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Effects of Fire on Bird Populations in Mixed-grass Prairie

Study Area

The Woodworth Study Area consists of 1,231 ha of mixed-grass prairie pothole habitat in east-central North Dakota (Higgins et al. 1992). It is situated on the Missouri Coteau, a morainal belt extending from southcentral to northwestern North Dakota. The rolling terrain contains 548 wetlands on the study area, totaling 10% of the land area. Most of these are seasonally flooded (classified according to Stewart and Kantrud 1971), but many temporary and semipermanent wetlands also are present.

Prior to purchase by the U.S. Fish and Wildlife Service in the mid 1960's, land use on the study area was a mixture of grazing by cattle, haying, and crop production (Bayha 1964). Those practices continue on the privately owned portions of the study area. On the Service-owned portions (87% of the area), management of the uplands since acquisition has emphasized restoration of grassland. Some formerly cropped fields were replanted to grasses or grass-legume mixtures. Unplowed grasslands have been managed mostly by prescribed burning.

Study plots were located in relatively homogeneous areas within seven different quarter-sections, the units that received various treatments under the management of the study area. I located study plots so as to avoid large wetlands, in order to concentrate on upland bird communities. Plots were measured and marked by use of compass and pacing. Surveyor's flags were placed at 40-m intervals on a grid throughout each plot to facilitate recording of bird locations. One plot served as a control. The other six plots were subjected to burning under different regimes; intervals between burns averaging 3 to 5 years. Spring burns were slightly more frequent than fall burns. The seven study plots were denoted by the quarter-section in which they were located (e.g., Plot 13 was located within Quarter-section 13).

Plot 13, the control, had been grazed from 1906 to 1961 but has been left idle subsequently. Originally 8.09 ha in size, Plot 13 was increased to 10.12 ha in 1973. All or portions of eight seasonal and two temporary wetland basins lie within the plot, totaling about 1.2 ha. Upland vegetation is a mixture of grasses, forbs, and woody plants. Common species are Kentucky bluegrass (Poa pratensis), needle-and-thread (Stipa comata), stiff sunflower (Helianthus rigidus), and Canada goldenrod (Solidago canadensis). Wolfberry (Symphoricarpos occidentalis), silverberry (Eleagnus commutata), and Woods' rose (Rosa woodsii) form shrubby patches of various sizes. One thicket of chokecherry (Prunus virginiana), surrounded by wolfberry and silverberry, became decadent during the years of the surveys.

Plot 2, 8.68 ha in size, was surveyed during 1977-82, after which the trail to it became difficult. It had been grazed or hayed from 1906 to 1967, then left idle except for five prescribed burns. Plot 2 contains all or portions of seven seasonal wetlands, totaling about 0.9 ha. Kentucky bluegrass is abundant, and needle-and-thread, yarrow (Achillea lanulosa), fringed sage (Artemisia frigida), and prairie wild rose (Rosa arkansana) are common. Stands of wolfberry and silverberry also occur.

Plot 7, 6.07 ha in extant, was hayed during 1904-55. It was plowed in 1956 for one year of crop production. Alfalfa (Medicago sativa) and possibly some tame grasses were planted in 1958, after which it was grazed or hayed until 1970. Plot 7 has since been subjected to four prescribed burns. Six seasonal and two temporary wetlands, covering 1.1 ha, are included in the plot. Dominant upland plants are needle-and-thread, green needlegrass (Stipa viridula), alfalfa, Kentucky bluegrass, rigid goldenrod (Solidago rigida), and stiff sunflower. Patches of silverberry and wolfberry have increased in area during the study.

Plot 9, 6.07 ha in size, consists of unbroken sod that had been hayed and probably grazed from 1908 to 1965. There have been five burns since 1965. Two small seasonal and one ephemeral wetland cover 0.2 ha. Dominant plant species are Kentucky bluegrass, needle-and-thread, yellow sweetclover (Melilotus officinalis), white prairie aster (Aster ericoides), and stiff sunflower. Wolfberry occurs in several patches.

Plot 11, 4.86 ha in size, had been cropped during 1917-27 and from 1934 to about 1940. It then reverted to grass and was grazed through 1970. Since then, it has been burned four times. Plot 11 contains one ephemeral and portions of two seasonal wetlands, totaling about 0.5 ha. The uplands are dominated by Kentucky bluegrass and, to a lesser extent, smooth brome (Bromus inermis).

Plot 16 is 6.07 ha in size. It was grazed from 1906 to 1968, after which it was treated with a total of six prescribed fires. Five seasonal wetlands and small portions of two seasonal to semipermanent wetlands lie within the plot, comprising about 0.6 ha. Common plants are Kentucky bluegrass, quackgrass (Agropyron repens), needle-and-thread, and little bluestem (Andropogon scoparius), as well as wolfberry and silverberry.

Plot 18, also 6.07 ha in size, is unbroken prairie sod that had been grazed from 1906 to 1968. The plot was burned seven times between 1969 and 1990. In addition, it was intensively grazed by sheep during 1973 and 1974. The plot contains four small wetlands--one ephemeral, two temporary, and one seasonal--covering <0.1 ha. At the beginning of the surveys, the plot had several thickets of chokecherry and hawthorn (Crataegus chrysocarpa). The various fire and grazing treatments, as well as nest-searching with a cable-chain device (Higgins et al. 1969), have reduced the thickets considerably. Other common plants include Kentucky bluegrass, blue grama (Bouteloua gracilis), and fringed sage.

I excluded Plot 18 in 1974-75 because it had been crowd-grazed by sheep during the previous growing seasons. I excluded two plots for one year each (Plot 9 in 1982, Plot 18 in 1993) because portions were burned during the survey period. In 1977, two plots were burned on 1 June. Several surveys of both plots had been completed before the burns, other surveys were conducted after the burns. The pre-burn and post-burn results were used separately in the analysis.

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