Effects of Climate on Numbers of Northern Prairie Wetlands
Introduction
The highly productive closed basins typical of the Prairie Pothole Region provide breeding and brood-rearing habitat for more than half of North America's waterfowl population (Batt, et al., 1989). Yearly population estimates of many duck species closely track the number of wet basins within the region (Johnson and Grier, 1988; Batt, et al., 1989). An understanding of the contribution climate makes to wetland numbers is increasingly important as we enter a period of potentially enhanced greenhouse warming. Although considerable uncertainty exists in projections of climatic change (Cess et al., 1993), general circulation models agree that temperatures will rise in the northern Great Plains (Smith and Tirpak, 1990; Karl, et al., 1991a). There is less agreement on potential changes in precipitation, but most climate models predict a slight net annual increase (Karl, et al., 1991a; see also Poiani and Johnson, 1991).
The number of wetland basins holding water during any year is a function of physical properties of the site such as soil permeability, groundwater flow, and basin size, as well as climatic effects such as temperature and precipitation (Meyboom, 1966; Shjeflo, 1968; Winter, 1989; Poiani and Johnson, 1991). The relative importance of each of these factors for water accumulation and retention has been studied for only a few wetlands (Winter, 1989) and may vary geographically. Because settling patterns of duck species throughout the Prairie Pothole Region vary and depend on local wetland conditions (Johnson and Grier, 1988), understanding geographic variation in wetland response to climate is a critical step in anticipating the effects a changing climate might have on waterfowl populations. The purpose of this paper is to (1) report on regression models relating number of wet basins to climate variables throughout the Prairie Pothole Region, (2) explore the potential effects of temperature and precipitation changes on number of basins holding water, (3) compare sensitivities of wetlands in different geographic areas of the Prairie Pothole Region to climate, and (4) examine implications for waterfowl breeding in the Prairie Pothole Region.
The following definitions apply in this paper. Prairie Pothole Region refers to the geographic area extending south from the border of the boreal forests in Manitoba, Saskatchewan, and Alberta to the southernmost extent of the Wisconsin glaciation in North Dakota and South Dakota. The region is unified by an abundance of glacial depressions, but can be subdivided by predominant vegetation: parkland, in which basins are surrounded by aspen and willow, and grassland, where mixed-grass prairie predominates. Wetland basins are glacially formed depressions, also known as potholes, that may or may not hold water in any given year; hereafter I refer to them simply as basins. Wet basins are those holding water during aerial surveys described below. Wetland classification follows Stewart and Kantrud (1971).
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