Tallgrass prairie fires historically occurred at a variety of frequencies and seasons. Currently, tallgrass prairie prescribed burns mostly occur annually in the spring and mostly for purposes of cattle forage production. Altering fire frequency and season is known to affect plant composition, but researchers are still uncertain how fire frequency and season interact. We present results of a long-term experiment examining the effects of a factorial combination of different burn frequencies (annual or quadrennial) and seasons (spring, summer, autumn, or variable season [rotated through seasons]) on the plant composition and species richness of an ungrazed, restored tallgrass prairie in eastern Nebraska, USA. The experimental plots were established in 1978 and surveyed for baseline data in 1979 and 1981. Experimental burn treatments were begun in 1982.
Results/Conclusions Multiple plant surveys were completed and these results will be presented, but all percent cover and visually estimated biomass values reported in the abstract are from 2011, the most recent survey. Annual spring and summer burns minimized forb percent cover (4% and 8%, respectively) while maximizing C4 graminoid visually estimated percent biomass (105% and 95%, respectively). The response to annual autumn burns was the opposite (30% forb cover and 81% C4 graminoid estimated percent biomass). Variable season and all quadrennial burns, regardless of burn season, were intermediate for both forb cover and C4 graminoid visually estimated percent biomass, suggesting that burn season primarily affected plant composition when burning occurred frequently. Total aboveground annual net primary production (ANPP) was measured in 2015, a year in which both annual and quadrennial burns occurred, and did not differ significantly between burn frequencies nor between spring and autumn burns (772 g m-2 average), although it was lower in summer burns (541 g m-2). We also found that C4 graminoid ANPP was higher in spring than autumn burns (795 g m-2 and 503 g m-2, respectively) while C3 graminoid ANPP was lower in spring than autumn burns (28 g m-2 and 116 g m-2, respectively). Overall, these results suggest autumn burns can increase C3 graminoid and forb abundance, without strongly affecting total ANPP relative to spring burns. Thus, land managers that burn annually in the spring may want to incorporate some autumn burns.