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Ecological Studies at the Woodworth Study Area

An Introduction to the Woodworth Study Area

Douglas H. Johnson1*, Kenneth F. Higgins2 and Robert O. Woodward1
National Biological Service, 1Northern Prairie Science Center,
8711 37th Street Southeast, Jamestown, ND 58401 and
2South Dakota Cooperative Fish and Wildlife Research Unit,
South Dakota State University, Wildlife and Fisheries
Sciences Department, Brookings, SD 57007

The Woodworth Study Area (WSA) was purchased by the U.S. Fish and Wildlife Service (FWS) during the early 1960's as a waterfowl production area. Unlike most such areas, its primary purpose was not to provide waterfowl breeding habitat directly, but instead it was dedicated for use as a research area to develop information for better management of upland and wetland habitats. This article provides some history of the area and background information about biological monitoring and research that have been conducted on the WSA. Unless otherwise stated, information included is derived from (1) or unpublished data on file at the Northern Prairie Science Center (NPSC).

The driving concept behind the WSA was to have a sizable tract of land within the Missouri Coteau and close to the Northern Prairie Wildlife Research Center (now the Northern Prairie Science Center) in Jamestown, North Dakota (2). The Center itself was established in 1963 by the FWS to investigate migratory birds, with an emphasis on waterfowl, their life history, habitat needs, and potential for management. The Woodworth site was intended to provide a representative area for long-term research on habitat changes and responses by prairie waterfowl.

The study area is located in northwestern Stutsman County about 5 km east of the city of Woodworth. Elevation ranges from 561 to 594 MSL. Of the 1,231 ha, 1,073 are federally owned. The remainder consists of privately owned property whose owners allowed research to be conducted. Study area managers were Leo M. Kirsch (deceased) from 1964 to 1979, Kenneth F. Higgins during 1979-85, and J. Michael Callow from 1985 to 1989. Since 1989, the area has been managed from the Jamestown Center. The FWS is established on site and, in conjunction with the NPSC, coordinates management of the area to facilitate research.

Geology and Soils

The Missouri Coteau is a geologic formation that extends from east-central South Dakota through southwestern Saskatchewan. The Coteau is an area of morainic hills that rises 91-152 m above the adjacent Drift Plain. The Coteau consists primarily of dead-ice moraine left from the extensive glacial stagnation that followed advances of late Wisconsin glaciers. The drift has been aged at 9,000-13,000 years.

The study area itself includes two major landforms: hummocky stagnation moraine and stagnation outwash moraine (3). Hummocky stagnation moraine is rugged with an average elevation of more than 564 m MSL, and consists mainly of knobs and kettles. The dominant material is glacial till. Stagnation outwash moraines are extensive areas underlain by glaciofluvial material deposited in association with stagnating dead ice.

Parent materials of the soils were deposited by glacial ice. They are considered glacial till, a mixture of sand, silt, clay, pebbles, and stones, without sorting by size. The major soil association is Buse-Barnes (4, 5). Svea, Renshaw, Fordville, Sioux, Parnell, Colvin, and Tetonka soils also occur, the latter three mainly in wetland basins.


The climate of the area is continental, characterized by a high evapo-transpiration ratio, cold winters, and warm summers. Average relative humidity is 68%. Precipitation records were taken in the city of Woodworth during 1961-66 and at the study area subsequently. Mean precipitation during 1964-81 was 41 cm; the long-term average for the general area was 44 cm. Precipitation in most years is greatest during summer.

Mean temperature for the area is 4 C, with January being the coldest month and July the warmest. The frost-free period averages 120 days, roughly between 20 May and 15 September. Prevailing winds are from the northwest, with an average speed of 16 km/h. Winds are typically strongest during the afternoon and calmest at night.

Land-Use History

The Woodworth area was settled by Europeans about 1900 (6). Prior to acquisition by the FWS, fields composing the study area were mostly used for grazing by cattle, sheep, and horses. About a fourth of the area had been cultivated, but some of it for only short periods.

Since acquisition, most of the study area, especially native prairie, was managed primarily by prescribed burning, grazing, and leaving idle. Areas that had been cultivated were planted, either to native grasses or to a mixture of tame grasses and legumes that were recommended for waterfowl nesting habitat. Reseeded areas were managed by fire, scarification, and to a limited extent, fertilization. Little management of wetlands has taken place, although in recent years several wetlands that had been drained have been restored by the FWS.


