The effect of fire season and frequency was evaluated in 2001 at a 65 ha restored tallgrass prairie in eastern Nebraska using permanent plots established in 1978. Treatments included annual and quadrennial burning in spring, summer, and fall as well as unburned plots. Overall, the number of species increased from 28 in 1979 to 30 in 2001 although Shannon diversity (H’) increased significantly across all treatments during this time reflecting the general shift from ruderal to native perennial species. Canopy cover of grasses (+19%) and forbs (+18%) increased significantly. However, only forbs showed a significant difference among treatments in 2001 with the lowest value occurring in the annual spring treatment (6%) and the highest value occurring in the annual variable-season treatment (35%). Native perennial grasses, such as big bluestem (Andropogon gerardii) and Indiangrass (Sorghastrum nutans), generally increased with more frequent burning, although there was considerable variation among seasonal treatments. Cool-season native sedges (Carex spp.) also increased significantly, although they did not show a differential response to treatment. In contrast, cool-season, non-native grasses, such as Kentucky bluegrass (Poa pratensis), increased significantly with quadrennial burns but remained low or absent with frequent fire, irrespective of season. The response of forb species differed from that of grasses in that higher cover generally occurred with less frequent burning, than with annual fall or variable-season burning.

This study suggests that variation in fire season and frequency in the tallgrass prairie has the potential to affect diversity, species composition, species groups, and individual species. Overall, this study shows that fire management with different seasons and frequencies of burning can be used to maintain prairie diversity. More specifically, the variation of plant responses suggests that, to provide high ecosystem diversity, fire management should be applied with a more random and less systematic season and frequency than is typically applied in prescribed burning of tallgrass prairies.