This paper reports preliminary response of sandhills prairie vegetation to spring and summer prescribed burns, and their interaction with bison Bison bison grazing. The study was conducted in 1991 and 1992 at the Niobrara Valley Preserve in Nebraska, north-central USA.
Experimental sites were located within a 3,000 ha pasture grazed year-round by about 250 bison since 1986. Sandhills prairie occupies 90% of the area (dominated by perennial, warm season grasses, primarily, sand bluestem Andropogon hallii, prairie sandreed Calamovilfa longifolia, switchgrass Panicum virgatum and little bluestem Schizachyrium scopurium).
Approximately 500 ha were burned in two prescribed burns in early May (spring) 1991 and another 100 ha in late July (summer) 1991.
Vegetation was sampled at ‘sands range’ sites only (i.e. the prevalent and most productive Sandhills upland sites). Plant species composition and standing crop were recorded in 40, 1 m² paired plots (bison excluded from one of each pair by a wire cage) randomly located in burned and unburned areas during the 1991 (early May burns only; regrowth in July burns was not considered sufficient to warrant sampling) and 1992 growing seasons. In 1992, the cages were relocated (May and July burned areas 14 paired plots each; unburned areas 12 pairs).
The main findings were that at the end of the growing season, standing crop of rhizomatous grasses was increased by burning whilst bunchgrasses generally decreased.
Summer burning did not affect rhizomatous grass standing crop, but substantialy reduced that of bunchgrasses (1992: spring burn 7.6 g/m² grazed, 22.5 g/m² ungrazed; summer burn 1.5 g/m² grazed, 4.6 g/m² ungrazed). On burned areas, bison grazing reduced bunchgrass standing crop by 56%, but reduced rhizomatous grass standing crop by only 18%. Forbs generally appeared unaffected by grazing (individual species were affected variously by burning).
Thus maintenance of Sandhills prairie bunchgrasses may be dependent on fire exclusion. With burns, replacement of bunchgrass by rhizomatous grasses may increase available forage, but accordingly may increases susceptibility to wind erosion.
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