The native vegetation on the WSA is xeric mixed-grass prairie, which has been invaded by woody species--especially wolfberry (Symphoricarpos occidentalis), silverberry (Elaeagnus commutata), chokecherry (Prunus virginiana), and Woods rose (Rosa woodsii)--and introduced grasses--notably Kentucky bluegrass (Poa pratensis), smooth brome (Bromus inermis), and quackgrass (Agropyron repens). Vegetation on the study area is discussed in more detail by Meyer (7).


The study area contains 548 wetland basins. The distribution by class for 344 basins that contained water during mid-May surveys is presented in Table 1, along with the total area for each class. Wetlands were either fresh (typically >40 - 500 mmhos/cm3) or slightly brackish (typically 500 - 2,000 mmhos/cm3), as defined by Stewart and Kantrud (8).

Long-Term Studies and Monitoring

A number of habitat and wildlife surveys have been conducted for long periods on the WSA. All wetlands were surveyed, usually five times each year, once in each of the following periods: 1-15 May, 1-15 June, 16-30 June, 1-15 July, and 1-15 August. Basins were classified as wet if they were at least 5% inundated and the water was 2.5 cm or more deep. Wetlands were classified (8) annually during 1963-73 and in 1975, 1980, 1983, and 1988.

Upland vegetation was monitored with a series of transects, and first flowering dates were recorded for many species (9). Each transect consisted of a number of Daubenmire quadrats. Twelve transects were established on the area in 1966, but two were discontinued after four years. During the first few years, only the end points of the transects were permanently marked, and individual quadrats were located by equidistant pacing. In 1973, all quadrats were marked. Each transect included 7-14 square quadrats, each 1.5 m on a side. Woodworth Field Station staff and volunteers surveyed the transects once each year during 1966-89, in summer or later. They recorded the plant species composition in each quadrat according to Daubenmire coverage classes. In addition, environmental variates were recorded for each quadrat, including soil type, moisture regime (upland, lowland, wet meadow, or wetland), slope, and aspect. Kirsch and Kruse (10) reported preliminary results based on four of the transects. Johnson (11) analyzed data for 1966-82, focusing on 15 of the most common plant species, of the 228 identified.

Photo stations were placed in 33 areas that represented the three major cover types on the study area: native grassland, seeded native grassland, and seeded dense nesting cover. Photos were taken during 1970-89 once in mid April or May before new plant growth began to dominate residual vegetation and again in August after the growing season. The photos visually depicted changes in vegetation structure and composition resulting from land-use treatments such as grazing, burning, and other types of habitat manipulation.

Breeding waterfowl were censused annually during 1965-89. Numerous counts were made during the earlier years, but beginning in 1972, two were considered adequate. These were made between 10 April and 10 July each year, but counts taken near 1 May and near 1 June ultimately were used for population determination. Censuses were made while walking or driving around the periphery of wetland basins. Total number of duck pairs ranged from 236 to 692, and averaged 492 during 1965-89. Averages by species are presented in Table 2.

Nests of waterfowl were searched for and monitored in most years. Three or four searches were made each year, usually beginning the first week in May and concluding by mid July. A cable-chain device, towed between two jeeps, was developed at the Station (12) and used to find nests by flushing attending females. (The device has since become a standard technique for waterfowl nesting studies.) Information recorded at each nest included number of eggs, stage of incubation, and habitat features of the nest site and surrounding field. Nests were revisited periodically to monitor fates of the clutches. During 1966-81, 3,832 duck nests were found, ranging from 65 to 424 per year (Table 2).

Broods of waterfowl were also counted, although the secretiveness of ducklings resulted in very minimal estimates of the total number (Table 2). Two brood counts were made each year, one in early July and one in early August. Counts involved visiting each wetland on the study area, and wading through wetlands to flush the birds. Species, age class, and number of ducklings were recorded.

Terrestrial bird communities were censused annually during 1972-95. Censuses were conducted on as many as 10 plots, but six plots were surveyed in all years. Census plots ranged in size from 4.86 to 10.12 ha. Each was visited about eight times each year, and locations of birds were recorded on field maps. After all surveys were done, territories were plotted and the number of pairs of each species was estimated. Details of these censuses are reported separately (13).

In addition, surveys were made each year of dancing grounds of sharp-tailed grouse (Tympanuchus phasianellus). Incidental observations were made of other species as well. For migratory birds, dates of first sighting each spring were recorded.

Beyond the monitoring programs described, numerous scientists conducted shorter-term studies based at Woodworth. Those done by NPSC staff are listed in Appendix 1. Table 3 mentions those performed by visiting scientists.

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