higher evapotranspiration from the willow ring than the centre of the wetland, subsurface seepage from the central pond (if there is one) to the depressed water table beneath the willow ring, and migration of moisture to the freezing front in winter. Over the long-term there is a net outward subsurface flow to the willow ring, which continues even in fall and winter after the trees stop transpiring. This outward flow finds expression in the wetland soils and the water chemistry of the wetland and the groundwater near it. Long-term water-level data on wetlands suggest that development of a willow ring has little impact on the water balance of the wetlands as a whole, but probably repartitions evapotranspiration between the centre of the wetland and the willow ring.