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Effects of Climate on Numbers of Northern Prairie Wetlands

Methods

Figure 1. Study area used in model construction and validation. Shaded areas are parkland; unshaded areas are grassland. Horizontal lines are aerial transect locations. Numbers are stratum designations.

Models were developed using data from the annual May waterfowl breeding population and habitat surveys conducted by the United States Fish and Wildlife Service and the Canadian Wildlife Service (Henny, et al., 1972). These surveys are made from aircraft flown at an elevation of 30 to 50 m above ground along transects traversing strata of similar habitat (Figure 1); both ducks and wet basins are counted. All seasonally and semipermanently flooded basins holding water are counted on one side of the transect to a distance of 200 m. Temporary wetlands and sheet water are not counted. Only those strata occurring within the Prairie Pothole Region for which transect end points could be obtained were used in model development (Table 1). I used data from strata 34 and 35 for model testing because the latitudes and longitudes of the transect end points became available after models were completed. Because I did not know the total number of basins on each transect, I set the largest number counted on each transect for the period covered by the data to 100%. Strata were classified as parkland or grassland according to Kiel (1972) as modified by Sargeant, et al. (1993). Grassland was further divided into Canadian and United States transects because counts of wet basins started later in the United States and the climate data for the two countries came from different sources and included somewhat different variables. I excluded strata that could not be categorized as either grassland or parkland (e.g., transition forest). The Canadian grassland and parkland models were developed with data from 1968 through 1990; the United States grassland model was developed with data from 1973 through 1987. Each data set included relatively wet and dry, and warm and cool years (Figure 2).

Table 1. Geographic areas within the Prairie Pothole Region used in model development

Model Region	n Transects	Maximum Basins¹	Area (km²)		Basin Density²
Model Region	n Transects	Maximum Basins¹	Total	Sampled	Basin Density²
Parkland	17	13,025	131,427	1259	10.35
Canadian Grassland	28	10,343	195,237	2506	4.13
United States Grassland	20	8,019	169,515	2319	3.46

¹ Maximum number of wet basins counted during aerial transect surveys for the time period covered by the model: 1968-90 for parkland and Canadian grassland, 1973-87 for United States grassland.
² Basins/km² within sampled area.

Figure 2. Average annual maximum temperature and total annual precipitation for study areas in Canada (red line) and the United States (blue line) during the years used in model development (1973-1987 for United States; 1968-1990 for Canada).

United States climate data were obtained from the Historic Climatology Network (Karl, et al., 1990) and included monthly minimum, maximum, and mean temperatures and monthly total precipitation. Canadian climate data were obtained from the Canadian Climate Centre and included the same variables as the United States data set plus monthly snowfall accumulations. I produced monthly grids for each variable for each year, using ordinary kriging (Isaaks and Srivastava, 1989) with the software package Surfer (Golden Software, Inc., 1991) to geographically interpolate data. From these grids I sampled 10 randomly selected points on each transect, whose values were then averaged to obtain a mean value for each month of each year on each transect. The same 10 locations on each transect were used throughout.

I used multiple linear regression to examine the relationship between climate variables and percentage of wet basins for the three geographic regions: parkland, Canadian grassland, and United States grassland. Explanatory variables included minimum, maximum, and mean monthly, seasonal, and annual temperature; total monthly, seasonal, and annual precipitation; monthly snowfall (in Canada); and the number of wet basins counted the previous May. The effects of one- and two-year lags were examined for the seasonal and annual data. Two moisture indices were also examined, the Thornthwaite moisture index (McCabe and Wolock, 1991) and a conserved soil moisture index (Boyd, 1981). These indices are essentially linear combinations of seasonally weighted temperature and precipitation values. In addition, I examined effects of monthly temperature ranges (mean maximum - mean minimum) for spring and fall. All calculations were based on a year defined to be May-to-April. Because the same transects were sampled each year, I treated transects as blocks. The MAXR selection method in SAS PROC REG (SAS Institute, Inc., 1988) was used to determine which climate variables best explained the variation in the wet basin data. Only variables resulting in a substantial increase in R² were included in the final models. When two or more variables produced similar increases in R², variables not resulting in multicollinearity were favored. Finally, models were inspected for presence of interactions and quadratic terms.

I tested the models in three ways. First, I randomly withheld 10% of the data, re-ran the models on the restricted data set, and compared observed and predicted values for percentage of basins holding water (hereafter, percent wet basins). Next, I withheld two randomly selected transects from each stratum, one transect at a time, re-ran the models on the restricted data sets, and again compared observed and predicted values for percent wet basins. Finally, I used the parkland model to predict percent wet basins on transects in Strata 34 and 35, which were not used in model development, and compared observed and predicted values.

To explore the potential effects of increasing temperatures on number of wet basins in the three regions, I increased temperature variables in the original climate data set by 3 °C and 6 °C but held precipitation at observed levels. Likewise, I increased and decreased precipitation by 10% but held temperatures at observed levels. Finally, I combined the 3 °C temperature increase and 10% precipitation increase and decrease to examine the potential for interactive effects in the models.

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