Like many grasslands globally, tallgrass prairies of the North American Great Plains were historically maintained by frequent fire. Today, managers may want to manage prairies for grass dominance to produce biomass for livestock or for forb dominance to increase plant diversity and pollinator habitat. Fire season and interval could both influence grass dominance but their interacting effects with each other and with grazing are not well-understood. For example, annual fires often increase grass dominance but annual fire is most often applied in the spring, timing that favors warm-season grasses.
I examined grass dominance in tallgrass prairies using long-term plant cover monitoring data from Konza Prairie LTER (Kansas), Clymer Meadow Preserve (Texas), and Tridens Prairie Preserve (Texas). These prairies are managed with fire and sometimes grazing. Konza is often burned in the spring while Clymer is often burned in the fall; Tridens was burned infrequently. Between the three sites, fire has been used from January through October and fire intervals have varied from 1 to 22 years. The varying fire management, combined with over 5000 observations in units with unique fire histories, allows me to examine the effects of fire frequency, fire season, and grazing on grass dominance.
Grass dominance (grass cover as a proportion of total herbaceous cover) was higher at Konza (69 ± 19% [mean ± standard deviation]) and the infrequently burned Tridens (65 ± 15%) compared to Clymer (54 ± 23%). Grass dominance was lower in sites with longer fire intervals (74% with annual fire vs ~60% at 18+ year intervals). Units burned in January-July had higher grass dominance (60-70%) compared to units burned in October (40%). Grazed units had lower grass dominance (60 ± 20%) compared to ungrazed units (73 ± 17%). Interestingly, grazing influenced vegetation responses to fire season and interval. In management units burned in spring (March-April), grass dominance in ungrazed units was lower in sites with longer fire intervals; in grazed units, grass dominance was higher or remained stable. For ungrazed units burned in fall (September/October), grass dominance was higher in sites with longer fire intervals (too few fall-burned sites were grazed to examine that interaction). In all cases, the range of grass dominance was very large, suggesting that additional, unexamined factors are also important. These results show that the effects of fire season, fire frequency, and grazing interact to determine plant community outcomes in southern tallgrass prairies.