The time of year a fire occurs is one of many characteristics of a fire regime that can impact the survival of individuals and the relative abundances of plant species. Where an invasive grass has reduced native biodiversity, prescribed fire is a potential tool to differentially control the invasive and thereby restoring native diversity and improving our understanding of the role of fire in community dynamics. Bothriochloa ischaemum var. songarica (King Ranch bluestem) is a perennial bunch grass from Eurasia that reduces biodiversity in south central USA. In central Texas, we imposed prescribed fires in four different seasons, plus an unburned control, to determine the effects of fire on the abundances of B. ischaemum and native herbaceous plants. Our hypotheses were that 1) species richness would be higher in burned plots than in unburned plots, 2) plots that were burned when B. ischaemum was actively growing would have less B. ischaemum than plots burned in its dormant season, and 3) plots with less B. ischaemum would have greater abundances of native herbaceous perennials.
Results/Conclusions
The time of year the fires occurred had a substantial impact. On average, a plot burned in August or October had significantly less B. ischaemum than a plot burned in February or April or left unburned. The average estimated cover of B. ischaemum ranged from 8% in the plots burned in August to 68% in the unburned plots. Native herbaceous perennials were more abundant in burned plots than in unburned plots, and they were especially abundant in plots burned in August (P < 0.0015). In particular, the most common native perennial grass (Nassella leucotricha, Texas wintergrass) and the most common native perennial forb (Stenaria nigricans var. nigricans, bluets; Rubiaceae) were significantly more abundant in the plots burned in August compared to the unburned plots. Species richness, a component of species diversity, tended to be lower in unburned plots, but the trend was non-significant. Our results indicate that the date of the burn, probably in conjunction with the weather and fuel conditions on the day of the burn, can have very different effects on community composition.